The OpenSSH agent permits its clients to load PKCS11 providers using the commands SSH_AGENTC_ADD_SMARTCARD_KEY and SSH_AGENTC_ADD_SMARTCARD_KEY_CONSTRAINED if OpenSSH was compiled with the ENABLE_PKCS11 flag (normally enabled) and the agent isn't locked. For these commands, the client has to specify a provider name. Th e agent passes this provider name to a subprocess (via ssh-agent.c:process_add_smartcard_key -> ssh-pkcs11-client.c:pkcs11_add_provider -> ssh-pkcs11-client.c:s end_msg), and the subprocess receives it and passes it to dlopen() (via ssh-pkcs 11-helper.c:process -> ssh-pkcs11-helper.c:process_add -> ssh-pkcs11.c:pkcs11_ad d_provider -> dlopen). No checks are performed on the provider name, apart from testing whether that provider is already loaded. This means that, if a user connects to a malicious SSH server with agent forwarding enabled and the malicious server has the ability to place a file with attacker-controlled contents in the victim's filesystem, the SSH server can execute code on the user's machine.
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