Samba chain_reply Memory Corruption (Linux x86)

Description

This exploits a memory corruption vulnerability present in Samba versions prior to 3.3.13. When handling chained response packets, Samba fails to validate the offset value used when building the next part. By setting this value to a number larger than the destination buffer size, an attacker can corrupt memory. Additionally, setting this value to a value smaller than 'smb_wct' (0x24) will cause the header of the input buffer chunk to be corrupted.

After close inspection, it appears that 3.0.x versions of Samba are not exploitable. Since they use an "InputBuffer" size of 0x20441, an attacker cannot cause memory to be corrupted in an exploitable way. It is possible to corrupt the heap header of the "InputBuffer", but it didn't seem possible to get the chunk to be processed again prior to process exit.

In order to gain code execution, this exploit attempts to overwrite a "talloc chunk" destructor function pointer.

This particular module is capable of exploiting the flaw on x86 Linux systems that do not have the nx memory protection.

NOTE: It is possible to make exploitation attempts indefinitely since Samba forks for user sessions in the default configuration.