A critical security risk has been identified in OpenSSL, coded as CVE-2023-3817, which may allow an attacker to cause a significant delay—potentially leading to a Denial of Service (DoS)—when applications perform insecure checks on Diffie-Hellman (DH) parameters coming from untrusted sources.

OpenSSL’s DH parameter check routines, including DH_check(), DH_check_ex(), and EVP_PKEY_param_check(), can become extremely slow when handling certain crafted, excessively large DH parameter values. This issue arises when a parameter called q is significantly larger than it should be, causing expensive number-theoretical computations that burn CPU cycles.

What's the Problem?

When an application (yours, or anyone else's that uses OpenSSL) receives DH parameters—say, from a network client, file, or other source—it might check them like this:

#include <openssl/dh.h>

int is_valid_dh(const DH *dh) {
    int codes = ;
    if(!DH_check(dh, &codes)) {
        return ;
    }
    return codes == ;
}

If malicious or badly formed DH parameters are supplied where q is excessively large (bigger than the modulus p), the checking logic inside DH_check() does far more work than needed. This can slow the function down drastically, freezing your app for seconds, minutes, or even longer—depending on system resources and how “evil” the parameters are.

Affected

- Apps using DH_check(), DH_check_ex(), or EVP_PKEY_param_check() on DH material from an untrusted source

Not Affected

- OpenSSL’s SSL/TLS protocol code (doesn’t use these checks on traffic)

Exploit Details

Let’s visualize an exploit: An attacker sends a crafted public key, DH parameter file, or some other input, containing an enormous q value (e.g., 1+ gigabyte in size—or in practice, whatever makes the server choke). There’s no need for the attacker to break or guess keys—the check itself is the target.

An adversary could hand you a PEM-encoded DH parameter with a monster q parameter

-----BEGIN DH PARAMETERS-----
MIIBCAKCAQEA7yCApTmO... (lots of junk, q very large)
-----END DH PARAMETERS-----

Handling this parameter with dhparam -check or in custom C code like this

DH *dh = PEM_read_bio_DHparams(bio, NULL, NULL, NULL);
int valid = DH_check(dh, &codes); // Here your app could freeze for minutes!

Results: Server or app locks up or throws an unresponsive-CPU moment. If your program is running as a service, attacker can keep hitting you on a loop: your server might quit responding, wasting system resources.

Why Does This Happen?

* Root Cause: The check routines fell into costly computation when q was too big, even though a basic comparison found in the OpenSSL fix can short-circuit the whole thing: if q > p, fail fast and return.

* Fixed in: All supported OpenSSL branches as of
 - OpenSSL 3.1.2 (Release note)
 - OpenSSL 3..10 (Release note)
 - OpenSSL 1.1.1v (Release note)

Ask yourself

- Do you accept DH keys or parameters from users/network/files you don’t fully trust?

Are you running a version of OpenSSL prior to 1.1.1v, 3..10, or 3.1.2?

If yes, you’re at risk. Even if the check eventually fails, your service could be bogged down until it recovers.

First, update OpenSSL NOW to a fixed version

- Latest releases: openssl.org/source

2. Basic Validations

If you must support older OpenSSL or want defense-in-depth, add length checks before calling the DH check routines:

if (q && BN_cmp(q, p) > ) {
    // Reject parameter: q is too big!
}
else {
    DH_check(dh, &codes);
}

3. Limit Input Size

Reject any crypto input from external sources with absurdly large sizes. Never accept kilometer-long DH params!

Real-World Impact

This CVE is about resource exhaustion, not direct data compromise. If your server or application quietly freezes on a bad parameter, you may be susceptible to easy DoS. Web services, APIs, VPN tools, chat apps, cryptographic gateways that “verify before use” can all be targets.

If an attacker can control the input and your app performs parameter validation, you are at risk of unintentional self-DDoS.

- OpenSSL Security Advisory: 2023-08-01
- CVE Entry: CVE-2023-3817 at Mitre
- OpenSSL Source Repository - CVE-2023-3817 patch

Conclusion

CVE-2023-3817 is a “silent killer” style DoS vector that affects anyone running parameter checks on untrusted DH crypto data. It’s easy to overlook and can be abused with a single malicious input. Make sure you’re running an updated OpenSSL, and always add simple size guards anytime you handle cryptographic inputs.

If your servers need to keep running smoothly, patch and protect yourself today!

Timeline

Published on: 07/31/2023 16:15:00 UTC
Last modified on: 08/18/2023 14:15:00 UTC