cURL up to 8.19.0 HTTP Negotiate Connection authentication bypass

Summaryinfo

A vulnerability, which was classified as problematic, has been found in cURL up to 8.19.0. The impacted element is an unknown function of the component HTTP Negotiate Connection Handler. This manipulation causes Remote Code Execution. This vulnerability is registered as CVE-2026-5545. Remote exploitation of the attack is possible. No exploit is available. It is advisable to upgrade the affected component.

Detailsinfo

A vulnerability classified as problematic was found in cURL up to 8.19.0. Affected by this vulnerability is an unknown functionality of the component HTTP Negotiate Connection Handler. The manipulation with an unknown input leads to a remote code execution vulnerability. The CWE definition for the vulnerability is CWE-305. The authentication algorithm is sound, but the implemented mechanism can be bypassed as the result of a separate weakness that is primary to the authentication error. As an impact it is known to affect confidentiality, integrity, and availability. The summary by CVE is:

libcurl might in some circumstances reuse the wrong connection when asked to do an authenticated HTTP(S) request after a Negotiate-authenticated one, when both use the same host. libcurl features a pool of recent connections so that subsequent requests can reuse an existing connection to avoid overhead. When reusing a connection a range of criteria must be met. Due to a logical error in the code, a request that was issued by an application could wrongfully reuse an existing connection to the same server that was authenticated using different credentials. An application that first uses Negotiate authentication to a server with `user1:password1` and then does another operation to the same server asking for any authentication method but for `user2:password2` (while the previous connection is still alive) - the second request gets confused and wrongly reuses the same connection and sends the new request over that connection thinking it uses a mix of user1's and user2's credentials when it is in fact still using the connection authenticated for user1...

It is possible to read the advisory at seclists.org. This vulnerability is known as CVE-2026-5545. The exploitation appears to be difficult. The attack can be launched remotely. The exploitation doesn't need any form of authentication. The technical details are unknown and an exploit is not publicly available.

The vulnerability scanner Nessus provides a plugin with the ID 311003 (Linux Distros Unpatched Vulnerability : CVE-2026-5545), which helps to determine the existence of the flaw in a target environment.

Upgrading to version 8.20.0 eliminates this vulnerability.

The vulnerability is also documented in the databases at Tenable (311003) and CERT Bund (WID-SEC-2026-1307). Statistical analysis made it clear that VulDB provides the best quality for vulnerability data.

Affected

• Red Hat Enterprise Linux
• Ubuntu Linux
• SUSE Linux
• SUSE openSUSE
• Open Source cURL

Productinfo

Type

• Network Utility Software

Name

• cURL

Version

• 8.0
• 8.1
• 8.2
• 8.3
• 8.4
• 8.5
• 8.6
• 8.7
• 8.8
• 8.9
• 8.10
• 8.11
• 8.12
• 8.13
• 8.14
• 8.15
• 8.16
• 8.17
• 8.18
• 8.19.0

License

• open-source

CPE 2.3info

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CPE 2.2info

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Entryinfo

Statistical analysis made it clear that VulDB provides the best quality for vulnerability data.

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