CVE-2026-15157

undici vulnerable to CRLF Injection via blob-like body 'type' property

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.

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():

} 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.

CVE-2026-15157 has a CVSS score of 4.2 (Medium). The vector is network-reachable, no privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (6.28.0, 7.29.0, 8.9.0); upgrading removes the vulnerable code path.

Kodem intelligence

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

1. What is CVE-2026-15157? CVE-2026-15157 is a medium-severity security vulnerability in undici (npm), affecting versions < 6.28.0. It is fixed in 6.28.0, 7.29.0, 8.9.0.
2. How severe is CVE-2026-15157? CVE-2026-15157 has a CVSS score of 4.2 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of undici are affected by CVE-2026-15157? undici (npm) versions < 6.28.0 is affected.
4. Is there a fix for CVE-2026-15157? Yes. CVE-2026-15157 is fixed in 6.28.0, 7.29.0, 8.9.0. Upgrade to this version or later.
5. Is CVE-2026-15157 exploitable, and should I be worried? Whether CVE-2026-15157 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-15157 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-15157?
   • Upgrade undici to 6.28.0 or later
   • Upgrade undici to 7.29.0 or later
   • Upgrade undici to 8.9.0 or later

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