Virus writers are actively exploiting a security vulnerability in the Microsoft Jet Database Engine that remains unpatched more than five months after it was first reported to the software giant.
The mail-borne exploit, which camouflages itself as a Microsoft Access file, infects Windows machines through a "highly critical" flaw in the Microsoft Jet database engine—the lightweight database widely used by applications such as Microsoft Office 2000, Office 2003, Access 2000 and Access 2003.
The vulnerability—along with proof-of-concept exploits—was first reported to Microsoft in March along with a warning that it could be used by malicious hackers to take complete control of a victims computer.
Microsoft has never publicly acknowledged the existence of the bug, which affects fully patched systems with Microsoft Access 2003 and Microsoft Windows XP, including Service Pack 2.
However, according to an advisory from Symantec Corp.s security response unit, the unpatched hole is being exploited to drop a malicious Trojan horse identified as "Backdoor.Hesive."
The exploit has been discovered in the wild. Symantec rates the distribution of the Trojan as "low" but warned that the potential for damage is significant.
At immediate risk at users of Windows 95, Windows 98, Windows ME, Windows NT, Windows 2000, Windows XP and Windows Server 2003.
The Trojan opens a back door on the compromised computer to allow a remote attacker unauthorized access. These types of malware are typically used to seed botnets for use as spam zombies and to launch spyware installation attacks.
A botnet is a collection of computers that have been hijacked by malicious hackers and controlled remotely via IRC (Inter Relay Chat) channels. Virus hunters have long warned the sophisticated hacking techniques are being used by botnet owners to maintain control of zombie drones and the latest exploit of an unpatched Windows flaw comes as no surprise.
The immediate fear is that a major spam attack could be launched with tainted Microsoft Access database e-mail attachments. Since those file types are mostly considered safe, most anti-virus engines may allow the virus through mail servers.
Officials at the Redmond, Wash-based software maker did not respond to queries at press time.
The Microsoft Jet Engine DB flaw, which was reported to Microsoft back in May, is described as a memory handling error when parsing database files. This can be exploited to execute arbitrary code by tricking a user into opening a specially crafted ".mdb" file in Microsoft Access.
The problem exists in "msjet40.dll," the main component of the Microsoft Jet Database Engine, which evaluates and carries out requests for data.
An advisory from HexView Security Research and Assessment, said instances where file data was not properly validated could cause system crashes, null pointer memory access conditions, and arbitrary code execution.
"Sufficient data validation is not performed when msjet40.dll parses the database file. As a result, it is possible to modify database file to cause a code of attackers choice to be launched when MS Jet database is opened," the company warned in an alert.
Only software products that utilize "msjet40.dll" are affected. HexView said Microsoft was notified of the vulnerability on March 30, 2005, but an acknowledgement only came in the form of an automated reply. "No human response was received," according to HexView.
In the absence of a patch from Microsoft, Secunia recommends that users avoid opening untrusted ".mdb" database files.
Symantec has posted removal instructions for "Backdoor.Hesive." Windows users are always advised to be wary of e-mail attachments, regardless of the file type.