CVE-2026-86000

Summary

soupsieve compiles CSS selector strings with a set of hand-written regular expressions. The shared IDENTIFIER sub-pattern (also embedded in VALUE, and therefore in attribute selectors) places two adjacent quantified groups over overlapping character classes: (?:[classA]|ESC)+(?:[classB]|ESC)*, where both classes match ordinary identifier characters such as a. When a selector contains a long identifier/value run that must ultimately fail to match (e.g. an attribute value with no closing ], or an identifier followed by an invalid character), the regex engine backtracks across all O(n) ways to split the run between the + group and the * group, giving O(n²) parse time. A single attacker-controlled selector of a few kilobytes stalls the interpreter for many seconds of CPU; tens of kilobytes reach minutes.

Trust model (Q0)

The selector string is the input. It reaches this code via soupsieve.compile(), soupsieve.select/iselect/match/filter, and — most commonly — BeautifulSoup's soup.select(selector) / soup.select_one(selector), which delegate to soupsieve. This is exploitable in any application that passes a user-controlled CSS selector to BeautifulSoup/soupsieve (scrapers that accept selectors, no-code extraction tools, admin/query UIs). Applications that only use hard-coded selectors are not affected.

Root cause (exact anchors) — src/soupsieve/css_parser.py

# lines 122-126
IDENTIFIER = fr'''
(?:(?:-?(?:[^\x00-\x2f\x30-\x40\x5B-\x5E\x60\x7B-\x9f]|{CSS_ESCAPES})+|--)
(?:[^\x00-\x2c\x2e\x2f\x3A-\x40\x5B-\x5E\x60\x7B-\x9f]|{CSS_ESCAPES})*)
'''
# line 129 — VALUE embeds IDENTIFIER (so attribute values inherit the pattern)
VALUE = fr'''(?:"(?:\\(?:.|{NEWLINE})|[^\\"\r\n\f])*?"|'...'|{IDENTIFIER})'''

• classA [^\x00-\x2f\x30-\x40\x5B-\x5E\x60\x7B-\x9f] excludes digits (0x30-0x39); classB [^\x00-\x2c\x2e\x2f\x3A-\x40\x5B-\x5E\x60\x7B-\x9f] allows digits. The intent is "first char not a digit, remaining chars may be digits."
• Both classes match ordinary letters (e.g. a = 0x61). The construct is therefore effectively (?:C)+(?:C)* over an overlapping class C — the canonical adjacent-quantifier shape that backtracks quadratically on a failing match.

The quadratic only manifests when the overall match must fail. IDENTIFIER matched greedily on "a"*n succeeds in linear time (1 ms at n=32000). Anchoring it so a following element is mandatory and fails (IDENTIFIER + "$" against "a"*n + "!") reproduces the O(n²) directly: n=2000 → 44 ms, 4000 → 257 ms, 8000 → 743 ms, 16000 → 2944 ms (×4 per ×2). Profiling compile("[a=" + "a"*4000) shows only 12 re.match calls consuming 2.685 s — i.e. the cost is inside a single regex match, confirming regex backtracking (not loop overhead).

Reproduction environment (discipline #12 — published artifact)

• git HEAD 751c57b (2.9, PYTHONPATH=src): cd src && python3 ../poc/poc_redos_compile.py.
• Published PyPI soupsieve 2.8.4 (fresh uv pip install soupsieve beautifulsoup4): cd poc && ../.venv-published/bin/python poc_redos_compile.py → same O(n²) (evidence: poc/evidence_redos_compile_PUBLISHED_2.8.4.log).
• Python 3.11.15 and 3.14.6 both reproduce.

PoC (poc/poc_redos_compile.py)

import sys, time
sys.path.insert(0, ".")
import soupsieve as sv

def compile_time(sel):
    t0 = time.perf_counter()
    try:
        sv.compile(sel)
        status = "ok"
    except Exception as e:
        status = type(e).__name__
    return (time.perf_counter() - t0), status

print(f"soupsieve {sv.__version__}\n")

print("Payload A: '[a=' + 'a'*n   (unterminated attribute value)")
for n in (1000, 2000, 4000, 8000):
    dt, st = compile_time("[a=" + "a" * n)
    print(f"  n={n:<6} len={3+n:<7} {dt*1000:9.1f} ms  [{st}]")

print("\nPayload B: 'a'*n + '!'   (identifier run + invalid trailing char)")
for n in (2000, 4000, 8000, 16000):
    dt, st = compile_time("a" * n + "!")
    print(f"  n={n:<6}…

Summary

soupsieve compiles CSS selector strings with a set of hand-written regular expressions. The shared IDENTIFIER sub-pattern (also embedded in VALUE, and therefore in attribute selectors) places two adjacent quantified groups over overlapping character classes: (?:[classA]|ESC)+(?:[classB]|ESC)*, where both classes match ordinary identifier characters such as a. When a selector contains a long identifier/value run that must ultimately fail to match (e.g. an attribute value with no closing ], or an identifier followed by an invalid character), the regex engine backtracks across all O(n) ways to split the run between the + group and the * group, giving O(n²) parse time. A single attacker-controlled selector of a few kilobytes stalls the interpreter for many seconds of CPU; tens of kilobytes reach minutes.

Trust model (Q0)

The selector string is the input. It reaches this code via soupsieve.compile(), soupsieve.select/iselect/match/filter, and — most commonly — BeautifulSoup's soup.select(selector) / soup.select_one(selector), which delegate to soupsieve. This is exploitable in any application that passes a user-controlled CSS selector to BeautifulSoup/soupsieve (scrapers that accept selectors, no-code extraction tools, admin/query UIs). Applications that only use hard-coded selectors are not affected.

Root cause (exact anchors) — src/soupsieve/css_parser.py

# lines 122-126
IDENTIFIER = fr'''
(?:(?:-?(?:[^\x00-\x2f\x30-\x40\x5B-\x5E\x60\x7B-\x9f]|{CSS_ESCAPES})+|--)
(?:[^\x00-\x2c\x2e\x2f\x3A-\x40\x5B-\x5E\x60\x7B-\x9f]|{CSS_ESCAPES})*)
'''
# line 129 — VALUE embeds IDENTIFIER (so attribute values inherit the pattern)
VALUE = fr'''(?:"(?:\\(?:.|{NEWLINE})|[^\\"\r\n\f])*?"|'...'|{IDENTIFIER})'''

• classA [^\x00-\x2f\x30-\x40\x5B-\x5E\x60\x7B-\x9f] excludes digits (0x30-0x39); classB [^\x00-\x2c\x2e\x2f\x3A-\x40\x5B-\x5E\x60\x7B-\x9f] allows digits. The intent is "first char not a digit, remaining chars may be digits."
• Both classes match ordinary letters (e.g. a = 0x61). The construct is therefore effectively (?:C)+(?:C)* over an overlapping class C — the canonical adjacent-quantifier shape that backtracks quadratically on a failing match.

The quadratic only manifests when the overall match must fail. IDENTIFIER matched greedily on "a"*n succeeds in linear time (1 ms at n=32000). Anchoring it so a following element is mandatory and fails (IDENTIFIER + "$" against "a"*n + "!") reproduces the O(n²) directly: n=2000 → 44 ms, 4000 → 257 ms, 8000 → 743 ms, 16000 → 2944 ms (×4 per ×2). Profiling compile("[a=" + "a"*4000) shows only 12 re.match calls consuming 2.685 s — i.e. the cost is inside a single regex match, confirming regex backtracking (not loop overhead).

Reproduction environment (discipline #12 — published artifact)

• git HEAD 751c57b (2.9, PYTHONPATH=src): cd src && python3 ../poc/poc_redos_compile.py.
• Published PyPI soupsieve 2.8.4 (fresh uv pip install soupsieve beautifulsoup4): cd poc && ../.venv-published/bin/python poc_redos_compile.py → same O(n²) (evidence: poc/evidence_redos_compile_PUBLISHED_2.8.4.log).
• Python 3.11.15 and 3.14.6 both reproduce.

PoC (poc/poc_redos_compile.py)

import sys, time
sys.path.insert(0, ".")
import soupsieve as sv

def compile_time(sel):
    t0 = time.perf_counter()
    try:
        sv.compile(sel)
        status = "ok"
    except Exception as e:
        status = type(e).__name__
    return (time.perf_counter() - t0), status

print(f"soupsieve {sv.__version__}\n")

print("Payload A: '[a=' + 'a'*n   (unterminated attribute value)")
for n in (1000, 2000, 4000, 8000):
    dt, st = compile_time("[a=" + "a" * n)
    print(f"  n={n:<6} len={3+n:<7} {dt*1000:9.1f} ms  [{st}]")

print("\nPayload B: 'a'*n + '!'   (identifier run + invalid trailing char)")
for n in (2000, 4000, 8000, 16000):
    dt, st = compile_time("a" * n + "!")
    print(f"  n={n:<6}…

Soup Sieve is a CSS selector library designed to be used with Beautiful Soup 4. Prior to 2.9, the selector parser in src/soupsieve/css_parser.py defines IDENTIFIER with adjacent quantified groups over overlapping character classes, and VALUE embeds IDENTIFIER for attribute selectors. When an attacker-controlled selector contains a long identifier or unquoted attribute-value run followed by input that makes the overall match fail, the regular expression engine explores quadratically many splits between the overlapping groups. User-controlled selectors can reach this path through soupsieve.compile(), soupsieve.select(), or BeautifulSoup.select(), while applications using only hard-coded selectors are unaffected. The resulting CPU consumption can hold the Python GIL, exhaust application workers, and stall a service; successful plain identifier matches are linear, and the issue does not cause memory corruption or code execution. The issue is fixed in version 2.9.

Soup Sieve is a CSS selector library designed to be used with Beautiful Soup 4. Prior to 2.9, the selector parser in src/soupsieve/css_parser.py defines IDENTIFIER with adjacent quantified groups over overlapping character classes, and VALUE embeds IDENTIFIER for attribute selectors. When an attacker-controlled selector contains a long identifier or unquoted attribute-value run followed by input that makes the overall match fail, the regular expression engine explores quadratically many splits between the overlapping groups. User-controlled selectors can reach this path through soupsieve.compile(), soupsieve.select(), or BeautifulSoup.select(), while applications using only hard-coded selectors are unaffected. The resulting CPU consumption can hold the Python GIL, exhaust application workers, and stall a service; successful plain identifier matches are linear, and the issue does not cause memory corruption or code execution. The issue is fixed in version 2.9.

These are all security issues fixed in the python313-soupsieve-2.9.2-1.1 package on the GA media of openSUSE Tumbleweed.

This update for python-soupsieve fixes the following issues:

• CVE-2026-85999: quadratic complexity when running the whitespace/comment trimming regex RE_WS_END can lead to DoS (bsc#1281119).
• CVE-2026-86000: quadratic complexity when running the IDENTIFIER / VALUE selector sub-patterns can lead to a DoS (bsc#1281120).

This update for python-soupsieve fixes the following issues:

• CVE-2026-85999: quadratic complexity when running the whitespace/comment trimming regex RE_WS_END can lead to DoS (bsc#1281119).
• CVE-2026-86000: quadratic complexity when running the IDENTIFIER / VALUE selector sub-patterns can lead to a DoS (bsc#1281120).

This update for python-soupsieve fixes the following issues:

• CVE-2026-85999: quadratic complexity when running the whitespace/comment trimming regex RE_WS_END can lead to DoS (bsc#1281119).
• CVE-2026-86000: quadratic complexity when running the IDENTIFIER / VALUE selector sub-patterns can lead to a DoS (bsc#1281120).