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UNIRAS Brief - 308/04 - Mitigating risks to Email based Trojans


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   UNIRAS (UK Govt CERT) Briefing Notice - 308/04 dated 18.06.04  Time: 14:33
  UNIRAS is part of NISCC (National Infrastructure Security Co-ordination Centre)
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  UNIRAS material is also available from its website at www.uniras.gov.uk and
         Information about NISCC is available from www.niscc.gov.uk
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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 computer. 

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 05/03
- - 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 strategy 

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

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

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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. 
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<End of UNIRAS Briefing>

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