PYSEC-2026-3855

Summary

kas persistently disables SSH host key checking for the invoking user when internal SSH key setup is triggered via SSH_PRIVATE_KEY or SSH_PRIVATE_KEY_FILE and no user-specific SSH configuration file exists so far.

When this path is used, kas creates ~/.ssh/config with a global Host * rule containing StrictHostKeyChecking no. This was intended to ease the use of kas in short-lived CI environments that lack a pre-configured set of known hosts. In case a local user had no SSH configuration file so far, this approach weakens SSH host authenticity verification beyond the lifetime and scope of the kas command, increasing the risk of successful man-in-the-middle attacks against future SSH connections made by the same user.

Patches

Such SSH configurations were created since the very first public release. The issue is addressed now by commit which is part of kas version 5.4.

Workarounds

Since kas 2.6.3, a local user's SSH configuration is only written if it didn't exist before. From that version on, the issue can be avoided by creating an own ~/.ssh/config prior to calling kas. If kas was already called, ~/.ssh/config should be inspected and undesired settings created by kas should be removed.