CVE-2026-45537: OpenSIPS: Global Buffer Overflow in construct_uri

OpenSIPS is a Session Initiation Protocol (SIP) server implementation. In versions prior to 3.6.6 and 4.0.0-rc1, the constructuri() 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 constructuri() 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 disable503translation, 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 contactbuilder(), 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.