CVE-2026-104844 is a medium-severity (CVSS 5.9) Uncontrolled Resource Consumption vulnerability in postcss-selector-parser. A fix is available for postcss-selector-parser — see the affected versions and patch details below.
PostCSS: Quadratic complexity in flat selector parsing allows CPU exhaustion
Exploitation Status
No confirmed exploitation observed yet
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-104844.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
CVE-2026-104844 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.
Where this sits among everything scored
Of 394,803 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.
Real-World Exposure
postcss-selector-parserReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects npm packages — download data is not available via public APIs for these ecosystems.
Description
Impact
. and # are not word delimiters in the tokenizer, so a flat selector such as
.a.a.a... reaches splitWord() as a single word token carrying n class or id
indexes. Three passes scanned those index arrays linearly for every index,
making the parse O(n^2) in the number of indexes rather than in input length:
uniqs(), the indices.forEach loop, and the Sass-interpolation filter.
Parsing a 400 KB flat selector took ~34 s on a modern laptop, fully occupying a
single thread. A benign selector of identical byte size parses in tens of
milliseconds, so the cost is driven by the index count, not the input size.
The nesting depth of such a selector is 0, so the maxNestingDepth guard added
in 7.1.3 offers no protection.
Reachability is deployment dependent. Only consumers that parse untrusted, attacker-supplied selectors synchronously in a request path are exposed, for example CSS sanitizers, CSS-in-JS services and online playgrounds. Ordinary build-time use on trusted sources is not affected.
Patches
Fixed in 7.1.6. The three passes now use Set membership tests, making parsing linear in the number of indexes. There is no behaviour change: parsing is byte-identical on a differential corpus of 8413 selectors.
Workarounds
Cap the size of selectors accepted from untrusted sources before parsing.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | postcss-selector-parser | all versions | 7.1.6npm install postcss-selector-parser@7.1.6 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for postcss-selector-parser, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update postcss-selector-parser to 7.1.6 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-104844 is resolved across your whole dependency graph.
Workarounds
Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.
Frequently Asked Questions
Is CVE-2026-104844 in your dependencies?
Find it across npm, including transitive dependencies.