undici's cache interceptor documents that only safe HTTP methods are cached, but its logic to skip caching is built by subtracting the configured methods from the set of safe methods, so an unsafe method such as POST, PUT, or DELETE is never placed in the skip list and instead falls through to the full cache-read path. The response-storage gate also lacked a method check, so a response to an unsafe request that is heuristically cacheable or carries an explicit Cache-Control directive is stored and later replayed from cache. Because response headers from a remote origin are untrusted, an origin can answer once with a cacheable status and then have the client's own subsequent state-changing requests to that path served from the stale cache entry without ever reaching the origin, an integrity failure that occurs under the interceptor's default configuration. This affects undici versions from 7.0.0 up to 7.29.1 and from 8.0.0 up to 8.10.2. Users should upgrade to undici 7.29.1 or 8.10.2.

Impact

When an application passes a duck-typed blob-like body to undici's HTTP/1.1 dispatcher (via request(), stream(), pipeline(), or dispatch()) with a .type derived from untrusted input, an attacker can inject CRLF sequences (\r\n) to append arbitrary HTTP headers and potentially smuggle a second request past the upstream.

The vulnerable branch in lib/dispatcher/client-h1.js pushes body.type directly into the outgoing headers with no validation, while every other header path in undici goes through isValidHeaderValue():

javascript } else if (util.isBlobLike(body) && request.contentType == null && body.type) { headers.push('content-type', body.type) // bypasses isValidHeaderValue() }

The bug requires a hand-rolled duck-typed blob object or a Blob subclass with a controlled .type. Native Blob is safe because its constructor strips CRLF from .type. fetch() is unaffected because it validates via the Headers class. Ecosystem consumers that build duck-typed blob shapes from user input include form-data-encoder, formdata-polyfill, and formdata-node.

Same defect class as CVE-2022-35948 (explicit content-type sink, fixed in undici 5.8.2) and CVE-2026-1527 (upgrade option sink, fixed in 6.24.0 / 7.24.0), both closed by adding isValidHeaderValue() on their respective sinks. This branch was missed.

Patches

Patched in undici v6.28.0, v7.29.0, and v8.9.0. Users should upgrade to one of these versions or later.

Workarounds

- Set an explicit, validated content-type header on the request options (skips the vulnerable branch). - Use a native Blob (or fetch-blob) instead of a hand-rolled duck-typed object. - Reject control characters in the MIME type before assigning it to .type. - Use fetch() instead of the non-fetch APIs.

Impact

The setCookie function has two attribute injection paths. validateCookieDomain does not reject semicolons (validateCookiePath already does at 0x3B), so a domain value like example.com; SameSite=None lands verbatim as Domain=example.com; SameSite=None. The unparsed array's loop only checks each entry contains = and does not sanitize values, so an entry like X-Custom=val; HttpOnly lands unchanged, injecting HttpOnly without the caller setting cookie.httpOnly = true.

Applications that pass user-controlled input to these fields, typically multi-tenant or reverse-proxy servers that scope session cookies to a tenant-supplied domain, can have SameSite CSRF protections bypassed, Secure or HttpOnly forced or stripped, or the intended SameSite tier overridden.

Patches

Patched in undici v6.28.0, v7.29.0, and v8.9.0.

Workarounds

- Sanitize domain values against the RFC 1034 letter-digit-hyphen set before passing to setCookie. - Do not pass user-controlled data to the unparsed field.