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CVE-2020-26950 is a critical use-after-free vulnerability in Mozilla's JavaScript JIT engine, specifically in the handling of the MCallGetProperty opcode. The vulnerability arises from the JIT compiler's misclassification of the idempotent property, which fails to account for write side effects. This can result in memory corruption and a use-after-free condition during certain JavaScript operations, enabling remote code execution in the Firefox content process. The issue was discovered as part of the Tianfu Cup 2020 exploit and affects Firefox versions 78 through 84, including ESR and GeckoView mobile builds.
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This repository contains a single Metasploit module: 'firefox_jit_use_after_free.rb', which exploits CVE-2020-26950, a use-after-free vulnerability in the Firefox JavaScript JIT engine. The exploit targets Firefox versions <= 79 (and related ESR/Thunderbird versions) on both Linux and Windows platforms. The module sets up an HTTP server that delivers a specially crafted HTML/JavaScript payload to the victim's browser. The JavaScript leverages the vulnerability to achieve arbitrary code execution by spraying memory and manipulating JIT-compiled code regions. The payload is customizable via the Metasploit framework and is injected as shellcode into the browser process. The exploit requires the target to have the Firefox content sandbox disabled (MOZ_DISABLE_CONTENT_SANDBOX=1). The repository structure is typical for a Metasploit module, with the main logic in a single Ruby file that generates and serves the exploit payload. No hardcoded IPs, domains, or external URLs are used in the attack delivery; the only fingerprintable endpoint is the HTTP server path ('/').
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