CVE-2021-45331: 
NixOS vulnerability analysis and mitigation

CVE-2021-45331 is an Authentication Bypass vulnerability affecting Gitea versions before 1.5.0. The vulnerability was disclosed in February 2022 and impacts the Two-Factor Authentication (2FA) mechanism of Gitea. The core issue allows a malicious user to reuse TOTP (Time-based One-Time Password) codes multiple times, effectively bypassing the one-time nature of the 2FA system (NVD).

The vulnerability stems from a design flaw in the TOTP implementation where the system fails to properly invalidate used TOTP codes. If an attacker captures a valid TOTP code, they can submit it multiple times within the same time window, which contradicts the fundamental security principle of one-time passwords. The vulnerability has been assigned a CVSS v3.1 base score of 9.8 (CRITICAL) with the vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H (NVD).

The vulnerability allows malicious users to gain elevated privileges by bypassing the two-factor authentication mechanism. Once a valid TOTP code is obtained, it can be reused to authenticate multiple times, effectively compromising the account's security and potentially leading to unauthorized access to sensitive information (NVD).

The vulnerability was fixed in Gitea version 1.5.0. The fix prevents the reuse of TOTP codes by implementing proper validation and tracking of used codes. Organizations running affected versions should upgrade to version 1.5.0 or later to protect against this vulnerability (Gitea Release, Gitea PR).