Monday, June 21, 2004

VIRUS: [INFOCON] UNIRAS Brief - 308/04 - Mitigating risks to Email based Trojans


UNIRAS (UK Govt CERT) Briefing Notice - 308/04 dated 18.06.04 Time:
UNIRAS is part of NISCC (National Infrastructure Security Co-ordination
UNIRAS material is also available from its website at www.uniras.gov.uk
Information about NISCC is available from www.niscc.gov.uk

Mitigating risks to Email based Trojans

Departmental and company security officers should be aware of the prevalence
of vulnerabilities that can be exploited in HTML in Microsoft Email products
that can be used to download malicious code onto computers of end users. The
techniques used are likely to bypass Email content checkers and anti-virus
products at organisational gateways because the Emails do not contain any
malicious code.
Up-to-date anti virus products may identify the malicious code when it is
downloaded on the end users' machines.

A common technique is to embed an HTML IFRAME into an Email with a link to a
web page that contains malicious code. When the Email is previewed or
opened, the HTML parser will attempt to open the embedded frame or image on
the malicious web page. It will then use an exploit of an Internet Explorer
vulnerability in order to download and execute code on the end user's

An exploit that is very commonly seen is the MHTML exploit discussed in
UNIRAS Briefing 157/04 that will cause a Compiled Help Module (.CHM) file to
be downloaded and executed in the local machine context. The vulnerability
behind this exploit was patched in Microsoft Security Bulletin MS04-13 (see
http://www.microsoft.com/technet/security/bulletin/ms04-013.mspx) issued in
April 04.
Although it has been patched, other vulnerabilities could potentially be
used, and patching needs to form part of a more general approach, as
indicated below:

- - Block high risk file types via Email content filters or mail servers
(including .CHM
files) as identified in NISCC Technical Note 03/04
- - Use an anti-spam product on your organisational gateway, see NISCC
Technical Note 02/04
- - Configure web browsers in line with the guidance in NISCC Technical Note
- - Deploy anti-virus software on desktop and keep signatures up to date
- - Apply a whitelist approach to HTML pages in Email, removing IFRAMEs in
particular if
possible in Email content filters
- - Apply patches to web browsers and Email clients as part of a patching


For additional information or assistance, please contact the HELP Desk by
telephone or Not Protectively Marked information may be sent via EMail to:

Office Hours:
Mon - Fri: 08:30 - 17:00 Hrs
Tel: +44 (0) 20 7821 1330 Ext 4511
Fax: +44 (0) 20 7821 1686

Outside of Office Hours:
On Call Duty Officer:
Tel: +44 (0) 20 7821 1330 and follow the prompts

Reference to any specific commercial product, process, or service by trade
name, trademark manufacturer, or otherwise, does not constitute or imply its
endorsement, recommendation, or favouring by UNIRAS or NISCC. The views and
opinions of authors expressed within this notice shall not be used for
advertising or product endorsement purposes.

Neither UNIRAS or NISCC shall also accept responsibility for any errors or
omissions contained within this briefing notice. In particular, they shall
not be liable for any loss or damage whatsoever, arising from or in
connection with the usage of information contained within this notice.

UNIRAS is a member of the Forum of Incident Response and Security Teams
(FIRST) and has contacts with other international Incident Response Teams
(IRTs) in order to foster cooperation and coordination in incident
prevention, to prompt rapid reaction to incidents, and to promote
information sharing amongst its members and the community at large.

Version: PGP 8.0.2


Information is the currency of victory on the battlefield.
GEN Gordon Sullivan, CSA (1993)

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