fast-xml-parser: Quadratic CPU consumption scanning unterminated stop-node tags

Exploitation

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

CVSS

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

EPSS

No dated EPSS signal available.

As published by the source; long descriptions are shortened.

Summary With the non-default stopNodes option, a stop node's body with many " " anywhere after makes readStopNodeData() rescan the entire remaining input from every " ` exists - `OrderedObjParser.js:812-818` — on failure, `i` isn't advanced, so the next ` ` lookahead is neither cached nor bounded — `n` copies of ` ` means `n` scans over the remaining input, O(n²) work, before it finally throws. Fix 1. Throw immediately in `readStopNodeData()` when `readTagExp()` returns `undefined` 2. Or break the loop on the first failed scan 3. Or find the next `>` once and reuse/bound that result 4. Add a test asserting near-linear scaling when payload size doubles

Proof of concept Not benchmarked — static-analysis finding, confidence 0.5. To confirm: configure `stopNodes` matching a root element. Build input as `' ' + ' `. Start around n=10,000 and double it — quadratic should show ~4x time per doubling, linear ~2x. Control: same input with a `>` present, or `stopNodes` unset. ###…

Published by the source

10 Nov 2026, 21:09 UTC

Last modified by the source

10 Nov 2026, 21:15 UTC

Source last checked

11 Nov 2026, 06:00 UTC

1. Revision 2 · amended · 11 Nov 2026

   Amended from osv record GHSA-474x-hrwr-fgx4

2. Revision 1 · created · 10 Nov 2026

   Published from ghsa record GHSA-474x-hrwr-fgx4

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# Patch Next advisory brief: CVE-2026-126066

Advisory: fast-xml-parser: Quadratic CPU consumption scanning unterminated stop-node tags
IDs: CVE-2026-126066, GHSA-474x-hrwr-fgx4
Record: https://patchnext.com/advisories/cve-2026-126066 (revision 2, source checked 11 Nov 2026, 06:00 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Low (CVSS 3.7)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: fast-xml-parser 5.12.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm fast-xml-parser: affected < 5.12.0; fixed in 5.12.0

## Suggested inspection (read-only)
- `npm ls fast-xml-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 fast-xml-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-474x-hrwr-fgx4 (GitHub-reviewed advisory GHSA-474x-hrwr-fgx4 lists affected ranges: npm fast-xml-parser >=0 <=5.11.2)
- Verified: https://osv.dev/vulnerability/GHSA-474x-hrwr-fgx4 (OSV record GHSA-474x-hrwr-fgx4 lists affected ranges: npm fast-xml-parser >=0 <5.12.0 (fixed))

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