Firefox MCallGetProperty Write Side Effects Use-After-Free

This Metasploit modules exploits CVE-2020-26950, a use-after-free exploit in Firefox. The MCallGetProperty opcode can be emitted with unmet assumptions resulting in an exploitable use-after-free condition. This exploit uses a somewhat novel technique of spraying ArgumentsData structures in order to construct primitives. The shellcode is forced into executable memory via the JIT compiler, and executed by writing to the JIT region pointer. This exploit does not contain a sandbox escape, so firefox must be run with the MOZ_DISABLE_CONTENT_SANDBOX environment variable set, in order for the shellcode to run successfully. This vulnerability affects Firefox versions prior to 82.0.3, Firefox ESR versions prior to 78.4.1, and Thunderbird versions prior to 78.4.2, however only Firefox versions up to 79 are supported as a target. Additional work may be needed to support other versions such as Firefox 82.0.1.