CVE-2020-12420: 
NixOS vulnerability analysis and mitigation

CVE-2020-12420 is a high-severity vulnerability discovered in Mozilla Firefox and Thunderbird that affects versions Firefox ESR < 68.10, Firefox < 78, and Thunderbird < 68.10.0. The vulnerability was discovered by Byron Campen and disclosed on June 30, 2020. The issue occurs when attempting to connect to a STUN server, where a race condition could trigger a use-after-free condition (Mozilla Advisory).

The vulnerability stems from a race condition that occurs during STUN server connection attempts, which could cause a use-after-free of a pointer. Specifically, if nr_ice_component_insert_pair fails, the candidate pair remains in the check_list, leading to potential memory corruption when the peer candidate is destroyed. This vulnerability has been assigned a CVSS v3.1 base score of 8.8 (High), with attack vector: Network, attack complexity: Low, privileges required: None, user interaction: Required, scope: Unchanged, and impact scores of High for confidentiality, integrity, and availability (Ubuntu Security).

The exploitation of this vulnerability could lead to memory corruption and a potentially exploitable crash. Given the high CVSS scores for confidentiality, integrity, and availability impacts, successful exploitation could potentially allow an attacker to execute arbitrary code on the affected system (Mozilla Advisory, Ubuntu Security).

The vulnerability requires network access and user interaction to exploit. While the attack complexity is rated as low, the requirement for user interaction provides some mitigation against automated attacks. The vulnerability was fixed before any known exploits in the wild were reported (Ubuntu Security).

The vulnerability was addressed in Firefox ESR 68.10, Firefox 78, and Thunderbird 68.10.0. Users are advised to upgrade to these versions or later. The fix involved modifications to the candidate pair insertion code to prevent the race condition that could lead to the use-after-free condition (Mozilla Advisory).