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Opera Telnet URI Handler File Creation/Truncation Vulnerability



Opera is a cross-platform web browser. More information is available
from http://www.opera.com/


Exploitation of an input validation vulnerability within Opera Software
ASA.'s Opera Web Browser could allow remote attackers to create or
truncate arbitrary files.

The problem specifically exists within the telnet URI handler. Opera
does not check for '-' at the beginning of hostname passed through the
handler, which lets options pass to the telnet program, allowing file
creation or overwriting. Under Windows XP, when telnet.exe is executed
with the '-f' option, the remainder of the argument is used as a
filename for logging the connection. Under Linux, the '-n' option
creates a 'tracefile' for the connection. These options create a file if
it does not exist, or truncates it if it does.

If a telnet: URI with the appropriate option is opened, a file will be
created in the current working directory of the Opera process if the
user has permission. In Windows, this defaults to the directory Opera
was installed in. Under Linux, the default is the user's home directory.


Windows XP: Creates or overwrites 'Filename' in Opera directory.


Under Linux: Creates or overwrites 'Filename' in user's home directory.


Under some previous versions of Opera, it was possible to create a file
anywhere on the filesystem, by hex encoding an absolute path in the
filename portion of the URI.


In Windows, depending on the privileges, it may be possible to make
Opera unavailable by overwriting files. Under Linux it is possible to
overwrite files in the current user's home directory (e.g. .bashrc,

Some versions or configurations of Windows may not be vulnerable, due to
the absence of the '-f' command line switch.


Opera 7.23 has been confirmed vulnerable, as have a variety of earlier
versions on multiple platforms. It is suspected that all earlier
versions are also vulnerable.



Disable the telnet URI handler from within Opera.

Click on the 'File' menu, then the 'Preferences...' item choose
'Programs and paths' from the view on the left. Select on 'telnet' from
the Protocols box and press the delete key. Do the same with the tn3270


The vulnerability has been addressed in Opera 7.50 (Windows, Mac, Linux).

Windows version downloadable from

Mac version downloadable from

Linux i386 version downloadable from


The Common Vulnerabilities and Exposures (CVE) project has assigned the
name CAN-2004-0473 to this issue. This is a candidate for inclusion in
the CVE list (http://cve.mitre.org), which standardizes names for
security problems.


Karol Wiesek and Greg MacManus are credited with this discovery.

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Copyright © 2004 Verisign, Inc.

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