HTTP Request Smuggling Affecting undici package, versions <6.28.1>=7.0.0 <7.29.1>=8.0.0 <8.10.2
Threat Intelligence
Snyk has a proof-of-concept or detailed explanation of how to exploit this vulnerability.
The probability is the direct output of the EPSS model, and conveys an overall sense of the threat of exploitation in the wild. The percentile measures the EPSS probability relative to all known EPSS scores. Note: This data is updated daily, relying on the latest available EPSS model version. Check out the EPSS documentation for more details.
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Test your applications- Snyk IDSNYK-JS-UNDICI-19635212
- published6 Sept 2026
- disclosed4 Sept 2026
- creditYuSheng Chen
Introduced: 4 Sep 2026
NewCVE-2026-18540 (opens in a new tab)How to fix?
Upgrade undici to version 6.28.1, 7.29.1, 8.10.2 or higher.
Overview
undici is an An HTTP/1.1 client, written from scratch for Node.js
Affected versions of this package are vulnerable to HTTP Request Smuggling in the interceptors.retry() interceptor, which resumes a partial response and appends the resumed bytes to an already-delivered body, so the delivered body can exceed the declared Content-Length. An attacker can inject bytes beyond the Content-Length into a forwarded response and split the downstream HTTP response by running an upstream that returns a partial response and then resumes it with a Range request, for example a 404 with Content-Length: 2 that sends one byte, closes, then appends 206 Partial Content bytes. This requires the application to enable interceptors.retry(), an untrusted or faulty upstream, and a downstream proxy or gateway that forwards the body without recalculating framing.
Workaround
This vulnerability can be avoided by removing or recalculating Content-Length before forwarding a response body, so a resumed over-length body cannot desynchronize the downstream response.