On arm64 architectures, a WebAssembly br_table instruction with a large number of entries could cause the label to be located too far from the instruction, leading to truncation and incorrect branch address computation. This flaw could potentially allow attackers to influence control flow in Firefox's WASM execution engine. The vulnerability was assigned CVE-2025-8028 and carries a CVSS 3.1 base score of 9.8 (critical). Mozilla addressed this issue in Firefox 141 and corresponding ESR and Thunderbird releases. The vendor advisory confirms the fix and provides references to the underlying bug (Bug 1971581).

The vulnerability affects the correctness of branch address computation in WASM code on arm64 platforms, which can lead to serious security consequences including potential arbitrary code execution or other high-impact outcomes. The CVSS score of 9.8 reflects critical impact on confidentiality, integrity, and availability. No exploits in the wild have been reported at this time.

Mozilla has released official patches fixing this vulnerability in Firefox 141, Firefox ESR 115.26, Firefox ESR 128.13, Firefox ESR 140.1, Thunderbird 141, Thunderbird 128.13, and Thunderbird 140.1. Users and administrators should update to these versions or later to remediate the issue. Since the vulnerability is patched, no additional mitigation steps are required beyond applying the official updates.