Bug 2490022 (CVE-2026-9679)

Summary:	CVE-2026-9679 undici: undici vulnerable to HTTP header injection via Set-Cookie percent-decoding
Product:	[Other] Security Response	Reporter:	OSIDB Bzimport <bzimport>
Component:	vulnerability	Assignee:	Product Security <prodsec-ir-bot>
Status:	NEW ---	QA Contact:
Severity:	medium	Docs Contact:
Priority:	medium
Version:	unspecified	CC:	aazores, abarbaro, alizardo, ataylor, cdrage, cmah, dbruscin, dschmidt, eaguilar, ebaron, erezende, jchui, jhe, jkoehler, jlanda, jolong, kaycoth, kshier, ktsao, kvanderr, lphiri, manissin, nboldt, oaljalju, pjindal, psrna, rhel-process-autobot, rushinde, sdawley, simaishi, smcdonal, stcannon, teagle, watson-tool-maintainers, yguenane
Target Milestone:	---	Keywords:	Security
Target Release:	---
Hardware:	All
OS:	Linux
Whiteboard:
Fixed In Version:		Doc Type:	---
Doc Text:	A flaw was found in undici. The cookie parser in the `parseSetCookie` function incorrectly decodes cookie values, which is contrary to standard specifications. This vulnerability allows an attacker-controlled upstream to inject arbitrary HTTP response headers, such as `Set-Cookie`, `Location`, or `Cache-Control`. The primary consequence is HTTP response header injection, which can lead to session fixation, open redirect, or cache poisoning.	Story Points:	---
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Verified Versions:		Category:	---
oVirt Team:	---	RHEL 7.3 requirements from Atomic Host:
Cloudforms Team:	---	Target Upstream Version:
Embargoed:
Bug Depends On:	2490200, 2490202, 2490203, 2490201
Bug Blocks:

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.