Title: 04 March 2002 Cumulative VM Update Released: 04 March 2002 Revised: 18 March 2002 (version 2.0) Software: Microsoft Virtual Machine Impact: Information Disclosure, run code of an attacker's choice Max Risk: Critical
Microsoft encourages customers to review the Security Bulletin at: http://www.microsoft.com/technet/security/bulletin/MS02-013.asp.
On March 4, 2002, Microsoft released the first version of this bulletin. On March 18, 2002, Microsoft re-released this bulletin to make customers aware of an additional vulnerability that is eliminated by the updated VM (Microsoft VM build 3805). Customers who have previously installed the new build do not need to take any additional action.
The Microsoft VM is a virtual machine for the Win32(r) operating environment. The Microsoft VM is available for Windows 95, Windows 98, ME, Windows NT(r) 4.0, Windows 2000, and Windows XP. It is also available as part of Internet Explorer 6 and earlier.
A new build of the VM (build 3805) is available, which eliminates two security vulnerabilities. The first vulnerability is the result of a flaw affecting how Java requests for proxy resources are handled. A malicious Java applet could exploit this flaw to re-direct web traffic once it has left the proxy server to a destination of the attacker's choice.
An attacker could use this flaw to send a user's Internet session to a system of his own control, without the user being aware of this. The attacker could then forward the information on to the intended destination, giving the appearance that the session was behaving normally. The attacker could then send his own malicious response, making it seem to come from the intended destination, or could discard the session information, creating the impression of a denial of service. Additionally, the attacker could capture and save the user's session information. This could enable him to execute a replay attack or to search for sensitive information such as user names or passwords.
A system is only vulnerable if IE is used in conjunction with a proxy server. Users whose browsers are not behind a proxy server are not vulnerable to this vulnerability. However, those users would be vulnerable if they changed their browser to use a proxy server at a later date.
The second vulnerability is a new discovered variant of the "Virtual Machine Verifier" issue first discussed in MS99-045. Like most programming languages, the Java language provides the means to convert types by means of casting operations. Most commonly, these are used to convert data types, although other more complex type conversion is possible. A flaw exists in the security checks on casting operations within the Microsoft VM. A vulnerability results because it is possible for an attacker to exploit this flaw and use it to execute code outside of the sandbox. This code would execute as in the context of the user, and would only be limited by those constraints which govern the user herself.
The flaw only affects Java applets, it does not affect Java applications. To mount a successful attack, the malicious user would have to specially craft a Java applet at the binary level, post it on his site, and entice the intended target to visit his site.
HTTP Proxy Redirection Vulnerability:
The vulnerability only affects configurations that utilize a proxy server. Customers who are not using a proxy server are not at risk from this vulnerability.
Best practices strongly recommend using SSL to encrypt sensitive information such as user names, passwords and credit card numbers. If this has been done, sensitive information will be protected from examination and disclosure by an attacker exploiting this vulnerability.
Virtual Machine Verifier Variant:
The vulnerability only affects Java applets, not Java applications.
Exploiting the vulnerability requires detailed specific knowledge and skills.
An attacker must lure an intended target to a site under his control where he has posted the malicious applet.
Java applets and other mobile code can be blocked at the firewall perimeter through the use of application filtering software.
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