Digital Rights Management (DRM) is built to protect video streams from piracy, but sometimes the armor itself has cracks. In 2021, a flaw tracked as CVE-2021-46851 was discovered in a DRM module—specifically in how it checks secure memory attributes. If an attacker understands this bug, they could cause your videos to glitch, freeze, or refuse to play altogether.

Let’s break down how this vulnerability works, see a bit of the underlying code, and explore how someone might exploit it.

What is CVE-2021-46851?

This vulnerability lurks inside a DRM module (exact DRM stack or vendor was not clearly named in the advisory). The bug comes from improper verification of secure memory attributes when handling protected content. As a result, an attacker with the right knowledge can trick the DRM module and make your videos play out of sync, scrambled, or not at all.

Attack Vector: Malicious app or crafted payload—local attack

Original reference:  
- NVD page for CVE-2021-46851
- Vendor security notice (example placeholder)

How Does This Bug Happen?

DRM modules often use a chunk of “secure memory” (like Trusted Execution Environment/TZ or TEE memory) to store video keys and decrypted content. Before using this memory, the DRM module is supposed to check if the memory region is *really* protected.

But in the case of CVE-2021-46851, this check is too relaxed. Let’s look at a simplified version of the code:

Vulnerable Code Example

int drm_check_secure_memory(void *ptr) {
    // Intended to check if memory is within secure region
    if (ptr != NULL) {
        // Only checks for NULL, does NOT check secure attribute
        return 1; // Considered valid!
    }
    return ;
}

What’s Wrong?

Instead of checking if ptr points to memory marked “secure”, the function only checks if it’s non-NULL. Any pointer (even to normal memory) passes the check.

A better check might look like this

int drm_check_secure_memory(void *ptr) {
    if (ptr_is_in_secure_region(ptr)) {
        return 1; // Secure memory
    }
    return ; // Not secure
}

How Can It Be Exploited?

A malicious app, or a local process, could supply a pointer to regular system memory. The DRM module will mistakenly treat it as secure and process DRM-protected data there. This can lead to several issues:

- Abnormal Video Playback: Decryption might fail, causing the video to appear corrupted, not play at all, or crash the player.

DoS (Denial of Service): If the DRM module writes or reads from invalid memory, it could crash.

- Potential Data Leakage: If decrypted video is written to non-secure memory, a malicious app could read and dump it.

Suppose a video player calls into the DRM module like this

void *video_buffer = malloc(x200); // Not secure memory!
if (drm_check_secure_memory(video_buffer)) {
    // DRM thinks this is secure, processes the buffer
    decrypt_and_play(video_buffer);
}

An attacker could patch a media player or craft an exploit to trigger the bug, causing the module to process untrusted or normal memory.

"Playback error: DRM content decryption failed."

In the worst case, decrypted frames end up where they don’t belong—which could expose pirated content.

Memory Checks: Developers should always validate memory attributes with trusted OS or TEE APIs.

- App Sandboxing: Limit what untrusted apps can do; don’t let them pass pointers directly to secure modules.

Want To Learn More?

- NIST NVD Details for CVE-2021-46851
- How TrustZone Secure World Works (ARM)
- How DRM Modules Use Secure Memory (Pattern Blog)
- Secure Programming for DRM (OWASP)

Conclusion

CVE-2021-46851 shows how a simple memory check—done carelessly—can have big consequences for video playback and user trust. If you’re running a device or app that plays protected content, check for updates and stay safe.


*This post is exclusive and was written in plain language to help everyone understand DRM and memory vulnerabilities. Be safe, patch often, and keep videos running smooth!*

Timeline

Published on: 11/09/2022 21:15:00 UTC
Last modified on: 11/10/2022 19:26:00 UTC