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Vulnerability Spotlight: Understanding CVE-2023-21608 PDF Reader Flaw
2/2/2024 - Brian O'Neill

In the final quarter of 2023, a use-after-free vulnerability was identified in (and before) several versions of Adobe Acrobat Reader. Without involving viruses or malware whatsoever, successful exploitation of this type of vulnerability could have very severe consequences.

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This vulnerability could, for one example, lead to arbitrary code execution in the context of the victim (current application user loading a PDF in Reader). If that victim had escalated privileges within the target application, the attacker could end up with access to large volumes of sensitive data.

Let’s take a closer look at what a “use-after-free” vulnerability really means, and let's understand how we can mitigate threatening content that might be used to exploit zero-day use-after-free vulnerabilities.

What is a use-after-free vulnerability?

To manage memory efficiently, computer programs usually release (deallocate) memory which was first dynamically allocated during the program’s runtime. Many relevant programming languages in this context use a function (such as free() or delete) to release memory, and once that release occurs, the computer program effectively “assumes” it won’t be accessing (or “pointing” to) that memory block again.

When there’s a use-after-free vulnerability in an application, it means there’s a flaw making it possible to force the program to continue “pointing” to a memory block that was already deallocated from use. Once the program starts trying to access deallocated memory, unpredictable things can happen. Attackers can take advantage of this flaw to execute arbitrary code and possibly change the program’s behavior entirely.

How could a threat actor exploit a use-after-free vulnerability like CVE-2023-21608?

To exploit a use-after-free vulnerability in a PDF reader application, a threat actor would start by crafting a malicious PDF file. This file would likely contain a specific set of characteristics (for example: special objects, structures, or scripts/instructions) designed to exploit insecure memory deallocation functions. The attacker would then disguise and possibly obfuscate this file’s content so it appeared as safe & innocuous as possible (likely in addition to leveraging social engineering techniques) before sharing it with a potential victim using a vulnerable version of the PDF reader application.

Once the victim opened the malicious PDF in the vulnerable PDF reader application, the PDF’s special elements could force the application to “point” to deallocated memory. The attacker could then take control of this freed memory execute arbitrary code within the vulnerable program. If the attacker chose their victim wisely, they could have escalated privileges within the vulnerable application and potentially take complete control of it.

If this all sounds a little too easy, we’re on the right track: once a sophisticated threat actor is aware of a use-after-free vulnerability in a particular application, they’ve already done the hardest work. Disguising and/or obfuscating malicious files & employing effective social engineering techniques to exploit targeted victims are often trivial tasks for experienced attackers, largely because no virus or malware signatures are involved. Traditional antivirus software very likely won’t find anything wrong with the malicious PDF, and the target user might assume the file was safe if it appeared to pass a standard antivirus scan.

How can we mitigate the exploitation of use-after-free vulnerabilities?

First and foremost, it’s critical that we update all our applications regularly. Security teams, researchers and bug bounty hunters are constantly at work trying to identify critical vulnerabilities in popular applications, and once those vulnerabilities are identified, new software versions are hastily released with relevant fixes.

To avoid becoming the subject of a zero-day use-after-free vulnerability exploit, however, we need to focus on rigorously verifying the content processed by our sensitive applications. Use-after-free vulnerability exploits might be driven by insecure coding practices, but they only result in outcomes like arbitrary code execution when malicious content isn’t properly validated before passing through program functions. Unintentional (non-malicious) use-after-free exploits can occur because of improperly validated content too, potentially leading to other unwanted outcomes such as Denial-of-Service.

Verifying content with the Cloudmersive Virus Scan API

The advanced version of the Cloudmersive Virus Scan API performs in-depth content verification checks on files & identifies the types of content threat actors use to exploit vulnerabilities like use-after-free. It can be configured to disallow content containing scripts, executables, invalid file content (relative to the file extension), JavaScript, HTML, and more.

In the context of protecting a potentially vulnerable PDF reader application, API request parameters could be customized to flag PDFs containing any content that did not fully conform to PDF standards. This means scripts, executables, and HTML/JavaScript objects would be called out before the file could reach the target application.

For more information on the Cloudmersive Virus Scan API, please feel free to reach out to a member of our team.

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