Samba is the most commonly used Windows interoperability suite of programs, used by Linux and Unix systems. It uses the SMB/CIFS protocol to provide a secure, stable, and fast file and print services. It can also seamlessly integrate with Active Directory environments and can function as a domain controller as well as a domain member (legacy NT4-style domain controller is supported, but the Active Directory domain controller feature of Samba 4 is not supported yet).

CVE-2015-0240 is a security flaw in the smbd file server daemon. It can be exploited by a malicious Samba client, by sending specially-crafted packets to the Samba server. No authentication is required to exploit this flaw. It can result in remotely controlled execution of arbitrary code as root.

We believe code execution is possible but we’ve not yet seen any working reproducers that would allow this.

This flaw arises because of an uninitialized pointer is passed to the TALLOC_FREE() funtion. It can be exploited by calling the ServerPasswordSet RPC api on the NetLogon endpoint, by using a NULL session over IPC.
Note: The code snippets shown below are from samba-3.6 shipped with Red Hat Enterprise Linux 6. (All versions of samba >= 3.5 are affected by this flaw)
In the _netr_ServerPasswordSet() function, cred is defined as a pointer to a structure. It is not initialized.

1203 NTSTATUS _netr_ServerPasswordSet(struct pipes_struct *p,   
1204                          struct netr_ServerPasswordSet *r)   
1205 {   
1206         NTSTATUS status = NT_STATUS_OK;   
1207         int i;  
1208         struct netlogon_creds_CredentialState *creds;

Later netr_creds_server_step_check() function is called with cred at:

1213  status = netr_creds_server_step_check(p, p->mem_ctx,   
1214                               r->in.computer_name,   
1215                               r->in.credential,   
1216                               r->out.return_authenticator,   
1217                               &creds);

If netr_creds_server_step_check function fails, it returns and cred is still not initialized. Later in the _netr_ServerPasswordSet() function, cred is freed using the TALLOC_FREE() function which results in an uninitialized pointer free flaw.
It may be possible to control the value of creds, by sending a number of specially-crafted packets. Later we can use the destructor pointer called by TALLOC_FREE() to execute arbitrary code.

As mentioned above, this flaw can only be triggered if netr_creds_server_step_check() fails. This is dependent on the version of Samba used.

In Samba 4.1 and above, this crash can only be triggered after setting “server schannel = yes” in the server configuration. This is due to the
adbe6cba005a2060b0f641e91b500574f4637a36 commit, which introduces NULL initialization into the most common code path. It is still possible to trigger an early return with a memory allocation failure, but that is less likely to occur. Therefore this issue is more difficult to exploit. Red Hat Product Security team has rated this flaw as having important impact on Red Hat Enterprise Linux 7.

In older versions of Samba (samba-3.6 as shipped with Red Hat Enterprise Linux 5 and 6. samba-4.0 as shipped with Red Hat Enterprise Linux 6) the above mentioned commit does not exist. An attacker could call _netr_ServerPasswordSet() function with a NULLED buffer, which could trigger this flaw. Red Hat Product Security has rated this flaw as having critical impact on all other versions of samba package shipped by Red Hat.

Lastly the version of Samba 4.0 shipped with Red Hat Enterprise Linux 6.2 EUS is based on an alpha release of Samba 4, which lacked the password change functionality and thus the vulnerability. The same is true for the version of Samba 3.0 shipped with Red Hat Enterprise Linux 4 and 5.

Red Hat has issued security advisories to fix this flaw and instructions for applying the fix are available on the knowledgebase.  This flaw is also fixed in Fedora 20 and Fedora 21.