CVE-2025-65497: Critical DTLS Vulnerability in Libcoap Exposes Systems to Denial of Service

Overview

CVE-2025-65497 is a security vulnerability affecting libcoap version 4.3.5, a popular library for implementing the Constrained Application Protocol (CoAP). This vulnerability stems from a NULL pointer dereference within the coap_dtls_generate_cookie() function, potentially leading to a Denial of Service (DoS) attack. An attacker can exploit this by sending a specially crafted DTLS handshake, causing SSL_get_SSL_CTX() to return NULL and triggering the dereference.

Technical Details

The vulnerability resides in src/coap_openssl.c within the coap_dtls_generate_cookie() function. Specifically, the function fails to properly handle a NULL return value from SSL_get_SSL_CTX() during DTLS handshake processing. This leads to a NULL pointer dereference when the code attempts to access members of the non-existent context, causing the application to crash and potentially disrupting service availability.

      // Example vulnerable code snippet (Illustrative - see referenced PR for precise fix)
      SSL_CTX *ctx = SSL_get_SSL_CTX(ssl);
      if (ctx) {
        // Vulnerable code: ctx could be NULL, but used without check
        //const EVP_MD *md = EVP_get_digestbyname("SHA256");
        //... use md from ctx
      } else {
        // Handle the error appropriately to avoid dereference.
      }
    

CVSS Analysis

As of the publication date (2025-11-24), a CVSS score is not yet available (N/A). However, given the potential for a remote attacker to trigger a Denial of Service, it is likely to be classified as at least a medium severity vulnerability. The base score will depend on factors such as attack complexity, privileges required, and the scope of the impact.

Possible Impact

The most significant impact of CVE-2025-65497 is the potential for a Denial of Service (DoS). A remote attacker could exploit this vulnerability to crash applications using libcoap, making services unavailable to legitimate users. This could be particularly problematic for IoT devices and other resource-constrained environments where CoAP is commonly used.

Mitigation and Patch Steps

The recommended mitigation is to upgrade to a patched version of libcoap. The issue has been addressed in a pull request and is likely included in a later release. Specifically, refer to the pull request below for the fix.

1. Upgrade libcoap: The most effective solution is to update libcoap to a version that includes the fix for this vulnerability. Check the official libcoap repository or your distribution’s package manager for updates.
2. Review DTLS Configuration: Evaluate your DTLS configuration to ensure it adheres to best practices and minimizes potential attack vectors.