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.
References
tndud042713 Reporter
mcollina Remediation developer
KhafraDev Remediation reviewer
UlisesGascon Remediation reviewer
Impact
undici's cookie parser in
parseSetCookiepercent-decodes cookie values viaqsUnescape, turning encoded sequences like%0D%0A,%00,%3B, and%3Dinto their literal byte equivalents. RFC 6265 §5.4 does not specify any decoding and browsers do not decode either.Applications that parse a
Set-Cookieheader 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 arbitrarySet-Cookie,Location, orCache-Controlheaders 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/getSetCookiesdirectly into response headers; sanitize the value first to strip or reject CR, LF, NUL,;, and=bytes.References