PostCSS: Path Traversal in Previous Source Map Auto-Loading (sourceMappingURL) leads to Arbitrary .map File Disclosure

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

7.5 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N

EPSS

• CVE-2026-73646: 0.53% estimated exploitation activity in the next 30 days; percentile 44.85 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Vulnerability Details File: lib/previous-map.js Line: 87-98 (loadFile), 129-144 (loadMap) Root Cause PostCSS auto-detects a /

sourceMappingURL=... / comment inside the CSS text it is asked to parse and, unless the caller explicitly passes map: false, attempts to load that path from disk as a "previous source map." This happens on every postcss.parse() / postcss().process() call by default (opt-out, not opt-in). loadMap() builds the candidate path via join(dirname(opts.from), annotation), where annotation is the raw, attacker-controlled string from the CSS comment. path.join() normalizes but does not sandbox .. segments, so a ../../../ prefix walks the resolved path outside the intended directory. If opts.from is not set at all, the annotation is used completely unmodified — an absolute path in the CSS comment is read verbatim. 8.5.12 already fixed a strictly worse variant of this (any file, any extension, could be read) by requiring the resolved path to end in .map (loadFile()).…

Published by the source

24 Jul 2026, 16:24 UTC

Last modified by the source

10 Sep 2026, 03:50 UTC

Source last checked

26 Oct 2026, 13:01 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-r28c-9q8g-f849

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-r28c-9q8g-f849

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# Patch Next advisory brief: CVE-2026-73646

Advisory: PostCSS: Path Traversal in Previous Source Map Auto-Loading (sourceMappingURL) leads to Arbitrary .map File Disclosure
IDs: CVE-2026-73646, GHSA-r28c-9q8g-f849
Record: https://patchnext.com/advisories/cve-2026-73646 (revision 2, source checked 26 Oct 2026, 13:01 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 7.5)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: postcss 8.5.18. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm postcss: affected <= 8.5.17; fixed in 8.5.18

## Suggested inspection (read-only)
- `npm ls postcss --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.
- 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://osv.dev/vulnerability/GHSA-r28c-9q8g-f849 (OSV record GHSA-r28c-9q8g-f849 lists affected ranges: npm postcss >=0 <8.5.18 (fixed))
- Verified: https://github.com/advisories/GHSA-r28c-9q8g-f849 (GitHub-reviewed advisory GHSA-r28c-9q8g-f849 lists affected ranges: npm postcss >=0 <=8.5.17)

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