MFA bypass in oauth flow in Firefly III

Vulnerability Description

Firefly III is a free and open source personal finance manager. In affected versions an MFA bypass in the Firefly III OAuth flow may allow malicious users to bypass the MFA-check. This allows malicious users to use password spraying to gain access to Firefly III data using passwords stolen from other sources. As OAuth applications are easily enumerable using an incrementing id, an attacker could try sign an OAuth application up to a users profile quite easily if they have created one. The attacker would also need to know the victims username and password. This problem has been patched in Firefly III v6.1.17 and up. Users are advised to upgrade. Users unable to upgrade should Use a unique password for their Firefly III instance and store their password securely, i.e. in a password manager.

References

• https://github.com/firefly-iii/firefly-iii/security/advisories/GHSA-4gm4-c4mh-4p7w
• https://owasp.org/www-community/attacks/Password_Spraying_Attack
• https://www.menlosecurity.com/what-is/highly-evasive-adaptive-threats-heat/mfa-bypass