Software Engineering Institute

Microsoft's Exchange Server supports a number of protocols for handling email, including the Simple Mail Transfer Protocol (SMTP) and SMTP extended verbs as defined by RFC2821. A buffer overflow error exists in the way that Exchange Server 2003 and Exchange 2000 Server handle a particular, but unspecified SMTP extended verb. This error results in a vulnerability that could allow a remote attacker to execute arbitrary code or cause a denial of service. According to Microsoft, this vulnerability could be exploited by:

On Exchange 2000, any anonymous user who could connect to an SMTP port on the Exchange Server and issue a specially crafted extended verb request.

On Exchange 2003, the level of authentication required to exploit this vulnerability is typically only granted to other Exchange Servers within the same organization. In this case, the attacker would have to connect to an SMTP port on the Exchange Server with the authority of another Exchange Server within the same organization and issue a specially crafted extended verb request.

NOTE: We are aware of publicly available exploit code for this vulnerability and have received reports of successful compromise of vulnerable systems.

Remote attackers may be able to cause a denial of service or execute code of their choosing on an affected system, depending on the version of Exchange Server being used. In the case of code execution, the code would be run with the privileges of the Exchange Server, typically Local System.

Apply a patch

Microsoft has published Microsoft Security Bulletin MS05-021 in response to this issue. Users are encouraged to review this bulletin and apply the patches it refers to.

Workarounds

In addition to the patches referenced above, Microsoft Security Bulletin MS05-021 also contains workaround information to mitigate this issue. Users, particularly those who are unable to apply the patches, should consider implementing these workarounds as appropriate.