CVE-2016-10009

Vulnerabilidad de ruta de búsqueda no confiable en ssh-agent.c en ssh-agent en OpenSSH en versiones anteriores a 7.4 permite a atacantes remotos ejecutar modulos locales PKCS#11 arbitrarios aprovechando el control sobre un agent-socket reenviado.

It was found that ssh-agent could load PKCS#11 modules from arbitrary paths. An attacker having control of the forwarded agent-socket on the server, and the ability to write to the filesystem of the client host, could use this flaw to execute arbitrary code with the privileges of the user running ssh-agent.

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.