Finjan Software Discovers a New Critical Vulnerability In Yahoo E-mail Service
Yahoo E-mail Service Vulnerability
Release Date:
December 10, 2003
Severity:
Critical (Potential web-based e-mail worm)
Systems Affected:
Other web-based e-mail systems may be vulnerable.
Internet Explorer and any software application used for reading Yahoo e-mail
messages.
Status:
Yahoo,Excite and Outblaze (Mail.com) have already patched their Web-based
e-mail services. Other web-based e-mail systems may be vulnerable.
Description:
Finjan Software identified a malicious script execution security vulnerability
in Yahoo's Web-based e-mail service. This vulnerability had the potential to
allow malicious hackers to automatically launch a worm or malicious mobile code
attack upon the opening of an e-mail message. The vulnerability was reported
to Yahoo and has been fixed. Malicious Script Execution flaws allow a malicious
hacker to input malicious script into a seemingly normal e-mail message. A
computer user opening an e-mail message containing an embedded malicious script
could automatically be hit with a malicious code attack if scripting has been
enabled on the Web browser. Malicious script can be written in various
languages including Java, JavaScript, VB Script, Active X, and HTML.
In addition to destroying files, malicious code attacks have the ability to
steal personal information such as usernames, passwords, credit card numbers,
and any other information a user inputs into the computer. It can also expose
restricted parts of a local area network, such as an Intranet, to the public.
"Web-based e-mails have become very popular due to its ability to provide
access to one?s e-mail messages from any computer connected to the Internet,"
said Brian Burke, program manager at IDC. "Malicious hackers are always looking
at ways to gain unauthorized access to personal information of their victims
for various reasons and Web e-mail services are certainly a potential target."
Other web-based e-mail systems may be vulnerable to this vulnerability.
Additional information about the malicious script execution security flaw can
be found at: http://www.kb.cert.org/vuls/id/707100
Technical details:
This was a cross-site scripting vulnerability of the Yahoo! Web-based e-mail
service. The purpose of Yahoo's active content filter is to block the injection
of any active content into Yahoo! messages. However, the basic failure that
allowed this vulnerability is that there was no blocking of a double encoding.
Yahoo?s filter removed only the first instance.
MCRC has inserted malformed encoded style to the ?input? HTML tag, using
several known encoding methods.
For example:
<input type="text" size=80 value="XSS Yahoo Mail" style="\000062
ackground-image:url('java\73 crip\t\3A ?
The ?input? tag can be used to call a JavaScript file.
The injected JavaScript code is responsible for:
-Automatic launching of malicious code.
-Getting personal information of users in Yahoo! address book and creating a
detailed commercial database to be used by spammers. (using the known cookies
decoder tool, created by i_n_f_o_w_a_r@xxxxxxxxxxx)
-Identity theft using a spoofed re-login window (suggested by
http-equiv@xxxxxxxxxxx).
-Read and Disclose User inbox & contacts.
-Sending an e-mail message.
The JavaScript code has been used for creating demos, but Finjan Software won't
reveal this source code.
The ActiveX control could have been used for a destructive payload of the
propagating worm. It also allows propagation to non-Hotmail users.
The basic attack does not require an ActiveX control. The ActiveX control is
the payload that can be used to extend the attack to non-web mail users, or to
perform any malicious activity, including formatting of the hard disk
Upon using the ActiveX control, end user may get a security warning. It depends
on the security setting of the browser. An example:
http://www.finjan.com/mcrc/demos/activex.cfm (Click on the 'test me' button
after reading the disclaimer)
The initial tip was received from "stardust (hoshikuzu)".
Protection:
This specific vulnerability has been eliminated by Yahoo based on Finjan
Software
notification. Finjan's content security products: SurfinGate for Web, SurfinGate
for E-mail, SurfinShield Corporate and SurfinGuard Pro, provided proactive
defense against this Hotmail vulnerability prior to its detection and
correction.
Finjan's patented behavior inspection engine will protect computer users from
similar future vulnerabilities and comparable potential exploits.
Credit: stardust (hoshikuzu), Dror Shalev and Menashe Eliezer.
Finjan Software MCRC
http://www.finjan.com/mcrc
Prevention is the best cure!