GHSA-m8rv-5g2x-5cg5

### 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.