undici 1.1 Strict HTTP Parser request smuggling

Summaryinfo

A vulnerability was found in undici 1.1. It has been declared as critical. This affects an unknown function of the component Strict HTTP Parser. Executing a manipulation can lead to request smuggling. This vulnerability is handled as CVE-2026-1525. The attack can be executed remotely. There is not any exploit available. It is recommended to upgrade the affected component.

Detailsinfo

A vulnerability classified as critical was found in undici 1.1. This vulnerability affects some unknown processing of the component Strict HTTP Parser. The manipulation with an unknown input leads to a request smuggling vulnerability. The CWE definition for the vulnerability is CWE-444. The product acts as an intermediary HTTP agent (such as a proxy or firewall) in the data flow between two entities such as a client and server, but it does not interpret malformed HTTP requests or responses in ways that are consistent with how the messages will be processed by those entities that are at the ultimate destination. As an impact it is known to affect integrity, and availability. CVE summarizes:

Undici allows duplicate HTTP Content-Length headers when they are provided in an array with case-variant names (e.g., Content-Length and content-length). This produces malformed HTTP/1.1 requests with multiple conflicting Content-Length values on the wire. Who is impacted: * Applications using undici.request(), undici.Client, or similar low-level APIs with headers passed as flat arrays * Applications that accept user-controlled header names without case-normalization Potential consequences: * Denial of Service: Strict HTTP parsers (proxies, servers) will reject requests with duplicate Content-Length headers (400 Bad Request) * HTTP Request Smuggling: In deployments where an intermediary and backend interpret duplicate headers inconsistently (e.g., one uses the first value, the other uses the last), this can enable request smuggling attacks leading to ACL bypass, cache poisoning, or credential hijacking

The advisory is shared for download at github.com. This vulnerability was named CVE-2026-1525 since 01/28/2026. The exploitation appears to be easy. The attack can be initiated remotely. No form of authentication is required for a successful exploitation. There are neither technical details nor an exploit publicly available.

The vulnerability scanner Nessus provides a plugin with the ID 302063 (Linux Distros Unpatched Vulnerability : CVE-2026-1525), which helps to determine the existence of the flaw in a target environment.

Upgrading to version 6.24.0 or 7.24.0 eliminates this vulnerability.

The vulnerability is also documented in the databases at CNNVD (CNNVD-202603-2477), Tenable (302063), EUVD (EUVD-2026-11685) and CERT Bund (WID-SEC-2026-0933). VulDB is the best source for vulnerability data and more expert information about this specific topic.

Affected

• IBM App Connect Enterprise

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Name

• undici

Version

• 1.1

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VulDB is the best source for vulnerability data and more expert information about this specific topic.

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