CVE
Soup Sieve polynomial-time denial of service in descendant combinator
Affects Soup Sieve versions before 4.0 when selectors with descendant combinators run against documents that untrusted users can influence. Fixed in 4.0.
Written by anthropic/claude-haiku-5.5 from the sources below. Verify against them.
- Exploited
- No known exploitation
- Fix
- Fixed in 4.0
- Who is exposed
- Default setups
Do first: Check every install: default setups are affected.
- 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? →
- 0.54% · 45th 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? →
- 50/100 · 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? →
- Default setups
- 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, contains a flaw in how it matches descendant combinators, letting a remote attacker without logging in cause a denial of service by submitting deeply nested HTML or XML. Each additional descendant combinator in a selector greatly increases the processing work, so applications that run such selectors against untrusted documents can tie up a worker for seconds or minutes.
Read the original description
Soup Sieve is a CSS selector library designed to be used with Beautiful Soup 4. Prior to 4.0, Soup Sieve repeatedly walks the same ancestors while matching descendant combinators, and each additional descendant combinator raises the polynomial degree of the work on deeply nested attacker-influenced HTML or XML. Applications using selectors such as section div div through select(), select_one(), or Beautiful Soup CSS APIs against untrusted documents can have a worker occupied for seconds or minutes, while trusted-only document processing is not affected. Selectors compiled with NOCACHE continue to use the old uncached behavior. This issue is fixed in version 4.0.
Who should check
Teams running soupsieve before 4.0 with Beautiful Soup 4, such as scrapers, HTML sanitizers or filters, and services that process uploaded or fetched markup with CSS selectors
Not fixed by: Selectors compiled with NOCACHE stay vulnerable after upgrading.
Read the source excerpt
“Applications are affected if they run selectors with descendant combinators against documents that untrusted users can influence”
cve.now Index
- Exploitation?/40Unknown. No CISA SSVC exploitation rating yet
- Exposure22/25Default setups, reachable over the network without a login
- Reach20/20soupsieve: 81.9M downloads a week
- Severity7.5/10CVSS 7.5
- No patch0/5No signal
What would change the score
- Would score about 90 if added to CISA KEV
- Would score about 85 if CISA's SSVC rates exploitation active
Impact
- Widely used
soupsieve has 81.9m weekly downloads.Source: pypistats.org
34.5k packages depend on soupsieve.Source: deps.dev
What to do
Check every install: default setups are affected.
Upgrade to a fixed version: 4.0.
Workaround: Use as few descendant combinators as you can in selectors that run on untrusted documents.
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-2026-126190 (Soup Sieve: Polynomial-time denial of service in the descendant combinator), 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-2026-126190 (entry last updated 2026-11-12):
Affected:
- facelessuser soupsieve: fixed in 4.0
- PyPI package soupsieve: fixed in 4.0.0
Fixed versions: 4.0.
Conditions (all must be true for the bug to be hit):
- Application runs selectors with descendant combinators against documents that untrusted users can influence (source: https://github.com/facelessuser/soupsieve/security/advisories/GHSA-mmhr-3c5m-f64f)
- Selectors compiled with the NOCACHE flag remain affected (source: https://github.com/facelessuser/soupsieve/security/advisories/GHSA-mmhr-3c5m-f64f)
Setup: the bug is reachable in a default setup; no special feature or setting is needed.
Attacker needs: network access, no login, no user action.
Workaround: Use as few descendant combinators as you can in selectors that run on untrusted documents. (source: https://github.com/facelessuser/soupsieve/security/advisories/GHSA-mmhr-3c5m-f64f)
CISA KEV: CVE-2026-126190 is not listed as of cve.now's last read.
This KEV status is for CVE-2026-126190 only. If you also check another CVE, look up its own KEV status; never assume it from this one.
How to check:
- Packages (PyPI): find every lockfile in each project and run the stack.check operation of the cve.now MCP server (execute) on each one. Compare the resolved versions, not the ranges a manifest declares, with the affected versions above. Include transitive dependencies.
Verify against the linked advisories before concluding, and cite the date you read each one:
- https://github.com/facelessuser/soupsieve/security/advisories/GHSA-mmhr-3c5m-f64f
- https://github.com/facelessuser/soupsieve/pull/351
- https://github.com/facelessuser/soupsieve/commit/ff1a5ce7e5addc31626d7b6053511fecdb4f6320
- https://github.com/facelessuser/soupsieve/releases/tag/4.0
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-2026-126190
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-2026-126190"}.Timeline
- CVE published
- First seen by cve.now
Affected versions
- facelessuser soupsieve
fixed in 4.0
- soupsievePyPI
fixed in 4.0.0
Fixed in 4.0.0 (released Nov 3, 2026, 9 days before disclosure).
The latest release, 4.1.0, is not affected.
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-2026-126190
Is CVE-2026-126190 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?
Matching selectors that use descendant combinators can take polynomial time on deeply nested documents. A small, deeply nested document can keep a CPU busy for seconds to minutes, which is a denial of service.Source: github.com
Which versions are affected by CVE-2026-126190?
soupsieve: before 4.0. soupsieve (PyPI): before 4.0.0.Source: cve.now feed data
Am I affected, and how do I check?
Soup Sieve versions before 4.0 are affected. The setup at risk is an application that runs selectors with descendant combinators against documents that untrusted users can influence, and selectors compiled with the NOCACHE flag remain affected. To check, run the version command for soupsieve (for example, pip show soupsieve) and look at the selector usage in the application.Source: github.com
How do I fix CVE-2026-126190?
Upgrade Soup Sieve to 4.0 or later, which contains the fix from facelessuser/soupsieve#351. Until upgrading, use as few descendant combinators as possible in selectors that run on untrusted documents.Source: github.com