Buffer Overflow in MS Outlook Email Clients

Security Advisory: Buffer Overflow in MS Outlook & Outlook Express Email Clients

Date: 18th July 2000
Author: Aaron Drew (mailto:[email protected])
Versions Affected: MS Outlook 97/2000 and MS Outlook Express 4/5

A bug in a shared component of Microsoft Outlook and Outlook Express mail
clients can allow a remote user to write arbitrary data to the stack. This
bug has been found to exist in all versions of MS Outlook and Outlook
Express on both Windows 95/98 and Windows NT 4.

The vulnerability lies in the parsing of the GMT section of the date field
in the header of an email. Bound checking on the token representing the GMT
is not properly handled. This bug can be witnessed by opening an email with
an exceptionally long string directly preceding the GMT specification in
the Date header field such as:

Date: Fri, 13 July 2000 14:16:06
+1000xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

The bug lies in the shared library INETCOMM.DLL and has been successfully
exploited on Windows 95, 98 and NT with both Outlook and Outlook Express.

The execution of this code is performed differently under each client. Under
Outlook Express, the buffer overflow occurs as soon as the user tries to
view the mail folder containing email with a malicious date header. Under
Microsoft Outlook, the overflow occurs when attempting to preview, read,
reply or forward any email with a malicious date header. Under MS Outlook a
user may delete or save an email to disk without exploitation.

Whilst some mail transport systems seem to modify 8-bit header data or lines
over 70 characters in length preventing direct exploitation, these
restrictions seem to be avoided by encoding a message with an exploit date
field as a MIME attachment in a Outlook's MIME attached message format.
These messages also overflow the stack when read, previewed, replied to or
forwarded.

Microsoft was notified of this bug on July 3.

Attached is a proof-of-point exploit that, when placed in the header
field of a message or MIME attached message, will download and execute
an executable from the web. (In this particular case it will launch MS Freecell)

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