CVE-2026-9679: undici vulnerable to HTTP header injection via Set-Cookie percent-decoding
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 #3789.
Patches
Upgrade to undici v6.27.0, 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.
Other sources
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.
— NVD
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
npm/undicito a version that resolves this vulnerability.Fixed in 8.5.0 - Upgrade
Upgrade
npm/undicito a version that resolves this vulnerability.Fixed in 7.28.0 - Upgrade
Upgrade
npm/undicito a version that resolves this vulnerability.Fixed in 6.27.0 - Upgrade
Upgrade
undicito a version that resolves this vulnerability.Fixed in 6.26.0 - Upgrade
Upgrade
undicito a version that resolves this vulnerability.Fixed in 7.28.0 - Upgrade
Upgrade
undicito a version that resolves this vulnerability.Fixed in 8.5.0 - Configuration
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.
undici cookie parsing Do not directly forward parsed cookie values into response headers = sanitize/strip bytes before forwarding
Event History
Frequently Asked Questions
What is the severity of CVE-2026-9679?
CVE-2026-9679 has a medium severity rating of 5.9.
What is the impact of CVE-2026-9679?
CVE-2026-9679 allows for HTTP header injection through undici's cookie parser by percent-decoding certain cookie values.
How do I fix CVE-2026-9679?
To fix CVE-2026-9679, it is recommended to update the undici library to the latest version that addresses this vulnerability.
What type of injection vulnerability is found in CVE-2026-9679?
CVE-2026-9679 is classified as a CRLF injection vulnerability.
What specific cookie values are affected by CVE-2026-9679?
CVE-2026-9679 affects cookie values that include encoded sequences such as %0D%0A, %00, %3B, and %3D.