Can you spot the vulnerability?

ossl110d

This code was introduced in OpenSSL 1.1.0d, which was released a couple of days ago. This is in the server SSL code, ssl/statem/statem_srvr.c, ssl_bytes_to_cipher_list()), and can easily be reached remotely. Can you spot the vulnerability? (read ahead for the answer)

So there is a loop, and within that loop we have an ‘if’ statement, that tests a number of conditions. If any of those conditions fail, OPENSSL_free(raw) is called. But raw isn’t the address that was allocated; raw is increment every loop. Hence, there is a remote invalid free vulnerability.. But not quite. None of those checks in the ‘if’ statement can actually fail; earlier on in the function, there is a check that verifies that the packet contains at least 1 byte, so PACKET_get_1 cannot fail. Furthermore, earlier in the function it is verified that the packet length is a multiple of 3, hence PACKET_copy_bytes and PACKET_forward cannot fail.

Nonetheless OpenSSL has acknowledged that the OPENSSL_free line needs a rewrite: https://github.com/openssl/openssl/pull/2312

PS I’m not posting this to ridicule the OpenSSL project or their programming skills. I just like reading code and finding corner cases that impact security, which is an effort that ultimately works in everybody’s best interest, and I like to share what I find. Programming is a very difficult enterprise and everybody makes mistakes.

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