Overview
CVE-2025-65500 describes a NULL pointer dereference vulnerability found in the coap_dtls_generate_cookie() function within src/coap_openssl.c of OISM libcoap version 4.3.5. This flaw allows a remote attacker to trigger a denial-of-service (DoS) condition by sending a specially crafted DTLS handshake. Specifically, the vulnerability occurs because the SSL_get_SSL_CTX() function can return NULL under certain conditions, which is then dereferenced, leading to the crash.
Technical Details
The vulnerability lies in the way libcoap handles DTLS handshake processes. During the generation of a cookie for the DTLS handshake, the coap_dtls_generate_cookie() function attempts to retrieve the SSL_CTX object using SSL_get_SSL_CTX(). If this function returns NULL (which can happen under certain error conditions or during malformed handshake attempts), the code proceeds to dereference this NULL pointer. This dereference results in a program crash, effectively causing a denial of service. The relevant code snippet in src/coap_openssl.c is where the NULL SSL_CTX object returned by SSL_get_SSL_CTX() gets used without validation.
CVSS Analysis
Currently, the CVSS score and severity for CVE-2025-65500 are listed as N/A. This is often the case when a vulnerability is newly discovered or is pending full analysis. However, given that it leads to a denial-of-service, a high availability impact is expected. When assigning a CVSS score, factors like attack complexity and scope will be considered. A base score likely in the medium to high range would be anticipated upon completion of its scoring, primarily due to the potential for remote exploitation.
Possible Impact
The most immediate impact of CVE-2025-65500 is a denial of service. An attacker can repeatedly send crafted DTLS handshake packets to a vulnerable libcoap instance, causing it to crash and become unavailable. This could disrupt services relying on libcoap for communication, potentially leading to significant operational downtime. The impact is amplified in scenarios where libcoap is used in critical infrastructure or IoT devices, potentially affecting a large number of connected systems.
Mitigation or Patch Steps
The recommended mitigation is to upgrade to a version of libcoap that includes the fix for this vulnerability. The fix is available in the pull request https://github.com/obgm/libcoap/pull/1750 which likely involves adding a check to ensure that SSL_get_SSL_CTX() does not return NULL before attempting to dereference the pointer. Users should monitor the https://github.com/obgm/libcoap/issues/1746 issue for further updates and guidance from the libcoap maintainers.
Until a patch is applied, consider implementing network-level filtering to mitigate potential attacks. This could involve rate-limiting DTLS handshake attempts or blocking traffic from suspicious sources. However, these measures might not be entirely effective and are intended as temporary workarounds.
