Soft Serve Public Key Authentication Bypass Vulnerability when Keyboard-Interactive SSH Authentication is Enabled

Vulnerability Description

Soft Serve is a self-hostable Git server for the command line. Prior to version 0.6.2, a security vulnerability in Soft Serve could allow an unauthenticated, remote attacker to bypass public key authentication when keyboard-interactive SSH authentication is active, through the `allow-keyless` setting, and the public key requires additional client-side verification for example using FIDO2 or GPG. This is due to insufficient validation procedures of the public key step during SSH request handshake, granting unauthorized access if the keyboard-interaction mode is utilized. An attacker could exploit this vulnerability by presenting manipulated SSH requests using keyboard-interactive authentication mode. This could potentially result in unauthorized access to the Soft Serve. Users should upgrade to the latest Soft Serve version `v0.6.2` to receive the patch for this issue. To workaround this vulnerability without upgrading, users can temporarily disable Keyboard-Interactive SSH Authentication using the `allow-keyless` setting.

References

• https://github.com/charmbracelet/soft-serve/security/advisories/GHSA-mc97-99j4-vm2v
• https://github.com/charmbracelet/soft-serve/issues/389
• https://github.com/charmbracelet/soft-serve/commit/407c4ec72d1006cee1ff8c1775e5bcc091c2bc89
• https://github.com/charmbracelet/soft-serve/releases/tag/v0.6.2