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

Severity: medium (CVSS 5.9 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.3% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-06-17 · updated 2026-06-25
Weakness: CWE-93

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.

Affected products

undici: before 6.27.0; from 7.0.0 before 7.28.0; from 8.0.0 before 8.5.0 — fixed in 6.27.0, 7.28.0, 8.5.0
Npm Undici: before 6.27.0; from 7.0.0 before 7.28.0; from 8.0.0 before 8.5.0 — fixed in 6.27.0, 7.28.0, 8.5.0

How to fix

Upgrade to a fixed version: undici 6.27.0, 7.28.0, 8.5.0; Npm Undici 6.27.0, 7.28.0, 8.5.0. Check the vendor advisory for mitigations.

References

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