Bug 2490022 (CVE-2026-9679) - CVE-2026-9679 undici: undici vulnerable to HTTP header injection via Set-Cookie percent-decoding
Summary: CVE-2026-9679 undici: undici vulnerable to HTTP header injection via Set-Cook...
Keywords:
Status: NEW
Alias: CVE-2026-9679
Product: Security Response
Classification: Other
Component: vulnerability
Version: unspecified
Hardware: All
OS: Linux
medium
medium
Target Milestone: ---
Assignee: Product Security
QA Contact:
URL:
Whiteboard:
Depends On: 2490200 2490201 2490202 2490203
Blocks:
TreeView+ depends on / blocked
 
Reported: 2026-06-17 19:04 UTC by OSIDB Bzimport
Modified: 2026-06-17 23:07 UTC (History)
35 users (show)

Fixed In Version:
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Description OSIDB Bzimport 2026-06-17 19:04:15 UTC 
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.
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