Root Cause:
In the ssh library of guacd, function ssh_client_thread(), when guac_ssh_get_user() fails to load private key for ssh authentication, it will return NULL. In this case, the subsequent call to guac_common_ssh_create_session() with parameter 'user=0x0' will cause guacd crash in function guac_common_ssh_authenticate() by accessing 'user->username'.
Solution:
- Update the comment of function guac_ssh_get_user() to document that NULL will be returned if fails to import key for the user.
- In function ssh_client_thread(), verify the return of guac_ssh_get_user(). If ssh_client->user is NULL, return NULL.
Test:
- Configured a ssh app with an encrypted private key and a wrong passphrase.
- Ran the ssh app from web portal and observed guacd crash.
- Applied the fix and reran the ssh app. Observed no crash.
