{"id":"CVE-2026-26209","aliases":["AZL-80541","DEBIAN-CVE-2026-26209","EUVD-2026-14478","GHSA-3c37-wwvx-h642","PYSEC-2026-2123","ROOT-APP-PYPI-CVE-2026-26209","SUSE-SU-2026:21139-1","UBUNTU-CVE-2026-26209","openSUSE-SU-2026:10425-1","openSUSE-SU-2026:20468-1"],"status":"published","title":"cbor2 has a Denial of Service via Uncontrolled Recursion in cbor2.loads","description":"cbor2 provides encoding and decoding for the Concise Binary Object Representation (CBOR) serialization format. Versions prior to 5.9.0 are vulnerable to a Denial of Service (DoS) attack caused by uncontrolled recursion when decoding deeply nested CBOR structures. This vulnerability affects both the pure Python implementation and the C extension `_cbor2`. The C extension relies on Python's internal recursion limits `Py_EnterRecursiveCall` rather than a data-driven depth limit, meaning it still raises `RecursionError` and crashes the worker process when the limit is hit. While the library handles moderate nesting levels, it lacks a hard depth limit. An attacker can supply a crafted CBOR payload containing approximately 100,000 nested arrays `0x81`. When `cbor2.loads()` attempts to parse this, it hits the Python interpreter's maximum recursion depth or exhausts the stack, causing the process to crash with a `RecursionError`. Because the library does not enforce its own limits, it allows an external attacker to exhaust the host application's stack resource. In many web application servers (e.g., Gunicorn, Uvicorn) or task queues (Celery), an unhandled `RecursionError` terminates the worker process immediately. By sending a stream of these small (\u003c100KB) malicious packets, an attacker can repeatedly crash worker processes, resulting in a complete Denial of Service for the application. Version 5.9.0 patches the issue.","severity":{"version":"3.0","score":7.5,"vector":"CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H","level":"high","assessor":"cna","from":"cve"},"epss":{"day":"2026-11-11","score":0.00406,"percentile":0.353,"model":"v2026.06.15"},"facts":{"in_kev":false,"known_exploited":false,"public_exploit":false,"fix_available":true},"cwes":["CWE-674"],"published":"2026-03-23T18:53:10Z","modified":"2026-11-05T05:36:30Z","fixes":[{"ecosystem":"PyPI","package":"cbor2","fixed_in":["5.9.0"],"sources":["ghsa","osv"]},{"ecosystem":"Debian:14","package":"cbor2","fixed_in":["5.9.0-1"],"sources":["osv"]},{"ecosystem":"Root:PyPI","package":"rootio-cbor2","fixed_in":["5.6.5+root.io.2","5.6.5+root.io.3"],"sources":["osv"]},{"ecosystem":"Root:PyPI","package":"cbor2","fixed_in":["5.6.5+aikido.3"],"sources":["osv"]},{"ecosystem":"openSUSE:Tumbleweed","package":"python-cbor2","fixed_in":["5.9.0-1.1"],"sources":["osv"]}],"affected":[{"source":"ghsa","ecosystem":"PyPI","package":"cbor2","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"last_affected":"5.8.0"},{"fixed":"5.9.0"}]}]},{"source":"osv","ecosystem":"Azure Linux:3","package":"python-cbor2","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"last_affected":"5.6.5-2"}]}]},{"source":"osv","ecosystem":"Debian:12","package":"cbor2","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["5.4.6-1","5.4.6-2","5.5.1-1","5.6.0-1","5.6.1-1","5.6.2-1","5.6.3-1","5.6.4-1","5.6.5-1","5.7.0-1","5.7.1-1","5.8.0-1","5.8.0-2","5.8.0-3","5.9.0-1","5.9.0-2","5.9.0-2~bpo13+1","5.9.0-2~bpo13+2","6.1.2-1","6.1.2-2"]},{"source":"osv","ecosystem":"Debian:13","package":"cbor2","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["5.6.5-1","5.7.0-1","5.7.1-1","5.8.0-1","5.8.0-2","5.8.0-3","5.9.0-1","5.9.0-2","5.9.0-2~bpo13+1","5.9.0-2~bpo13+2","6.1.2-1","6.1.2-2"]},{"source":"osv","ecosystem":"Debian:14","package":"cbor2","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"5.9.0-1"}]}],"versions":["5.6.5-1","5.7.0-1","5.7.1-1","5.8.0-1","5.8.0-2","5.8.0-3"]},{"source":"osv","ecosystem":"PyPI","package":"cbor2","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"5.9.0"}]}],"versions":["1.0.0","1.1.0","2.0.0","3.0.0","3.0.1","3.0.2","3.0.3","3.0.4","4.0.0","4.0.1","4.1.0","4.1.1","4.1.2","5.0.0","5.0.1","5.1.0","5.1.1","5.1.2","5.2.0","5.2.0.post1","5.3.0","5.4.0","5.4.1","5.4.2","5.4.2.post1","5.4.3","5.4.4","5.4.5","5.4.6","5.5.0","5.5.1","5.6.0","5.6.1","5.6.2","5.6.3","5.6.4","5.6.5","5.7.0","5.7.1","5.8.0"]},{"source":"osv","ecosystem":"PyPI","package":"cbor2","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"5.9.0"}]}],"versions":["1.0.0","1.1.0","2.0.0","3.0.0","3.0.1","3.0.2","3.0.3","3.0.4","4.0.0","4.0.1","4.1.0","4.1.1","4.1.2","5.0.0","5.0.1","5.1.0","5.1.1","5.1.2","5.2.0","5.2.0.post1","5.3.0","5.4.0","5.4.1","5.4.2","5.4.2.post1","5.4.3","5.4.4","5.4.5","5.4.6","5.5.0","5.5.1","5.6.0","5.6.1","5.6.2","5.6.3","5.6.4","5.6.5","5.7.0","5.7.1","5.8.0"]},{"source":"osv","ecosystem":"Root:PyPI","package":"rootio-cbor2","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"5.6.5+root.io.2"},{"fixed":"5.6.5+root.io.3"}]}]},{"source":"osv","ecosystem":"Root:PyPI","package":"cbor2","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"5.6.5+aikido.3"}]}]},{"source":"osv","ecosystem":"SUSE:Linux Enterprise Server 16.0","package":"python-cbor2","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"5.6.5-160000.4.1"}]}],"bundled":2},{"source":"osv","ecosystem":"SUSE:Linux Enterprise Server for SAP applications 16.0","package":"python-cbor2","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"5.6.5-160000.4.1"}]}],"bundled":2},{"source":"osv","ecosystem":"Ubuntu:22.04:LTS","package":"cbor2","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["5.2.0-4","5.2.0-4build1","5.4.2-1","5.4.2-1build1"]},{"source":"osv","ecosystem":"Ubuntu:24.04:LTS","package":"cbor2","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["5.4.6-2","5.4.6-2build1","5.5.1-1","5.6.0-1","5.6.1-1","5.6.2-1","5.6.2-1build1"]},{"source":"osv","ecosystem":"Ubuntu:25.10","package":"cbor2","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["5.6.5-1build1"]},{"source":"osv","ecosystem":"Ubuntu:26.10","package":"cbor2","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["5.8.0-2ubuntu1","6.1.2-2"]},{"source":"osv","ecosystem":"Ubuntu:26.04:LTS","package":"cbor2","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["5.6.5-1build1","5.7.0-1","5.7.1-1","5.8.0-1","5.8.0-2ubuntu1"]},{"source":"osv","ecosystem":"openSUSE:Tumbleweed","package":"python-cbor2","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"5.9.0-1.1"}]}]},{"source":"osv","ecosystem":"openSUSE:Leap 16.0","package":"python-cbor2","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"5.6.5-160000.4.1"}]}],"bundled":2}],"products":[{"vendor":"agronholm","product":"cbor2"}],"references":[{"url":"https://github.com/agronholm/cbor2/security/advisories/GHSA-3c37-wwvx-h642","tags":["x_refsource_CONFIRM","Exploit","Vendor Advisory","advisory","web","evidence","report"]},{"url":"https://github.com/agronholm/cbor2/pull/275","tags":["x_refsource_MISC","Issue Tracking","Patch","fix","web","report"]},{"url":"https://github.com/agronholm/cbor2/commit/e61a5f365ba610d5907a0ae1bc72769bba34294b","tags":["x_refsource_MISC","Patch","fix","web","report"]},{"url":"https://github.com/agronholm/cbor2/releases/tag/5.9.0","tags":["x_refsource_MISC","Product","Release Notes","web","advisory","report"]},{"url":"https://nvd.nist.gov/vuln/detail/CVE-2026-26209","tags":["web","advisory"]},{"url":"https://github.com/advisories/GHSA-3c37-wwvx-h642"},{"url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/26xxx/CVE-2026-26209.json","tags":["advisory"]},{"url":"https://security-tracker.debian.org/tracker/CVE-2026-26209","tags":["advisory"]},{"url":"https://github.com/agronholm/cbor2","tags":["package"]},{"url":"https://www.suse.com/support/update/announcement/2026/suse-su-202621139-1/","tags":["advisory"]},{"url":"https://bugzilla.suse.com/1255783","tags":["report"]},{"url":"https://bugzilla.suse.com/1260367","tags":["report"]},{"url":"https://www.suse.com/security/cve/CVE-2025-68131","tags":["web"]},{"url":"https://www.suse.com/security/cve/CVE-2026-26209","tags":["web"]},{"url":"https://ubuntu.com/security/CVE-2026-26209","tags":["report"]},{"url":"https://www.cve.org/CVERecord?id=CVE-2026-26209","tags":["report"]},{"url":"","tags":["advisory"]}],"sources":{"cve":[{"id":"CVE-2026-26209","title":"cbor2 has a Denial of Service via Uncontrolled Recursion in cbor2.loads","description":"cbor2 provides encoding and decoding for the Concise Binary Object Representation (CBOR) serialization format. Versions prior to 5.9.0 are vulnerable to a Denial of Service (DoS) attack caused by uncontrolled recursion when decoding deeply nested CBOR structures. This vulnerability affects both the pure Python implementation and the C extension `_cbor2`. The C extension relies on Python's internal recursion limits `Py_EnterRecursiveCall` rather than a data-driven depth limit, meaning it still raises `RecursionError` and crashes the worker process when the limit is hit. While the library handles moderate nesting levels, it lacks a hard depth limit. An attacker can supply a crafted CBOR payload containing approximately 100,000 nested arrays `0x81`. When `cbor2.loads()` attempts to parse this, it hits the Python interpreter's maximum recursion depth or exhausts the stack, causing the process to crash with a `RecursionError`. Because the library does not enforce its own limits, it allows an external attacker to exhaust the host application's stack resource. In many web application servers (e.g., Gunicorn, Uvicorn) or task queues (Celery), an unhandled `RecursionError` terminates the worker process immediately. By sending a stream of these small (\u003c100KB) malicious packets, an attacker can repeatedly crash worker processes, resulting in a complete Denial of Service for the application. Version 5.9.0 patches the issue.","status":"published","published":"2026-03-23T18:53:10.268Z","modified":"2026-03-24T18:35:35.486Z","cvss":[{"version":"3.0","score":7.5,"vector":"CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H","level":"high","assessor":"cna"}],"cwes":["CWE-674"],"updated_at":"2026-11-02T13:38:20.971157Z"}],"euvd":[{"id":"EUVD-2026-14478","description":"cbor2 provides encoding and decoding for the Concise Binary Object Representation (CBOR) serialization format. Versions prior to 5.9.0 are vulnerable to a Denial of Service (DoS) attack caused by uncontrolled recursion when decoding deeply nested CBOR structures. This vulnerability affects both the pure Python implementation and the C extension `_cbor2`. The C extension relies on Python's internal recursion limits `Py_EnterRecursiveCall` rather than a data-driven depth limit, meaning it still raises `RecursionError` and crashes the worker process when the limit is hit. While the library handles moderate nesting levels, it lacks a hard depth limit. An attacker can supply a crafted CBOR payload containing approximately 100,000 nested arrays `0x81`. When `cbor2.loads()` attempts to parse this, it hits the Python interpreter's maximum recursion depth or exhausts the stack, causing the process to crash with a `RecursionError`. Because the library does not enforce its own limits, it allows an external attacker to exhaust the host application's stack resource. In many web application servers (e.g., Gunicorn, Uvicorn) or task queues (Celery), an unhandled `RecursionError` terminates the worker process immediately. By sending a stream of these small (\u003c100KB) malicious packets, an attacker can repeatedly crash worker processes, resulting in a complete Denial of Service for the application. Version 5.9.0 patches the issue.","status":"published","published":"2026-03-23T18:53:10Z","modified":"2026-03-24T18:35:35Z","cvss":[{"version":"3.0","score":7.5,"vector":"CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H","level":"high"}],"updated_at":"2026-11-02T19:25:29.933437Z"}],"ghsa":[{"id":"GHSA-3c37-wwvx-h642","title":"cbor2 has a Denial of Service via Uncontrolled Recursion in cbor2.loads","description":"### Summary\n\n- The `cbor2` library is vulnerable to a Denial of Service (DoS) attack caused by uncontrolled recursion when decoding deeply nested CBOR structures.\n- This vulnerability affects both the pure Python implementation and the C extension (`_cbor2`). The C extension correctly uses Python's C-API for recursion protection (`Py_EnterRecursiveCall`), but this mechanism is designed to prevent a stack overflow by raising a `RecursionError`. In some environments, this exception is not caught, thus causing the service process to terminate.\n- While the library handles moderate nesting, it lacks a configurable, data-driven depth limit independent of Python's global recursion setting. An attacker can supply a crafted CBOR payload containing thousands of nested arrays (e.g., `0x81`). When `cbor2.loads()` attempts to parse this, it hits the interpreter's recursion limit, causing the call to raise a `RecursionError`.\n- By sending a stream of small (\u003c100KB) malicious packets, an attacker can repeatedly crash worker processes faster than they can be restarted, resulting in a complete and sustained Denial of Service.\n\n### Details\n\n- The vulnerability stems from the recursive design of the `CBORDecoder` class, specifically how it decodes nested container types like Arrays and Maps.\n- Inside `decode_array` (and similarly `decode_map`), the decoder iterates through the number of elements specified in the CBOR header. For each element, it calls `self.decode()` again to parse the nested item. This recursive call lacks a depth-tracking mechanism.\n- Vulnerable Code Locations:\n  - `cbor2/decoder.py` (Pure Python implementation)\n  - `source/decoder.c` (C extension implementation)\n- Execution Flow:\n  1. The `cbor2.loads()` function initializes a `CBORDecoder` and calls its `decode()` method.\n  2. The `decode()` method reads the initial byte and dispatches control to a specific handler based on the major type. For an Array (Major Type 4), it calls `decode_array`.\n  3. `decode_array` loops and calls `self.decode()` for each item, leading to deep recursion when parsing a payload like `[...[...[1]...]...]`.\n\n### PoC\n\n```\nimport cbor2\n\nDEPTH = 1000\n\npayload = b'\\x81' * DEPTH + b'\\x01'\nprint(f\"[*] Payload size: {len(payload) / 1024:.2f} KB\")\nprint(\"[*] Triggering decoder...\")\n\ntry:\n    cbor2.loads(payload)\n    print(\"[+] Parsed successfully (Not Vulnerable)\")\nexcept RecursionError:\n    print(\"\\n[!] VULNERABLE: RecursionError triggered!\")\nexcept Exception as e:\n    print(f\"\\n[-] Unexpected Error: {type(e).__name__}: {e}\")\n```\n\n### Impact\n\n- Scope: This vulnerability affects any application using `cbor2` to parse untrusted data. Common use cases include IoT data processing, WebAuthn (FIDO2) authentication flows, and inter-service communication over COSE (CBOR Object Signing and Encryption).\n- Attack Vector: A remote, unauthenticated attacker can achieve a full Denial of Service with a highly efficient, low-bandwidth attack. A payload under 100KB is sufficient to reliably terminate a Python worker process.\n\n### Credit\n\nThis issue was discovered by Kevin Tu of TMIR at ByteDance. The patch was developed by @agronholm.","status":"published","published":"2026-03-23T20:23:57Z","modified":"2026-03-25T20:38:41Z","cvss":[{"version":"3.0","score":7.5,"vector":"CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H","level":"high"}],"cwes":["CWE-674"],"updated_at":"2026-11-02T14:39:42.712607Z"}],"nvd":[{"id":"CVE-2026-26209","description":"cbor2 provides encoding and decoding for the Concise Binary Object Representation (CBOR) serialization format. Versions prior to 5.9.0 are vulnerable to a Denial of Service (DoS) attack caused by uncontrolled recursion when decoding deeply nested CBOR structures. This vulnerability affects both the pure Python implementation and the C extension `_cbor2`. The C extension relies on Python's internal recursion limits `Py_EnterRecursiveCall` rather than a data-driven depth limit, meaning it still raises `RecursionError` and crashes the worker process when the limit is hit. While the library handles moderate nesting levels, it lacks a hard depth limit. An attacker can supply a crafted CBOR payload containing approximately 100,000 nested arrays `0x81`. When `cbor2.loads()` attempts to parse this, it hits the Python interpreter's maximum recursion depth or exhausts the stack, causing the process to crash with a `RecursionError`. Because the library does not enforce its own limits, it allows an external attacker to exhaust the host application's stack resource. In many web application servers (e.g., Gunicorn, Uvicorn) or task queues (Celery), an unhandled `RecursionError` terminates the worker process immediately. By sending a stream of these small (\u003c100KB) malicious packets, an attacker can repeatedly crash worker processes, resulting in a complete Denial of Service for the application. Version 5.9.0 patches the issue.","status":"published","published":"2026-03-23T19:16:39.53Z","modified":"2026-06-17T10:25:56.8Z","cvss":[{"version":"3.0","score":7.5,"vector":"CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H","level":"high","assessor":"secondary"}],"cwes":["CWE-674"],"updated_at":"2026-11-02T14:12:10.568062Z"}],"osv":[{"id":"CVE-2026-26209","title":"cbor2 has a Denial of Service via Uncontrolled Recursion in cbor2.loads","description":"cbor2 provides encoding and decoding for the Concise Binary Object Representation (CBOR) serialization format. Versions prior to 5.9.0 are vulnerable to a Denial of Service (DoS) attack caused by uncontrolled recursion when decoding deeply nested CBOR structures. This vulnerability affects both the pure Python implementation and the C extension `_cbor2`. The C extension relies on Python's internal recursion limits `Py_EnterRecursiveCall` rather than a data-driven depth limit, meaning it still raises `RecursionError` and crashes the worker process when the limit is hit. While the library handles moderate nesting levels, it lacks a hard depth limit. An attacker can supply a crafted CBOR payload containing approximately 100,000 nested arrays `0x81`. When `cbor2.loads()` attempts to parse this, it hits the Python interpreter's maximum recursion depth or exhausts the stack, causing the process to crash with a `RecursionError`. Because the library does not enforce its own limits, it allows an external attacker to exhaust the host application's stack resource. In many web application servers (e.g., Gunicorn, Uvicorn) or task queues (Celery), an unhandled `RecursionError` terminates the worker process immediately. By sending a stream of these small (\u003c100KB) malicious packets, an attacker can repeatedly crash worker processes, resulting in a complete Denial of Service for the application. Version 5.9.0 patches the issue.","status":"published","published":"2026-03-23T18:53:10.268Z","modified":"2026-10-28T11:09:48.226560784Z","cvss":[{"version":"3.0","score":7.5,"vector":"CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H","level":"high"}],"cwes":["CWE-674"],"updated_at":"2026-11-02T15:44:59.80315Z"},{"id":"DEBIAN-CVE-2026-26209","description":"cbor2 provides encoding and decoding for the Concise Binary Object Representation (CBOR) serialization format. Versions prior to 5.9.0 are vulnerable to a Denial of Service (DoS) attack caused by uncontrolled recursion when decoding deeply nested CBOR structures. This vulnerability affects both the pure Python implementation and the C extension `_cbor2`. The C extension relies on Python's internal recursion limits `Py_EnterRecursiveCall` rather than a data-driven depth limit, meaning it still raises `RecursionError` and crashes the worker process when the limit is hit. While the library handles moderate nesting levels, it lacks a hard depth limit. An attacker can supply a crafted CBOR payload containing approximately 100,000 nested arrays `0x81`. When `cbor2.loads()` attempts to parse this, it hits the Python interpreter's maximum recursion depth or exhausts the stack, causing the process to crash with a `RecursionError`. Because the library does not enforce its own limits, it allows an external attacker to exhaust the host application's stack resource. In many web application servers (e.g., Gunicorn, Uvicorn) or task queues (Celery), an unhandled `RecursionError` terminates the worker process immediately. By sending a stream of these small (\u003c100KB) malicious packets, an attacker can repeatedly crash worker processes, resulting in a complete Denial of Service for the application. Version 5.9.0 patches the issue.","status":"published","published":"2026-03-23T19:16:39.53Z","modified":"2026-10-01T03:03:21.529015299Z","cvss":[{"version":"3.0","score":7.5,"vector":"CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H","level":"high"}],"updated_at":"2026-11-02T15:45:53.877573Z"},{"id":"GHSA-3c37-wwvx-h642","title":"cbor2 has a Denial of Service via Uncontrolled Recursion in cbor2.loads","description":"### Summary\n\n- The `cbor2` library is vulnerable to a Denial of Service (DoS) attack caused by uncontrolled recursion when decoding deeply nested CBOR structures.\n- This vulnerability affects both the pure Python implementation and the C extension (`_cbor2`). The C extension correctly uses Python's C-API for recursion protection (`Py_EnterRecursiveCall`), but this mechanism is designed to prevent a stack overflow by raising a `RecursionError`. In some environments, this exception is not caught, thus causing the service process to terminate.\n- While the library handles moderate nesting, it lacks a configurable, data-driven depth limit independent of Python's global recursion setting. An attacker can supply a crafted CBOR payload containing thousands of nested arrays (e.g., `0x81`). When `cbor2.loads()` attempts to parse this, it hits the interpreter's recursion limit, causing the call to raise a `RecursionError`.\n- By sending a stream of small (\u003c100KB) malicious packets, an attacker can repeatedly crash worker processes faster than they can be restarted, resulting in a complete and sustained Denial of Service.\n\n### Details\n\n- The vulnerability stems from the recursive design of the `CBORDecoder` class, specifically how it decodes nested container types like Arrays and Maps.\n- Inside `decode_array` (and similarly `decode_map`), the decoder iterates through the number of elements specified in the CBOR header. For each element, it calls `self.decode()` again to parse the nested item. This recursive call lacks a depth-tracking mechanism.\n- Vulnerable Code Locations:\n  - `cbor2/decoder.py` (Pure Python implementation)\n  - `source/decoder.c` (C extension implementation)\n- Execution Flow:\n  1. The `cbor2.loads()` function initializes a `CBORDecoder` and calls its `decode()` method.\n  2. The `decode()` method reads the initial byte and dispatches control to a specific handler based on the major type. For an Array (Major Type 4), it calls `decode_array`.\n  3. `decode_array` loops and calls `self.decode()` for each item, leading to deep recursion when parsing a payload like `[...[...[1]...]...]`.\n\n### PoC\n\n```\nimport cbor2\n\nDEPTH = 1000\n\npayload = b'\\x81' * DEPTH + b'\\x01'\nprint(f\"[*] Payload size: {len(payload) / 1024:.2f} KB\")\nprint(\"[*] Triggering decoder...\")\n\ntry:\n    cbor2.loads(payload)\n    print(\"[+] Parsed successfully (Not Vulnerable)\")\nexcept RecursionError:\n    print(\"\\n[!] VULNERABLE: RecursionError triggered!\")\nexcept Exception as e:\n    print(f\"\\n[-] Unexpected Error: {type(e).__name__}: {e}\")\n```\n\n### Impact\n\n- Scope: This vulnerability affects any application using `cbor2` to parse untrusted data. Common use cases include IoT data processing, WebAuthn (FIDO2) authentication flows, and inter-service communication over COSE (CBOR Object Signing and Encryption).\n- Attack Vector: A remote, unauthenticated attacker can achieve a full Denial of Service with a highly efficient, low-bandwidth attack. A payload under 100KB is sufficient to reliably terminate a Python worker process.\n\n### Credit\n\nThis issue was discovered by Kevin Tu of TMIR at ByteDance. The patch was developed by @agronholm.","status":"published","published":"2026-03-23T20:23:57Z","modified":"2026-09-10T03:50:36.691215585Z","cvss":[{"version":"3.0","score":7.5,"vector":"CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H","level":"high"}],"cwes":["CWE-674"],"updated_at":"2026-11-02T15:46:35.406455Z"},{"id":"PYSEC-2026-2123","description":"cbor2 provides encoding and decoding for the Concise Binary Object Representation (CBOR) serialization format. Versions prior to 5.9.0 are vulnerable to a Denial of Service (DoS) attack caused by uncontrolled recursion when decoding deeply nested CBOR structures. This vulnerability affects both the pure Python implementation and the C extension `_cbor2`. The C extension relies on Python's internal recursion limits `Py_EnterRecursiveCall` rather than a data-driven depth limit, meaning it still raises `RecursionError` and crashes the worker process when the limit is hit. While the library handles moderate nesting levels, it lacks a hard depth limit. An attacker can supply a crafted CBOR payload containing approximately 100,000 nested arrays `0x81`. When `cbor2.loads()` attempts to parse this, it hits the Python interpreter's maximum recursion depth or exhausts the stack, causing the process to crash with a `RecursionError`. Because the library does not enforce its own limits, it allows an external attacker to exhaust the host application's stack resource. In many web application servers (e.g., Gunicorn, Uvicorn) or task queues (Celery), an unhandled `RecursionError` terminates the worker process immediately. By sending a stream of these small (\u003c100KB) malicious packets, an attacker can repeatedly crash worker processes, resulting in a complete Denial of Service for the application. Version 5.9.0 patches the issue.","status":"published","published":"2026-03-23T19:16:39.53Z","modified":"2026-07-13T07:15:16.97873212Z","cvss":[{"version":"3.0","score":7.5,"vector":"CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H","level":"high"}],"updated_at":"2026-11-02T15:52:55.537808Z"},{"id":"ROOT-APP-PYPI-CVE-2026-26209","title":"CVE-2026-26209 in rootio-cbor2 - Patched by Root","description":"Root has patched CVE-2026-26209 in the rootio-cbor2 package for Root:PyPI. Multiple fixed versions available.","status":"published","published":"2026-06-03T14:50:19Z","modified":"2026-06-03T19:30:07.731266038Z","cvss":[{"version":"3.0","score":7.5,"vector":"CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H","level":"high"}],"updated_at":"2026-11-02T15:53:27.940619Z"},{"id":"UBUNTU-CVE-2026-26209","description":"cbor2 provides encoding and decoding for the Concise Binary Object Representation (CBOR) serialization format. Versions prior to 5.9.0 are vulnerable to a Denial of Service (DoS) attack caused by uncontrolled recursion when decoding deeply nested CBOR structures. This vulnerability affects both the pure Python implementation and the C extension `_cbor2`. The C extension relies on Python's internal recursion limits `Py_EnterRecursiveCall` rather than a data-driven depth limit, meaning it still raises `RecursionError` and crashes the worker process when the limit is hit. While the library handles moderate nesting levels, it lacks a hard depth limit. An attacker can supply a crafted CBOR payload containing approximately 100,000 nested arrays `0x81`. When `cbor2.loads()` attempts to parse this, it hits the Python interpreter's maximum recursion depth or exhausts the stack, causing the process to crash with a `RecursionError`. Because the library does not enforce its own limits, it allows an external attacker to exhaust the host application's stack resource. In many web application servers (e.g., Gunicorn, Uvicorn) or task queues (Celery), an unhandled `RecursionError` terminates the worker process immediately. By sending a stream of these small (\u003c100KB) malicious packets, an attacker can repeatedly crash worker processes, resulting in a complete Denial of Service for the application. Version 5.9.0 patches the issue.","status":"published","published":"2026-03-23T19:16:00Z","modified":"2026-10-17T12:56:13.077699258Z","cvss":[{"version":"3.0","score":7.5,"vector":"CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H","level":"high"}],"updated_at":"2026-11-02T16:56:08.726452Z"},{"id":"AZL-80541","title":"CVE-2026-26209 affecting package python-cbor2 5.6.5-2","description":"cbor2 provides encoding and decoding for the Concise Binary Object Representation (CBOR) serialization format. Versions prior to 5.9.0 are vulnerable to a Denial of Service (DoS) attack caused by uncontrolled recursion when decoding deeply nested CBOR structures. This vulnerability affects both the pure Python implementation and the C extension `_cbor2`. The C extension relies on Python's internal recursion limits `Py_EnterRecursiveCall` rather than a data-driven depth limit, meaning it still raises `RecursionError` and crashes the worker process when the limit is hit. While the library handles moderate nesting levels, it lacks a hard depth limit. An attacker can supply a crafted CBOR payload containing approximately 100,000 nested arrays `0x81`. When `cbor2.loads()` attempts to parse this, it hits the Python interpreter's maximum recursion depth or exhausts the stack, causing the process to crash with a `RecursionError`. Because the library does not enforce its own limits, it allows an external attacker to exhaust the host application's stack resource. In many web application servers (e.g., Gunicorn, Uvicorn) or task queues (Celery), an unhandled `RecursionError` terminates the worker process immediately. By sending a stream of these small (\u003c100KB) malicious packets, an attacker can repeatedly crash worker processes, resulting in a complete Denial of Service for the application. Version 5.9.0 patches the issue.","status":"published","published":"2026-03-23T19:16:39Z","modified":"2026-11-05T05:36:30Z","updated_at":"2026-11-05T16:41:24.800038Z"},{"id":"openSUSE-SU-2026:10425-1","title":"python311-cbor2-5.9.0-1.1 on GA media","description":"These are all security issues fixed in the python311-cbor2-5.9.0-1.1 package on the GA media of openSUSE Tumbleweed.","status":"published","published":"2026-03-25T00:00:00Z","modified":"2026-03-26T09:02:47.04752Z","updated_at":"2026-11-02T16:58:38.415593Z"},{"id":"openSUSE-SU-2026:20468-1","title":"Security update for python-cbor2","description":"This update for python-cbor2 fixes the following issues:\n\n- CVE-2025-68131: CBORDecoder reuse across trust boundaries can lead to leak of shareable values from previous decode\n  calls via attacker-controlled messages (bsc#1255783).\n- CVE-2026-26209: uncontrolled recursion via crafted CBOR payloads can cause a denial of service (bsc#1260367).\n","status":"published","published":"2026-04-07T11:57:38Z","modified":"2026-04-10T18:33:46.117801Z","updated_at":"2026-11-02T16:58:41.463445Z"},{"id":"SUSE-SU-2026:21139-1","title":"Security update for python-cbor2","description":"This update for python-cbor2 fixes the following issues:\n\n- CVE-2025-68131: CBORDecoder reuse across trust boundaries can lead to leak of shareable values from previous decode\n  calls via attacker-controlled messages (bsc#1255783).\n- CVE-2026-26209: uncontrolled recursion via crafted CBOR payloads can cause a denial of service (bsc#1260367).\n","status":"published","published":"2026-04-07T11:57:32Z","modified":"2026-04-22T18:23:39.526684Z","updated_at":"2026-11-02T15:54:02.312556Z"}]}}