libgit2, when compiled using the optional, included libssh2 backend, fails to verify SSH keys by default.
When using an SSH remote with the optional, included libssh2 backend, libgit2 does not perform certificate checking by default. Prior versions of libgit2 require the caller to set the certificate_check field of libgit2's git_remote_callbacks structure - if a certificate check callback is not set, libgit2 does not perform any certificate checking. This means that by default - without configuring a certificate check callback, clients will not perform validation on the server SSH keys and may be subject to a man-in-the-middle attack.
Beginning in libgit2 v1.4.5 and v1.5.1, libgit2 will now perform host key checking by default. Users can still override the default behavior using the certificate_check function.
The libgit2 security team would like to thank the Julia and Rust security teams for responsibly disclosing this vulnerability and assisting with fixing the vulnerability.
Users are encouraged to upgrade to v1.4.5 or v1.5.1.