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Medium, CVSS 5.9amended

PostCSS: Quadratic complexity in flat selector parsing allows CPU exhaustion

CVE-2026-104844GHSA-rj75-hqrm-r3gf

What to do

Vendor fix available. Upgrade each affected package to the fixed version on its release line (postcss-selector-parser 7.1.6, 6.1.5), redeploy, then verify what is actually installed.

This page has no inventory attached, so it cannot say whether you are affected. A vendor fix is not proof that your deployment is fixed.

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Affected packages

Sources

Severity and exploitation
Exploitation
No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.
CVSS
5.9 · CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
EPSS
  • CVE-2026-104844: 0.39% estimated exploitation activity in the next 30 days; percentile 33.03 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.
Source description

As published by the source; long descriptions are shortened.

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…

Dates and revisions
Published by the source
Last modified by the source
Source last checked
  1. Revision 5 · amended ·

    Amended from osv record GHSA-rj75-hqrm-r3gf

  2. Revision 4 · amended ·

    Amended from ghsa record GHSA-rj75-hqrm-r3gf

  3. Revision 3 · amended ·

    Amended from ghsa record GHSA-rj75-hqrm-r3gf

  4. Revision 2 · amended ·

    Amended from osv record GHSA-rj75-hqrm-r3gf

  5. Revision 1 · created ·

    Published from ghsa record GHSA-rj75-hqrm-r3gf

Agent brief

Preview the exact text before you copy it. Commands are read-only suggestions; review them before running anything.

# Patch Next advisory brief: CVE-2026-104844

Advisory: PostCSS: Quadratic complexity in flat selector parsing allows CPU exhaustion
IDs: CVE-2026-104844, GHSA-rj75-hqrm-r3gf
Record: https://patchnext.com/advisories/cve-2026-104844 (revision 5, source checked 27 Oct 2026, 06:00 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.9)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: postcss-selector-parser 7.1.6, 6.1.5. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm postcss-selector-parser: affected >= 7.0.0, < 7.1.6; < 6.1.5; fixed in 6.1.5, 7.1.6

## Suggested inspection (read-only)
- `npm ls postcss-selector-parser --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on postcss-selector-parser.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-rj75-hqrm-r3gf (GitHub-reviewed advisory GHSA-rj75-hqrm-r3gf lists affected ranges: npm postcss-selector-parser >=0 <7.1.6)
- Verified: https://osv.dev/vulnerability/GHSA-rj75-hqrm-r3gf (OSV record GHSA-rj75-hqrm-r3gf lists affected ranges: npm postcss-selector-parser >=0 <7.1.6 (fixed))
- Verified: https://github.com/advisories/GHSA-rj75-hqrm-r3gf (GitHub-reviewed advisory GHSA-rj75-hqrm-r3gf lists affected ranges: npm postcss-selector-parser >=7.0.0 =0 <6.1.5)
- Verified: https://osv.dev/vulnerability/GHSA-rj75-hqrm-r3gf (OSV record GHSA-rj75-hqrm-r3gf lists affected ranges: npm postcss-selector-parser >=7.0.0 =0 <6.1.5 (fixed))

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