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DLL Hijacking Flaw Found in Bitdefender Antivirus Free 2020

A DLL hijacking vulnerability affecting Bitdefender Antivirus Free 2020 could have been exploited for privilege escalation and other malicious purposes, SafeBreach researchers revealed on Wednesday.

The vulnerability, tracked as CVE-2019-15295 with a CVSS score of 5.9 (medium severity), impacted Bitdefender Antivirus Free 2020 versions prior to 1.0.15.138, which patches the vulnerability. Bitdefender, which described the flaw as an untrusted search path issue, said the mitigation was delivered to affected users automatically.

According to SafeBreach, the security hole is related to the ServiceInstance.dll library, which is loaded by the BitDefender Update Service (updatesrv.exe) and the BitDefender Security Service (vsserv.exe), both of which are signed by Bitdefender and run with SYSTEM privileges. ServiceInstance.dll in turn loads a library called RestartWatchDog.dll.

The problem, SafeBreach said, was that RestartWatchDog.dll was not loaded safely and the antivirus application did not ensure that the loaded library file was signed. This allowed an attacker with access to a system running Bitdefender Antivirus Free 2020 to plant a malicious version of the library that would be executed instead of the legitimate version.

However, it’s important to note that for the attack to work a user or process with administrative privileges would first need to modify the system PATH variable to include the folder where the attacker wants to plant the malicious DLL, and to set the permissions for that directory so that non-admin users can write files to it.

Ensuring that a malicious library is loaded instead of the legitimate component can help attackers achieve various goals. For example, once the PATH modifications are made by an admin, a regular user can plant the malicious DLL, escalating its privileges to SYSTEM.

“After an attacker gains access to a computer, he might have limited privileges which can limit access to certain files and data. The service provides him with the ability to operate as NT AUTHORITY\SYSTEM which is the most powerful user in Windows, so he can access almost every file and process which belongs to the user on the computer,” SafeBreach said in a blog post.

Since the Bitdefender Security Service and the Update Service are loaded every time the computer starts, the attack can be useful as a persistence mechanism for malware, which would get executed by these services on every boot.

SafeBreach says attackers could have also exploited the vulnerability to load and execute their malicious payloads using a signed service, which can be useful for bypassing application whitelisting mechanisms.

The vulnerability was reported to Bitdefender on July 17 and a patch was rolled out roughly one month later.

Earlier this month, SafeBreach reported finding a very similar vulnerability in Trend Micro’s Password Manager.

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Eduard Kovacs (@EduardKovacs) is a contributing editor at SecurityWeek. He worked as a high school IT teacher for two years before starting a career in journalism as Softpedia’s security news reporter. Eduard holds a bachelor’s degree in industrial informatics and a master’s degree in computer techniques applied in electrical engineering.