In September 2022, a security vulnerability tracked as CVE-2022-39830 was discovered in the Samsung mTower software, up to version .3.. This flaw arises from a missing return value check on a critical cryptographic operation in the sign_pFwInfo function. Attackers can leverage this oversight to crash the application, causing a Denial of Service (DoS). This post breaks down the bug in simple terms, shows you vulnerable code, and explains how it can be exploited.

What Is the Vulnerability?

The function sign_pFwInfo in Samsung mTower is responsible for signing information, likely related to firmware updates or device authentication. It uses Elliptic Curve Cryptography (ECC) and, specifically, calls the following OpenSSL function when loading a public key:

int EC_KEY_set_public_key_affine_coordinates(
    EC_KEY *key, const BIGNUM *x, const BIGNUM *y);

But here's the problem: In mTower (versions ≤.3.), the return value of this function is not checked. The function can fail for many reasons (invalid coordinates, allocation errors, etc.). If it fails and the error is not handled, subsequent code might run with an invalid key, triggering a crash or other unexpected behavior.

Here's a simplified version of the vulnerable C source logic

EC_KEY *ec_key = EC_KEY_new_by_curve_name(NID_X9_62_prime256v1);
/* x_val and y_val are attacker-controlled */
EC_KEY_set_public_key_affine_coordinates(ec_key, x_val, y_val);

/* ... code continues here, assuming ec_key is valid ... */

What's missing?
The call to EC_KEY_set_public_key_affine_coordinates returns 1 on success, on failure. The program should always check for success, like this:

if (EC_KEY_set_public_key_affine_coordinates(ec_key, x_val, y_val) != 1) {
    /* Handle error: Possibly free ec_key and exit */
    fprintf(stderr, "Invalid public key coordinates!\n");
    EC_KEY_free(ec_key);
    return ERR_INVALID_KEY;
}

Exploit Details: How Could This Be Used?

Because there's no return check, an attacker can supply invalid or specially-crafted x_val and y_val values. If the library fails to set the key but the program continues, subsequent operations on an invalid ec_key can cause a crash — effectively taking down the service.

Steps to Exploit

1. Find a service endpoint that wraps or calls the sign_pFwInfo function, exposing x/y parameters (for instance, a firmware signature check API).

Supply malformed or deliberately invalid elliptical curve coordinate values.

3. Force the vulnerable code path — when the invalid key is used, OpenSSL can fail or assert internally, leading to a process crash (segfault or abort).

This is a Denial of Service attack: There's no data leak or code execution, but it's still a significant disruption.

Impacted Software: Samsung mTower ≤.3.

- Fixed In: (Check Samsung Security Updates)
According to the CVE entry, this was patched after responsible disclosure.

References and Further Reading

- CVE-2022-39830 entry
- Samsung mTower Info (archived)
- OpenSSL EC_KEY_set_public_key_affine_coordinates Documentation
- OSS-Fuzz/Google Project Zero: Common C Crypto Mistakes

How to Mitigate

- Update: If you use Samsung mTower (especially embedded in devices or infrastructure), update to the latest available version.

For OpenSSL and ECC, failures can indicate bad user data, library errors, or outright attacks.

- Harden Public-Facing APIs: Validate all user inputs. Never process cryptographic materials without thorough input checks.

Conclusion

CVE-2022-39830 shows how a simple coding oversight — not checking a return value — can lead to dangerous vulnerabilities. In security-critical code, always assume things can go wrong, and handle all possible errors. If you use Samsung mTower, patch immediately, and if you're a developer, double-check your crypto code!


Got comments or need help analyzing your own code for similar bugs? Feel free to reply or reach out for more resources on secure C/C++ coding!

Timeline

Published on: 09/05/2022 04:15:00 UTC
Last modified on: 09/08/2022 03:49:00 UTC