fast-xml-parser vulnerable to Prototype Pollution through tag or attribute name

Exploitation

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

CVSS

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

EPSS

• CVE-2021-26920: 9.50% estimated exploitation activity in the next 30 days; percentile 95.45 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.
• CVE-2023-26920: 1.15% estimated exploitation activity in the next 30 days; percentile 66.98 (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 As a part of this vulnerability, user was able to se code using `proto` as a tag or attribute name. ```js const { XMLParser, XMLBuilder, XMLValidator} = require("fast-xml-parser"); let XMLdata = " hacked " const parser = new XMLParser(); let jObj = parser.parse(XMLdata); console.log(jObj.polluted) // should return hacked ```

Patches The problem has been patched in v4.1.2

Workarounds User can check for "proto" in the XML string before parsing it to the parser.

References https://gist.github.com/Sudistark/a5a45bd0804d522a1392cb5023aa7ef7

Published by the source

13 Jun 2023, 12:44 UTC

Last modified by the source

8 Jul 2026, 06:49 UTC

Source last checked

24 Oct 2026, 04:13 UTC

1. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-x3cc-x39p-42qx

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# Patch Next advisory brief: CVE-2021-26920

Advisory: fast-xml-parser vulnerable to Prototype Pollution through tag or attribute name
IDs: CVE-2021-26920, GHSA-x3cc-x39p-42qx
Record: https://patchnext.com/advisories/cve-2021-26920 (revision 1, source checked 24 Oct 2026, 04:13 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 6.5)
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 4.1.2. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm fast-xml-parser: affected < 4.1.2; fixed in 4.1.2

## 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-x3cc-x39p-42qx (OSV record GHSA-x3cc-x39p-42qx lists affected ranges: npm fast-xml-parser >=0 <4.1.2 (fixed))

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