OpenSIPS up to 3.6.5 URI Construction construct_uri Username buffer overflow

Summaryinfo

A vulnerability was found in OpenSIPS up to 3.6.5 and classified as very critical. This affects the function construct_uri of the component URI Construction. The manipulation of the argument Username results in buffer overflow. This vulnerability was named CVE-2026-45537. The attack may be performed from remote. There is no available exploit.

Detailsinfo

A vulnerability was found in OpenSIPS up to 3.6.5. It has been classified as very critical. Affected is the function construct_uri of the component URI Construction. The manipulation of the argument username with an unknown input leads to a buffer overflow vulnerability. CWE is classifying the issue as CWE-120. The product copies an input buffer to an output buffer without verifying that the size of the input buffer is less than the size of the output buffer, leading to a buffer overflow. This is going to have an impact on confidentiality, integrity, and availability. CVE summarizes:

OpenSIPS is a Session Initiation Protocol (SIP) server implementation. In versions prior to 3.6.6 and 4.0.0-rc1, the construct_uri() function concatenates multiple URI components (protocol, username, domain, port, params) into a fixed 1024-byte global BSS buffer without any bounds checking. When a routing script calls construct_uri() with an attacker-controlled username, a combined component length exceeding 1024 bytes overflows the buffer, corrupting adjacent global data with attacker-controlled content. The overflow reaches disable_503_translation, a global flag controlling SIP 503 response handling, allowing an attacker to deterministically set the flag via the URI username and alter the server's routing behavior for subsequent messages. Because the same buffer is shared with contact_builder(), the overflow also corrupts that function's data, and without a memory sanitizer the adjacent globals are silently overwritten on every request containing a long username. This issue has been fixed in versions 3.6.6 and 4.0.0-rc1.

The advisory is available at github.com. This vulnerability is traded as CVE-2026-45537 since 05/12/2026. The exploitability is told to be easy. It is possible to launch the attack remotely. Technical details are known, but there is no available exploit. The structure of the vulnerability defines a possible price range of USD $0-$5k at the moment (estimation calculated on 08/05/2026).

Upgrading to version 3.6.6 or 4.0.0-rc1 eliminates this vulnerability.

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Productinfo

Name

• OpenSIPS

Version

• 3.6.0
• 3.6.1
• 3.6.2
• 3.6.3
• 3.6.4
• 3.6.5

Website

• Product: https://github.com/OpenSIPS/opensips/

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