CVE-2026-18540

undici vulnerable to downstream response splitting via retry interceptor

Workarounds

• Disable interceptors.retry() for untrusted upstreams, or set maxRetries: 0.
• Remove or recalculate Content-Length before forwarding a response body assembled by Undici.

Undici's interceptors.retry() can resume a request after a partial response and append the resumed bytes to an already partially delivered body, while the application still receives the original response's status and headers. When that response carried a Content-Length, the application can receive a longer body. Applications that forward Undici's status, headers, and body downstream without recalculating framing, for example proxy or gateway applications, may emit a response whose body exceeds the forwarded Content-Length, and the excess bytes can be read as the start of a subsequent HTTP response (downstream response splitting or desynchronization).

For example, a 404 Not Found with Content-Length: 2 that sends one byte then closes can be resumed with an open-ended Range request, and the resumed 206 Partial Content bytes are appended, so the application receives more than two body bytes while still seeing Content-Length: 2. The bug requires interceptors.retry() enabled, an attacker-controlled or faulty upstream, and a downstream forwarder that does not recalculate Content-Length.

CVE-2026-18540 has a CVSS score of 3.7 (Low). The vector is network-reachable, no privileges required, and no user interaction. 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 (6.28.1, 7.29.1, 8.10.2); upgrading removes the vulnerable code path.

Kodem intelligence

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Patched in undici v6.28.1, v7.29.1, and v8.10.2. Upgrade to one of these or later.

1. What is CVE-2026-18540? CVE-2026-18540 is a low-severity security vulnerability in undici (npm), affecting versions < 6.28.1. It is fixed in 6.28.1, 7.29.1, 8.10.2.
2. How severe is CVE-2026-18540? CVE-2026-18540 has a CVSS score of 3.7 (Low). 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-2026-18540? undici (npm) versions < 6.28.1 is affected.
4. Is there a fix for CVE-2026-18540? Yes. CVE-2026-18540 is fixed in 6.28.1, 7.29.1, 8.10.2. Upgrade to this version or later.
5. Is CVE-2026-18540 exploitable, and should I be worried? Whether CVE-2026-18540 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-2026-18540 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-2026-18540?
   • Upgrade undici to 6.28.1 or later
   • Upgrade undici to 7.29.1 or later
   • Upgrade undici to 8.10.2 or later

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