Man-in-the-middle attack via SSH host key bypass

Summary

When a libcurl-based application performs transfers via `SCP://` or `SFTP://` and utilizes the `CURLOPT_SSH_KEYFUNCTION` callback, it may silently accept an untrusted server. This vulnerability occurs when a server presents a host key type that does not match the specific key type already recorded for that host in the `known_hosts` file. Instead of rejecting the mismatch, the callback mechanism fails to properly enforce the restriction, allowing the connection to succeed without warning and risking a potential man-in-the-middle attack. A flaw was found in curl. When a libcurl-based application uses SCP:// or SFTP:// for transfers and employs the CURLOPT_SSH_KEYFUNCTION callback, it may silently accept an untrusted server. This occurs if the server's host key type differs from the one stored in the known_hosts file. The callback mechanism fails to enforce the host key restriction, enabling a connection to an untrusted server and risking a man-in-the-middle attack. This is an Important flaw where libcurl-based applications performing SCP or SFTP transfers with the CURLOPT_SSH_KEYFUNCTION callback may silently accept an untrusted server. This occurs when a server presents a host key type that differs from the one recorded in the known_hosts file, bypassing a critical host key verification and enabling potential man-in-the-middle attacks. The vulnerability requires specific application callback usage, not a general curl client default. Red Hat severity: Important — CVSS 7.4 (CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N). Weakness: CWE-347. No fixing RHSA erratum has published yet; monitor the Red Hat CVE page and patch when it ships.