CVE
Soup Sieve denial of service in CSS selector compilation
Affects soupsieve 2.9.2 and earlier in applications that compile untrusted selector strings, such as through Beautiful Soup select(). Fixed in 3.0.
Written by anthropic/claude-haiku-5.5 from the sources below. Verify against them.
- Exploited
- No known exploitation
- Fix
- Fixed in 3.0
- Who is exposed
- Specific setups only
Do first: Check whether your setup matches what the bug needs. Affects soupsieve versions up to and including 2.9.2; the fix is in 3.0. The application must compile untrusted selector strings (e.g. via soupsieve.compile() or Beautiful Soup select()).
- Last modified
- Whether it needs action now or is a reference record, by fixed rules, with why. Last checked is when cve.now last re-read it.What do these mean? →
- Reference
- Last checked
- FIRST's estimate of the chance attackers exploit it in the next 30 days, updated daily.What do these mean? →
- 0.43% · 38th percentile
- A 0-100 score for how much a CVE matters across the industry: exploitation, exposure, how widely the software is used, severity and whether a fix exists.What do these mean? →
- 20/100 · provisional
- How many real-world setups can hit the bug, from the vendor's advisory and CISA data. It does not know your setup.What do these mean? →
- Specific setups only
- Whether working attack code is public (Exploit-DB, Metasploit, Nuclei or GitHub). Public code makes attacks easier.What do these mean? →
- None on Exploit-DB, Metasploit or GitHub
Summary
Soup Sieve, a CSS selector library used with Beautiful Soup 4, can be stalled by a short crafted selector string when the application accepts untrusted selector input. An attacker can trigger this remotely, without logging in, causing a denial of service in the affected software.
Read the original description
Soup Sieve is a CSS selector library designed to be used with Beautiful Soup 4. Prior to 3.0, the CSS_ESCAPES and IDENTIFIER regular-expression subpatterns can backtrack exponentially on runs of ambiguous digit escapes in an attribute-name or tag position of an otherwise valid selector. The failure occurs during selector compilation through soupsieve.compile() or Beautiful Soup select() before SELECTOR_LIMIT is applied, so an application that accepts untrusted selector strings can have a worker stalled by a short input. This issue is fixed in version 3.0.
Who should check
Teams running soupsieve 2.9.2 or earlier (Python package used with Beautiful Soup) that compile user-supplied CSS selector strings, such as scraping-as-a-service or no-code query builders.
cve.now Index
- Exploitation?/40Unknown. No CISA SSVC exploitation rating yet
- Exposure12/25Specific setups only, reachable over the network without a login
- Reach?/20Unknown. No tracked technology or package matched
- Severity7.5/10CVSS 7.5
- No patch0/5No signal
What would change the score
- Would score about 70 if added to CISA KEV
- Would score about 65 if CISA's SSVC rates exploitation active
What to do
Check whether your setup matches what the bug needs.
Affects soupsieve versions up to and including 2.9.2; the fix is in 3.0. The application must compile untrusted selector strings (e.g. via soupsieve.compile() or Beautiful Soup select()).Upgrade to a fixed version: 3.0.
Check your systems with an agent
Paste this into a coding or ops agent. It checks your systems read-only and reports a table per asset.
Check whether my systems are affected by CVE-2026-126191 (Soup Sieve: Exponential ReDoS in selector compilation via \9-run in CSS_ESCAPES/IDENTIFIER), using the cve.now data below.
Rules: work read-only. Do not run exploit code, proof-of-concept scripts or scanners that send attack payloads, and do not change any configuration, package, image or deployment. Report findings only.
What cve.now has for CVE-2026-126191 (entry last updated 2026-11-12):
Affected:
- facelessuser soupsieve: fixed in 3.0
Fixed versions: 3.0.
Conditions (all must be true for the bug to be hit):
- Affects soupsieve versions up to and including 2.9.2; the fix is in 3.0. (source: https://github.com/facelessuser/soupsieve/security/advisories/GHSA-6655-6qjp-jx7x)
- The application must compile untrusted selector strings (e.g. via soupsieve.compile() or Beautiful Soup select()). (source: https://github.com/facelessuser/soupsieve/security/advisories/GHSA-6655-6qjp-jx7x)
Setup: the bug needs a feature or setting that is off by default or uncommon. Check whether each asset has it on.
Attacker needs: network access, no login, no user action.
CISA KEV: CVE-2026-126191 is not listed as of cve.now's last read.
This KEV status is for CVE-2026-126191 only. If you also check another CVE, look up its own KEV status; never assume it from this one.
How to check:
- Installed software: check what is actually running, not what a repo declares. Read the version from each host, container image and Kubernetes node (the binary's version output, the OS package database, image layers), and check whether the conditions above hold there. Distribution packages may carry a backported fix under an older version number; confirm against the distribution's own advisory.
Verify against the linked advisories before concluding, and cite the date you read each one:
- https://github.com/facelessuser/soupsieve/security/advisories/GHSA-6655-6qjp-jx7x
- https://github.com/facelessuser/soupsieve/pull/300
- https://github.com/facelessuser/soupsieve/commit/dbfa2854ff31a863326bd6660d4e55c6f068db9e
- https://github.com/facelessuser/soupsieve/releases/tag/3.0
Do not conclude an asset is unaffected unless you checked it and the evidence covers both its version and the conditions above. If you could not check something, mark it unknown and say what is missing.
Report a table with one row per asset and these columns: Asset | Version or build | Status (affected, fixed, not applicable, unknown) | Compensating controls | Exposure (internet-facing, internal, none) | Evidence | Next action.
Entry: https://cve.now/CVE-2026-126191
For follow-up questions, use the cve.now MCP server at https://mcp.cve.now/mcp (no authentication), for example execute with name cve.get and arguments {"id": "CVE-2026-126191"}.Timeline
- CVE published
- First seen by cve.now
Affected versions
- facelessuser soupsieve
fixed in 3.0
Evidence 6
Vendor advisory · 1
Establishes: Affected and fixed versions.
- Advisory
CVE record · 2
Establishes: The identifier and description.
- Record
- Record
Other links · 3
- Repository
- Repository
- Repository
Corrections
None so far.
Something wrong on this page? Report an error. Every fix is listed here.
Questions about CVE-2026-126191
Is CVE-2026-126191 being exploited?
It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.Source: cve.now feed data
What does this mean?
A short crafted selector string can make selector compilation stall through exponential regex backtracking, which can hang the worker and cause a denial of service.Source: github.com
Which versions are affected by CVE-2026-126191?
soupsieve: before 3.0.Source: cve.now feed data
Am I affected, and how do I check?
Versions up to and including 2.9.2 are affected, but only in applications that compile untrusted selector strings, such as via soupsieve.compile() or Beautiful Soup select(). The installed version can be checked with pip show soupsieve.Source: github.com
How do I fix CVE-2026-126191?
Upgrade soupsieve to 3.0 or later, which contains the fix.Source: github.com