[Full-Disclosure] Finjan Software Discovers a New Critical Vulnerability In Yahoo E-mail Service

From: Menashe Eliezer (menashe_at_finjan.com)
Date: 12/14/03

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    To: <full-disclosure@lists.netsys.com>
    Date: Sun, 14 Dec 2003 15:18:24 +0200

    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.


    Yahoo,Excite and Outblaze (Mail.com) have already patched their Web-based e-mail services. Other web-based e-mail systems may be vulnerable.



    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@hotmail.com)

    -Identity theft using a spoofed re-login window (suggested by http-equiv@malware.com).

    -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-Yahoo 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)".



    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 Yahoo 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


    Prevention is the best cure!


    Full-Disclosure - We believe in it.
    Charter: http://lists.netsys.com/full-disclosure-charter.html

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