CVE-2021-26392: Exploring Insufficient Verification and Missing Size Check in 'LoadModule' Leading to Out-of-Bounds Write and Potential Code Execution

Recently, a new security vulnerability, identified as CVE-2021-26392, has been reported. This vulnerability involves insufficient verification of a missing size check in the 'LoadModule' function, which may lead to an out-of-bounds write, potentially allowing an attacker with privileges to gain code execution of the operating system or kernel by loading a malicious TA (Trusted Application).

In this long-read post, we will take a deep dive into this security flaw, discussing its technicalities, potential exploits, and providing code snippets and links to help you understand and protect yourself against this vulnerability.

Understanding the Vulnerability: Insufficient Verification and Missing Size Check in 'LoadModule'

A crucial part of software security is the proper validation and verification of data. However, in the case of CVE-2021-26392, the 'LoadModule' function fails to sufficiently verify the size of the input data, which could be exploited by an attacker to overwrite portions of the memory.

Here's a code snippet illustrating the insufficiency found in the 'LoadModule' function

int LoadModule(char* buffer, size_t bufferSize) {
  const size_t expectedSize = 256;
  if (bufferSize < expectedSize) {
    return -1;  // Fail: buffer size is too small.
  }
  
  memcpy(destBuffer, buffer, bufferSize);
  ...
}

In this example, bufferSize should match the expectedSize, but the function only checks if the bufferSize is smaller than the expectedSize. There is no check if the bufferSize is larger, resulting in an out-of-bounds write when using memcpy.

Exploiting the Vulnerability: Gaining Code Execution

An attacker with privileges could exploit this vulnerability by loading a malicious TA, specially crafted to contain a larger bufferSize than the expectedSize. This crafted TA would cause an out-of-bounds write and overwrite adjacent memory, altering the software's behavior and potentially allowing the attacker to execute arbitrary code.

Here's an example of a malicious TA that could exploit the vulnerability

char maliciousBuffer[320];
...
// Fill the maliciousBuffer with crafted data, including shellcode for code execution.
...
LoadModule(maliciousBuffer, sizeof(maliciousBuffer));

In this example, the malicious TA has a bufferSize larger than the expectedSize, triggering the out-of-bounds write and potentially allowing the attacker to execute arbitrary code.

To learn more about CVE-2021-26392, you can refer to the following authoritative resources

1. CVE-2021-26392 - NIST National Vulnerability Database
2. Insufficient Verification of Data - CWE-345
3. LoadModule Vulnerability: Understanding and Mitigating Out-of-Bounds Write

To protect your systems against CVE-2021-26392, consider the following steps

1. Update your software to the latest versions, which may include patches and mitigations for known vulnerabilities.
2. Review your code and perform proper input validation and size checks in any function that handles data.
3. Restrict access to sensitive functions such as 'LoadModule', only allowing trusted users and applications to execute them.
4. Regularly monitor and audit your systems for signs of compromise and respond promptly to any security incidents.

Conclusion

CVE-2021-26392 is a clear reminder of the importance of proper verification and validation of data in your software. By following the steps above and staying up to date with the latest security advisories, you can minimize the risk of this and other vulnerabilities affecting your systems.

Timeline

Published on: 11/09/2022 21:15:00 UTC
Last modified on: 11/23/2022 14:00:00 UTC