Microsoft Internet Explorer URI Decoding Vulnerability



Date Discovered

February 8, 2005


Microsoft Internet Explorer is prone to a vulnerability related to URI decoding. A bug in how the browser parses encoded URI data may allow zone bypass. As a result, it is possible to force the browser to interpret Web content in the Local Zone. This could be exploited to execute arbitrary code in the context of the user who is currently logged in. Cross-site scripting attacks are also possible due to this issue, as well as partial address bar obfuscation. This vulnerability is similar to the zone bypass attack described in BID 10517.

Technologies Affected

  • Microsoft Internet Explorer 5.0.1
  • Microsoft Internet Explorer 5.0.1 SP1
  • Microsoft Internet Explorer 5.0.1 SP2
  • Microsoft Internet Explorer 5.0.1 SP3
  • Microsoft Internet Explorer 5.0.1 SP4
  • Microsoft Internet Explorer 5.5
  • Microsoft Internet Explorer 5.5 SP1
  • Microsoft Internet Explorer 5.5 SP2
  • Microsoft Internet Explorer 6.0
  • Microsoft Internet Explorer 6.0 SP1
  • Microsoft Internet Explorer 6.0 SP2 do not use
  • Nortel Networks IP softphone 2050
  • Nortel Networks Mobile Voice Client 2050
  • Nortel Networks Optivity Telephony Manager (OTM)
  • Nortel Networks Symposium Web Center Portal (SWCP)
  • Nortel Networks Symposium Web Client


Run all software as a nonprivileged user with minimal access rights.
Perform all non-administrative tasks, such as browsing the Web and reading e-mail, as an unprivileged user with minimal access rights. This will limit the consequences of successful exploitation.

Do not follow links provided by unknown or untrusted sources.
Web users should be wary of visiting links provided by untrusted or unfamiliar sources since this issue may be exploited by a malicious Web page.

Set web browser security to disable the execution of script code or active content.
Disable support for client-side scripting and active content in the Internet Zone. This may limit exposure to exploitation of this and other vulnerabilities.

Modify default configuration files to disable any unwanted behavior.
Configuring email clients so that all email messages are displayed in plain-text format only may help close a malicious email attack vector.

Microsoft has released a cumulative update for Internet Explorer to address this and other vulnerabilities. Updates for Internet Explorer on Microsoft Windows 98/98SE/ME may be obtained through Windows Update. Nortel Networks has released security advisory 2005005511-2 acknowledging this issue. Please the referenced advisory for further information.



Discovery is credited to Jouko Pynnönen.

© 1995- Symantec Corporation

Permission to redistribute this alert electronically is granted as long as it is not edited in any way unless authorized by Symantec Security Response. Reprinting the whole or part of this alert in any medium other than electronically requires permission from


The information in the advisory is believed to be accurate at the time of publishing based on currently available information. Use of the information constitutes acceptance for use in an AS IS condition. There are no warranties with regard to this information. Neither the author nor the publisher accepts any liability for any direct, indirect, or consequential loss or damage arising from use of, or reliance on, this information.

Symantec, Symantec products, Symantec Security Response, and are registered trademarks of Symantec Corp. and/or affiliated companies in the United States and other countries. All other registered and unregistered trademarks represented in this document are the sole property of their respective companies/owners.