CVE-2026-115779

Effect is a TypeScript framework that consists of several packages that work together to help build TypeScript applications. From @effect/platform 0.1.0 until 0.97.1 and effect 4.0.0-beta.0 until 4.0.0-beta.103, the name, domain, and path fields passed to makeCookie() allow semicolon delimiters that serializeCookie() writes verbatim into Set-Cookie headers. A caller that supplies attacker-controlled values can append cookie attributes, including a broader Domain or, when the caller has not set those options itself, SameSite=None or Max-Age. Encoded cookie values and CR or LF characters remain unaffected. This can widen a cookie's scope or add otherwise-unset cookie attributes. This issue is fixed in @effect/platform 0.97.1 and effect 4.0.0-beta.103.

Effect: Cookie attribute injection via domain, path, and name fields in @effect/platform makeCookie()

Impact

The name, domain, and path fields of makeCookie() were validated with a regex that allows ;, the attribute separator in Set-Cookie headers. Because serializeCookie() writes those fields verbatim, a caller who passes untrusted input into any of them can append arbitrary attributes to the header.

For example, a domain of "legit.com; Domain=.parent.tld" serializes to session=token; Domain=legit.com; Domain=.parent.tld. RFC 6265 §5.3 takes the last occurrence, so the injected domain wins and the cookie widens to the parent domain. The same trick sets SameSite=None or Max-Age when the caller has not set those options itself.

The value field is unaffected: it passes through encodeURIComponent, which encodes ;. CR and LF are rejected by the existing regex, so header splitting is not possible.

Patches

Fixed in #6763 (v4) and #6765 (v3): the shared regex is split into per-field validators that reject ;, and serializeCookie() now validates before writing the header.

Workarounds

Do not pass user input into name, domain, or path; set them from application constants.

Effect is a TypeScript framework that consists of several packages that work together to help build TypeScript applications. From @effect/platform 0.1.0 until 0.97.1 and effect 4.0.0-beta.0 until 4.0.0-beta.103, the name, domain, and path fields passed to makeCookie() allow semicolon delimiters that serializeCookie() writes verbatim into Set-Cookie headers. A caller that supplies attacker-controlled values can append cookie attributes, including a broader Domain or, when the caller has not set those options itself, SameSite=None or Max-Age. Encoded cookie values and CR or LF characters remain unaffected. This can widen a cookie's scope or add otherwise-unset cookie attributes. This issue is fixed in @effect/platform 0.97.1 and effect 4.0.0-beta.103.

Effect is a TypeScript framework that consists of several packages that work together to help build TypeScript applications. From @effect/platform 0.1.0 until 0.97.1 and effect 4.0.0-beta.0 until 4.0.0-beta.103, the name, domain, and path fields passed to makeCookie() allow semicolon delimiters that serializeCookie() writes verbatim into Set-Cookie headers. A caller that supplies attacker-controlled values can append cookie attributes, including a broader Domain or, when the caller has not set those options itself, SameSite=None or Max-Age. Encoded cookie values and CR or LF characters remain unaffected. This can widen a cookie's scope or add otherwise-unset cookie attributes. This issue is fixed in @effect/platform 0.97.1 and effect 4.0.0-beta.103.

Impact

The name, domain, and path fields of makeCookie() were validated with a regex that allows ;, the attribute separator in Set-Cookie headers. Because serializeCookie() writes those fields verbatim, a caller who passes untrusted input into any of them can append arbitrary attributes to the header.

For example, a domain of "legit.com; Domain=.parent.tld" serializes to session=token; Domain=legit.com; Domain=.parent.tld. RFC 6265 §5.3 takes the last occurrence, so the injected domain wins and the cookie widens to the parent domain. The same trick sets SameSite=None or Max-Age when the caller has not set those options itself.

The value field is unaffected: it passes through encodeURIComponent, which encodes ;. CR and LF are rejected by the existing regex, so header splitting is not possible.

Patches

Fixed in #6763 (v4) and #6765 (v3): the shared regex is split into per-field validators that reject ;, and serializeCookie() now validates before writing the header.

Workarounds

Do not pass user input into name, domain, or path; set them from application constants.