The Open Phone Abstraction Library (opal), as used by (1) Ekiga before 2.0.10 and (2) OpenH323 before 2.2.4, allows remote attackers to cause a denial of service (crash) via an invalid Content-Length header field in Session Initiation Protocol (SIP) packets, which causes a \0 byte to be written to an "attacker-controlled address."
The product receives input or data, but it does not validate or incorrectly validates that the input has the properties that are required to process the data safely and correctly.