HDCP-wned: Breaking Samsung's TrustZone via Digital Rights Management

Welcome back to the world of ARM TrustZone, where manufacturers promise us a "Secure World" that is mathematically isolated from the chaotic, malware-ridden "Non-Secure World" (Android). In theory, this is a fortress. The Trusted Execution Environment (TEE) holds the keys to the kingdom—fingerprint data, encryption keys, and payment tokens. It runs tiny, specialized programs called Trustlets (Trusted Applications) to handle these secrets.

But here is the irony: the more complexity you shove into a secure enclave, the larger the attack surface becomes. Enter HDCP (High-bandwidth Digital Content Protection). Its job is to ensure you aren't pirating 4K streams over HDMI. To do this, it needs deep access to hardware and memory.

CVE-2025-20936 is a classic tale of "who watches the watchmen?" The HDCP trustlet, a privileged component inside the Samsung TEE, forgot to check who was asking it to perform sensitive operations. It turns out, if you just ask nicely (or maliciously), it will oblige, regardless of whether you are the kernel or a low-level shell user.

So, where did the developers go wrong? The vulnerability is classified as CWE-285: Improper Authorization. In the context of a TEE, this is usually fatal. Trustlets communicate with the Android OS via a specific driver (like /dev/qseecom on Qualcomm chipsets or /dev/tzic on Exynos). When Android sends a command buffer to the Trustlet, the Trustlet must validate the origin and the context of that command.

The flaw in CVE-2025-20936 is a missing permission check on specific command handlers. The HDCP Trustlet exposes a set of functions (Command IDs). Some of these are benign, like "Get Version." Others are dangerous, like "Write to Protected Buffer" or "Map Physical Memory."

Samsung's implementation essentially treated all incoming commands as equal. It didn't enforce a check to see if the caller had the requisite privileges to invoke the critical functions. This is the digital equivalent of a bank vault that opens for anyone wearing a uniform, without checking if they actually work there.

Because Samsung TEE code is proprietary and closed-source, we don't have a public Git commit to point at. However, based on the vulnerability class and previous exploits like CVE-2023-30679, we can reconstruct the logic failure with high accuracy. Below is a pseudo-code representation of what the vulnerable command handler likely looked like versus the patched version.

The Vulnerable Logic:

// Inside the HDCP Trustlet (Secure World)
void command_handler(void* cmd_buffer, uint32_t cmd_len) {
    struct hdcp_cmd *cmd = (struct hdcp_cmd *)cmd_buffer;
    
    switch (cmd->id) {
        case CMD_INIT_SESSION:
            // Benign
            initialize_session();
            break;
            
        case CMD_DBG_WRITE_MEM: 
            // CRITICAL: No check on who is calling this!
            // This allows writing to arbitrary secure memory ranges
            write_secure_memory(cmd->addr, cmd->val);
            break;
            
        default:
            return ERR_UNKNOWN_CMD;
    }
}

The Patched Logic:

// The Fix: Validate the caller or disable debug commands in production
void command_handler(void* cmd_buffer, uint32_t cmd_len) {
    struct hdcp_cmd *cmd = (struct hdcp_cmd *)cmd_buffer;
 
    switch (cmd->id) {
        case CMD_INIT_SESSION:
            initialize_session();
            break;
            
        case CMD_DBG_WRITE_MEM:
            // FIX: Explicit authorization check
            if (!is_caller_privileged() || !is_debug_device()) {
                 return ERR_ACCESS_DENIED;
            }
            write_secure_memory(cmd->addr, cmd->val);
            break;
    }
}

The patch likely involves either removing the dangerous command handlers entirely from production builds or adding a strict check (is_caller_privileged) to ensure only the kernel or specific system daemons can invoke them.

How do we weaponize this? The path from shell to root via a TEE vulnerability is complex but standardized. Since the vulnerability allows us to interact with the Trustlet without proper checks, we can abuse the functionality to corrupt kernel memory.

Step 1: The Setup First, we gain initial access. This could be via ADB (developer mode) or by exploiting a vulnerable app (like a browser) to get a shell user (UID 2000). We then open a handle to the TEE driver. On Samsung devices, this is often /dev/qseecom.

Step 2: The Trigger We craft a specific QSEECom_send_cmd struct containing the ID of the vulnerable command (e.g., the hypothetical CMD_DBG_WRITE_MEM mentioned above). We pass this to the kernel driver via ioctl.

Step 3: The Corruption The Trustlet receives our command. Because of the missing check, it executes the memory write. But here is the trick: The Trustlet often has access to physical memory that the Android Kernel uses. If we can instruct the Trustlet to overwrite a specific address in kernel memory—say, the credentials structure of our current process—we win.

Step 4: The Escalation We target the kernel's struct cred for our process. We instruct the HDCP Trustlet to overwrite our UID and GID with 0 (root). When the ioctl returns, we check id in our shell. Suddenly, we are root. We have bypassed all kernel security mechanisms (SELinux, KASLR) by attacking from below the kernel.

This isn't just about getting root on a phone to install a custom theme. A TEE vulnerability is a "Scope Changed" (S:C) event in CVSS terms. It means a compromise in one component (the Trustlet) impacts a completely different security scope (the Android Kernel and User Data).

With root access gained via TEE:

1. Data Exfiltration: Attackers can bypass file permissions to steal photos, messages, and app data.
2. Persistence: They can install rootkits that survive factory resets (if they can write to the system partition or persist in the TEE storage).
3. DRM Bypass: Ironically, since this is the HDCP trustlet, an attacker could likely dump the DRM keys used to decrypt 4K content, rendering the protection useless.

This vulnerability affects a wide range of Samsung devices running Android 13, 14, and 15, making it a high-value target for spyware vendors and sophisticated actors.

Samsung released a fix in the SMR April-2025 Release 1. The patch involves updating the TrustZone firmware (specifically the HDCP Trustlet binary).

Unlike standard Android app updates, TEE updates are handled by the bootloader/firmware update process. You cannot patch this by just updating an app from the Play Store. You must install the full OTA (Over-The-Air) system update.

Verification: After updating, verify your patch level:

1. Go to Settings > About Phone > Software Information.
2. Ensure Android Security Patch Level says April 1, 2025 or later.

For enterprise fleet managers, enforce a compliance policy that blocks access to corporate resources for devices with a security patch level older than April 2025.