A vulnerability (CVE-2024-9143) in OpenSSL’s binary elliptic curve cryptography APIs can allow attackers to trigger out-of-bounds memory access by supplying custom, “exotic” curve parameters. Thankfully, in most real-world usage scenarios, this bug is difficult to exploit. Still, awareness and careful use of affected APIs is recommended.

What Happened?

OpenSSL offers support for elliptic curves over binary fields (GF(2^m))—a mathematical foundation for encryption and digital signatures. Some rarely used low-level APIs let developers define explicit curve parameters rather than picking from OpenSSL’s built-in “named curves.”

The problem? If an attacker can trick an application into using a specially crafted binary field polynomial (with a zero constant term), the APIs might read from or write to unintended memory spaces. This can lead to application crashes or, theoretically, remote code execution.

Relevant APIs:

Functions prefixed with BN_GF2m_*()

CVE Record:
- NVD - CVE-2024-9143
- OpenSSL Security Advisory

Most Are Safe

The vast majority of applications use named curves, or they only allow curve definitions following strict standards (like X9.62 encoding). In these cases, problematic polynomials can’t be supplied, and there’s no vulnerability.

OpenSSL’s FIPS modules (versions 3.3, 3.2, 3.1, 3.) are not affected.

Where’s the Risk

If a third-party library or protocol you depend on uses these low-level APIs with untrusted input.

Real-World Example:
Protocols like TLS, X.509 certificates for ECC, and modern cryptographic libraries do NOT let you define such “exotic” curves. You might only be at risk if you are creating or accepting exotic ECC definitions.

Sample Vulnerable Usage

Suppose an application lets a user select arbitrary binary field polynomials and uses them via EC_GROUP_new_curve_GF2m():

#include <openssl/ec.h>
#include <openssl/bn.h>
#include <stdio.h>

void create_exotic_curve(const char *poly_hex) {
    BN_CTX *ctx = BN_CTX_new();
    BIGNUM *p = NULL;
    EC_GROUP *group = NULL;

    // Dangerous: Accept user-supplied polynomial! (for demonstration only)
    BN_hex2bn(&p, poly_hex);

    // Vulnerable function, if 'p' is crafted (e.g., zero constant term)
    group = EC_GROUP_new_curve_GF2m(p, NULL, NULL, ctx);

    if (!group) {
        printf("Failed to create curve\n");
    } else {
        printf("Curve created!\n");
        EC_GROUP_free(group);
    }

    BN_free(p);
    BN_CTX_free(ctx);
}

int main() {
    // Potentially malicious polynomial: zero constant term
    create_exotic_curve("800000000000000000000000000000000000000000000001");

    return ;
}

Impact:
If a user (or attacker) supplies a specifically crafted polynomial (e.g., with a zero constant term), the underlying OpenSSL code may read/write outside the bounds of its internal arrays.

To exploit, you’d have to

- Find an application that lets you configure explicit curve parameters for GF(2^m) elliptic curves, AND

Get your input *processed* (not just validated).

Most protocols, standards, and popular libraries use named curves and validated encodings, so actual exploitation opportunities are rare.

Use named curves whenever possible.

2. Never accept untrusted explicit curve parameters unless you trust their source and validate them.

Update OpenSSL as soon as a patch is available, especially if you suspect use of exotic curves.

4. Audit third-party code/libraries that use OpenSSL’s low-level ECC APIs with custom parameters.

- NVD - CVE-2024-9143
- OpenSSL Security Advisory (Mar 2024)
- RFC 3279 - X9.62 ECC encoding
- OpenSSL Documentation: EC_GROUP_new_curve_GF2m()

In Summary

CVE-2024-9143 is a genuine vulnerability in OpenSSL’s handling of exotic binary curve parameters with custom field polynomials. Most applications and users are NOT affected, as standards only allow “named curves” or safe encodings. If you write crypto code that works with raw curve parameters, check your usage today—otherwise, you’re probably safe but should keep your OpenSSL installation patched and up to date.

Stay safe, audit code, and use cryptographic primitives wisely!

Timeline

Published on: 10/16/2024 17:15:18 UTC
Last modified on: 11/08/2024 16:35:21 UTC