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DLL Hijacking Vulnerability Found in Realtek HD Audio Driver

A vulnerability in the Realtek HD Audio Driver package could be abused to execute arbitrary payloads with elevated privileges on a vulnerable machine, SafeBreach Labs has discovered.

Tracked as CVE-2019-19705, the vulnerability could be leveraged to evade defenses and achieve persistence by loading an arbitrary, unsigned DLL into a signed process.

The Realtek HD Audio Driver package is present on all Windows machines that feature a Realtek sound card, rendering all of them vulnerable to attacks. Exploitation, however, requires the attacker to have administrator privileges.

While analyzing the RAVBg64.exe process, which essentially is the background process of the Realtek driver, SafeBreach’s security researchers discovered that it runs with System privileges, but that it attempts to load a couple of specific DLLs in an unsafe manner.

The issue, the researchers explain, is not only that the process attempts to load DLLs that are not present at the expected location, but it also executes these, although they are supposed to be data-only DLLs.

RAVBg64.exe is an MFC application, meaning that its developers can load resource-only DLLs. However, instead of loading said DLLs with a specific flag, it loads them as regular DLLs, which results in their execution.

There are two root causes of the problem, SafeBreach says, the first being that Realtek used Visual Studio 2005 to compile the binary, which resulted in the implementation of the wrong behavior in the MFC application.

Second, no digital signature validation was being performed on the loaded DLLs, thus allowing for the loading of unsigned, arbitrary libraries.

An attacker targeting the vulnerability could load and execute malicious payloads within the context of the Realtek signed process. This allows them to evade defenses and achieve persistence.

The vulnerability was reported to the vendor in July last year and was addressed with the release of Realtek HD Audio Driver package version in December.

Realtek published an advisory (DOCX) detailing this vulnerability only in January, confirming the potential to load unexepected code into the process.

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