Microsoft Outlook / Microsoft Exchange TNEF Decoding Remote Code Execution Vulnerability



Date Discovered

January 10, 2006


Microsoft Exchange Server and Outlook email clients are prone to a remote code-execution vulnerability. This vulnerability presents itself when the applications decode a message containing a specially crafted TNEF MIME attachment. Successful exploitation may result in arbitrary code execution facilitating a remote compromise. An attack against Microsoft Exchange Server could lead to a SYSTEM-level remote compromise, while attacks against Outlook would result in arbitrary code execution in the context of the current user.

Technologies Affected

  • Microsoft Exchange Server 2000 SP3
  • Microsoft Exchange Server 5.0
  • Microsoft Exchange Server 5.0 SP1
  • Microsoft Exchange Server 5.0 SP2
  • Microsoft Exchange Server 5.5
  • Microsoft Exchange Server 5.5 SP1
  • Microsoft Exchange Server 5.5 SP2
  • Microsoft Exchange Server 5.5 SP3
  • Microsoft Exchange Server 5.5 SP4
  • Microsoft Office 2000 SP3
  • Microsoft Office 2003 SP1
  • Microsoft Office 2003 SP2
  • Microsoft Office XP SP3
  • Microsoft Outlook 2000 SP3
  • Microsoft Outlook 2002 SP3
  • Microsoft Outlook 2003
  • Nortel Networks Optivity Telephony Manager for SL-100
  • Nortel Networks Passport Multiservice Data Manager (MDM)
  • Nortel Networks Self-Service


Block external access at the network boundary, unless external parties require service.
Deploy network access controls to permit access to the server for trusted or internal networks and computers only. This could include permitting remote access to the server through a VPN connection. Establish access controls at the network perimeter as well to explicitly deny access by all unauthorized computers and networks.

Run all software as a nonprivileged user with minimal access rights.
Running the affected Outlook client as a user with least privileges possible may help mitigate the impact of a successful attack.

Deploy network intrusion detection systems to monitor network traffic for malicious activity.
Deploy NIDS to monitor network traffic for signs of anomalous or suspicious activity. Monitor server logs regularly for any signs of unusual activity, such as repeated crashes of services.

Run all software as a nonprivileged user with minimal access rights.
If possible, run the server as an unprivileged user with minimal access rights. This may limit the consequences of successful exploitation of this and other latent vulnerabilities.

Do not accept communications that originate from unknown or untrusted sources.
Users should avoid trusting incoming email messages of enticing or suspicious nature.

Implement multiple redundant layers of security.
Memory-protection schemes (such as non-executable stack/heap configurations and randomly mapped memory segments) may complicate exploitation of memory-corruption vulnerabilities.

Microsoft has released fixes for supported applications. Nortel has released an advisory (2006006583) to identify vulnerable products. The vendor advises customers to follow Microsoft's recommendations and install fixes supplied by Microsoft.



Discovered by John Heasman and Marc Litchfield of NGS Software.

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