Is CVE-2026-18540 exploited in the wild?

No. Junglewise Threat Intelligence has no report of CVE-2026-18540 being exploited in the wild.

How severe is CVE-2026-18540?

Low severity, with a CVSS score of 3.7, published 4 September 2026.

Which products and versions are affected?

undici (npm) SEMVER:introduced=0,fixed=6.28.1.

Is there a fix for CVE-2026-18540?

Yes. undici (npm): upgrade to 6.28.1 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

How is CVE-2026-18540 attacked?

It is attacked over the network, in a software library.

Am I affected by CVE-2026-18540?

Check whether you run undici (npm) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

CVE-2026-18540: Node.js undici downstream response splitting via retry interceptor

CVE-2026-18540 · Severity: low · CVSS 3.7 · Published 2026-09-04

undici is a popular HTTP client library for Node.js used in web servers and proxy applications. A flaw in its automatic retry logic can cause it to combine bytes from multiple server responses into a single response body while maintaining the original response's headers, allowing attackers to inject malicious content that appears to be part of a legitimate response. Applications forwarding this malformed response downstream without recalculating content framing can emit HTTP smuggling attacks that breach security boundaries between requests.

Technologies: undici (npm). Vendors: npm.

Quick answers

Is CVE-2026-18540 exploited in the wild?: No. Junglewise Threat Intelligence has no report of CVE-2026-18540 being exploited in the wild.
How severe is CVE-2026-18540?: Low severity, with a CVSS score of 3.7, published 4 September 2026.
Which products and versions are affected?: undici (npm) SEMVER:introduced=0,fixed=6.28.1.
Is there a fix for CVE-2026-18540?: Yes. undici (npm): upgrade to 6.28.1 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-18540 attacked?: It is attacked over the network, in a software library.
Am I affected by CVE-2026-18540?: Check whether you run undici (npm) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

How to fix

undici (npm): upgrade to 6.28.1 or later
```
npm install undici@6.28.1
```

Technical details

A response splitting vulnerability exists in undici's retry interceptor when retrying partial responses. When an upstream server delivers a partial response (e.g., 404 or 206 Partial Content) and then closes, undici may retry the request with a Range header. The interceptor incorrectly appends bytes from the resumed response to the already-delivered partial body while preserving the original response's status, headers, and Content-Length. An attacker controlling the upstream server can deliver a response with mismatched Content-Length and actual body size, causing downstream proxies or gateways to interpret excess bytes as a new HTTP response, enabling HTTP request/response smuggling (CWE-444). Exploitation requires the retry interceptor to be enabled, an attacker-controlled upstream server, and a downstream application that forwards responses without normalizing content framing. Patches are available in undici 6.28.1, 7.29.1, and 8.10.2.

Affected products

OpenJS Foundation undici before 6.28.1, 7.0.0 through 7.29.0, 8.0.0 through 8.10.1

Identifiers

CVE-2026-18540
GHSA-r53p-7pc4-xj5r

Timeline

2026-09-04: disclosed
2026-09-04: patched: undici 6.28.1, 7.29.1, and 8.10.2 released

References

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