fast-xml-parser: Quadratic CPU consumption scanning unterminated stop-node tags
- Exploited
- No known exploitation
- Fix
- Fixed in 5.12.0
- Who is exposed
- Not reported
Do first: Upgrade to a fixed version: 5.12.0.
- Last modified
- Whether it needs action now or is a reference record, by fixed rules, with why. Last checked is when cve.now last re-read it.What do these mean? →
- Reference
- Last checked
- A 0-100 score for how much a CVE matters across the industry: exploitation, exposure, how widely the software is used, severity and whether a fix exists.What do these mean? →
- 41/100 · provisional
- The share of the registry's downloads of each affected package in the window that were versions inside the affected ranges. Downloads, not users or installs: CI runs and mirrors inflate the counts.
- 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
- Whether working attack code is public (Exploit-DB, Metasploit, Nuclei or GitHub). Public code makes attacks easier.What do these mean? →
- None on Exploit-DB, Metasploit or GitHub
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.
Written by anthropic/claude-haiku-5.5 from the sources below. Verify against them.
Read the original 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.
cve.now Index
- Exploitation?/40Unknown. No CISA SSVC exploitation rating yet
- Exposure17/25No login or user action needed per the CVSS vector, reachable over the network without a login · Unknown: No advisory context yet
- Reach20/20fast-xml-parser: 114.5M downloads a week
- Severity3.7/10CVSS 3.7
- No patch0/5No signal
What would change the score
- Would score about 86 if added to CISA KEV
- Would score about 81 if CISA's SSVC rates exploitation active
- Would score about 46 if advisories confirm default setups are exposed
Impact
What to do
Upgrade to a fixed version: 5.12.0.
Check your systems with an agent
Paste this into a coding or ops agent. It checks your systems read-only and reports a table per asset.
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.
Timeline
- CVE published
- First seen by cve.now
Affected versions
- NaturalIntelligence fast-xml-parser
fixed in 5.12.0
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.
Evidence 5
Vendor advisory · 1
Establishes: Affected and fixed versions.
- Advisory
CVE record · 2
Establishes: The identifier and description.
- Record
- Record
Other links · 2
- Repository
- Repository
Corrections
None so far.
Something wrong on this page? Report an error. Every fix is listed here.
Questions about CVE-2026-126066
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: cve.now feed data
Which versions are affected by CVE-2026-126066?
fast-xml-parser: before 5.12.0. fast-xml-parser (npm): before 5.12.0.Source: cve.now feed data
How do I fix CVE-2026-126066?
Upgrade to a fixed version: 5.12.0.Source: cve.now feed data