CVE-2025-65493: Libcoap Vulnerability Exposes Systems to Denial of Service

Overview

CVE-2025-65493 describes a NULL pointer dereference vulnerability found in OISM libcoap version 4.3.5. This flaw resides in the src/coap_openssl.c file and can be exploited by remote attackers. By sending a specially crafted DTLS/TLS connection request, an attacker can trigger the BIO_get_data() function to return NULL. This results in a NULL pointer dereference, ultimately leading to a denial-of-service (DoS) condition.

Technical Details

The vulnerability stems from improper handling of the return value of BIO_get_data() within the coap_openssl.c file. The code doesn’t adequately check if BIO_get_data() returns NULL before attempting to dereference the pointer. A malicious actor can leverage this by crafting a DTLS/TLS handshake that causes BIO_get_data() to return a NULL value. Subsequent attempts to use this NULL pointer will cause the application to crash, resulting in a denial of service.

Here’s a simplified illustration of the vulnerable code pattern (this is for demonstration and might not be the exact code):

            
            data = BIO_get_data(bio);
            // Missing NULL check here
            use_data(data); // Potential NULL pointer dereference
            
        

CVSS Analysis

As of the publication of this article, a CVSS score has not been assigned to CVE-2025-65493. However, given the potential for remote denial of service, a moderate to high severity score is anticipated. The ease of exploitation via network communication significantly increases the risk associated with this vulnerability.

Possible Impact

The primary impact of CVE-2025-65493 is a denial-of-service condition. Successful exploitation can lead to:

• Disruption of services relying on the affected libcoap instance.
• Application crashes and system instability.
• Potential for further exploitation if the denial of service is used as a precursor to more sophisticated attacks.

Mitigation or Patch Steps

The recommended mitigation is to upgrade to a patched version of libcoap that addresses this vulnerability. A patch has been submitted and is available for review.

1. **Upgrade libcoap:** Update to the latest version of libcoap containing the fix. Check the project’s website and release notes for the patched version number.
2. **Apply the Patch:** If an immediate upgrade is not possible, apply the patch available at https://github.com/obgm/libcoap/pull/1750.
3. **Monitor for Exploitation:** Implement intrusion detection systems (IDS) and intrusion prevention systems (IPS) to monitor network traffic for attempts to exploit this vulnerability.

References

• CVE: CVE-2025-65493 (link only valid domain if entry created)
• GitHub Issue: https://github.com/obgm/libcoap/issues/1743
• GitHub Pull Request (Patch): https://github.com/obgm/libcoap/pull/1750