CVE-2019-5436: Heap Buffer Overflow at lib/tftp.c

How it was found

Summary:

A heap buffer overflow can occur at line 1114 in file lib/tftp.c due to the fact of state->blksize containing the default size instead of containing the one specified in the --tftp-blksize parameter.

This bug could lead to a crash or maybe to RCE in the case the attacker also had a memory leak.

Vulnerable line:

state->rbytes = (int)recvfrom(state->sockfd,
                                (void *)state->rpacket.data,
                                state->blksize + 4,
                                0,
                                (struct sockaddr *)&fromaddr,
                                &fromlen);

Steps To Reproduce:

1. Download the server script
2. Run it and bind to an address: $ python evil-server.py IP PORT
3. Connect to that server with curl: $ curl --tftp-blksize N tftp://IP:PORT Where N should be a number lower than 293.

Impact

• An attacker would also need a memory leak in order to gain full RCE.
• The victim should explicitly set the --blksize argument to a value inferior to 293.

Thus, the impact is not very high but it's still quite dangerous to not release a patch.