Software Engineering Institute

Microsoft Windows Remote Procedure Call (RPC) "... is a powerful, robust, efficient, and secure interprocess communication (IPC) mechanism that enables data exchange and invocation of functionality residing in a different process. That different process can be on the same machine, on the local area network, or across the Internet." Distributed COM (DCOM) "...extends the Component Object Model (COM) to support communication among objects on different computers -- on a LAN, a WAN, or even the Internet."

Based on publicly available exploit code, there is a vulnerability in the way the RPCSS service handles DCOM/RPC messages. This vulnerability is different than those described in CA-2003-16 (VU#568148/MS03-026) and CA-2003-23 (VU#254236/VU#483492/MS03-039). As in the previous vulnerabilities, this flaw appears to occur in functions related to DCOM object activation. A remote attacker could attempt to exploit this vulnerability using crafted RPC packets.

Internet Security Systems (ISS) X-Force has published an advisory stating that this vulnerability "...manifests as a result of a separate multi-threaded race condition when processing incoming RPC requests." Depending on variables such as network latency and CPU load, one RPCSS thread may free a memory buffer before another thread has finished processing the same buffer. This causes memory corruption that can lead to termination of the RPCSS process.

An unauthenticated, remote attacker could cause a denial of service or possibly execute arbitrary code with SYSTEM privileges. In tests, the public exploit code crashes the RPCSS service on Windows 2000 and Windows XP systems patched with MS03-039. The exploit executes code on Windows 2000 systems that do not have the MS03-039 patch.

The CERT/CC is currently unaware of a practical solution to this problem.

Until patches are available, the following workarounds can be used to reduce possible attack vectors. These workarounds are not complete solutions and may affect network and application operation. Research and test before making changes to production systems.

• Using a network or host-based firewall, block RPC network traffic (ports 135/tcp, 139/tcp, 445/tcp, 593/tcp and 135/udp, 137/udp, 138/udp, 445/udp).
• Disable COM Internet Services (CIS) and RPC over HTTP as described in Microsoft Knowledge Base Article 825819.
• Disable DCOM as described Microsoft Knowledge Base Article 825750.