fast-xml-parser vulnerable to Regex Injection via Doctype Entities

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:N/I:N/A:H

EPSS

• CVE-2023-34104: 1.13% estimated exploitation activity in the next 30 days; percentile 66.34 (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.

Impact "fast-xml-parser" allows special characters in entity names, which are not escaped or sanitized. Since the entity name is used for creating a regex for searching and replacing entities in the XML body, an attacker can abuse it for DoS attacks. By crafting an entity name that results in an intentionally bad performing regex and utilizing it in the entity replacement step of the parser, this can cause the parser to stall for an indefinite amount of time. Patches The problem has been resolved in v4.2.4 Workarounds Avoid using DOCTYPE parsing by processEntities: false option.

Published by the source

6 Jun 2023, 17:33 UTC

Last modified by the source

9 Mar 2026, 16:01 UTC

Source last checked

26 Oct 2026, 12:43 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-6w63-h3fj-q4vw

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-6w63-h3fj-q4vw

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# Patch Next advisory brief: CVE-2023-34104

Advisory: fast-xml-parser vulnerable to Regex Injection via Doctype Entities
IDs: CVE-2023-34104, GHSA-6w63-h3fj-q4vw
Record: https://patchnext.com/advisories/cve-2023-34104 (revision 2, source checked 26 Oct 2026, 12:43 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: fast-xml-parser 4.2.4. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm fast-xml-parser: affected >= 4.1.3, < 4.2.4; fixed in 4.2.4

## 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://osv.dev/vulnerability/GHSA-6w63-h3fj-q4vw (OSV record GHSA-6w63-h3fj-q4vw lists affected ranges: npm fast-xml-parser >=4.1.3 <4.2.4 (fixed))
- Verified: https://github.com/advisories/GHSA-6w63-h3fj-q4vw (GitHub-reviewed advisory GHSA-6w63-h3fj-q4vw lists affected ranges: npm fast-xml-parser >=4.1.3 <4.2.4)

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