CVE-2025-15468: 
OpenSSL vulnerability analysis and mitigation

CVE-2025-15468 is a NULL pointer dereference vulnerability in OpenSSL's QUIC protocol implementation that causes a Denial of Service (DoS) when an application using SSL_CIPHER_find() receives an unknown or unsupported cipher suite from a peer. The vulnerable code was introduced in OpenSSL 3.2 with the addition of QUIC protocol support. Affected versions include OpenSSL 3.3.0–3.3.5, 3.4.0–3.4.3, 3.5.0–3.5.4, and 3.6.0; OpenSSL 3.0, 1.1.1, and 1.0.2 are not affected. The vulnerability was publicly disclosed on January 27, 2026, and carries a CVSS v3.1 base score of 5.9 (Medium) (OpenSSL Advisory, Red Hat Bugzilla).

The root cause is a missing NULL pointer check (CWE-476) in the ossl_quic_get_cipher_by_char() function within ssl/quic/quic_impl.c. When ssl3_get_cipher_by_char() returns NULL for an unknown or unsupported cipher ID, the code proceeds to dereference the NULL pointer by accessing ciph->algorithm2, causing a crash. The vulnerability is triggered when an application calls SSL_CIPHER_find() from a client_hello_cb callback on a QUIC SSL object and the peer sends an unrecognized cipher suite ID. The fix, applied across all affected branches, adds a two-line NULL guard (if (ciph == NULL) return NULL;) before the dereference (GitHub Commit b253963, GitHub Commit 1f08e54). The FIPS modules in affected versions are not impacted because the QUIC implementation resides outside the OpenSSL FIPS module boundary (OpenSSL Advisory).

Successful exploitation causes abnormal termination of the affected process, resulting in a Denial of Service. The impact is limited strictly to availability — there is no confidentiality or integrity compromise possible. Only applications that explicitly call SSL_CIPHER_find() from a client_hello_cb callback within a QUIC protocol context are vulnerable, which OpenSSL assessed as an uncommon usage pattern, contributing to the Low severity rating (OpenSSL Advisory, Red Hat Bugzilla). Downstream products incorporating affected OpenSSL versions — including IBM API Connect, IBM Netezza Appliance, IBM watsonx Orchestrate, and various Linux distributions — are also affected (IBM Advisory).

1. Identify target: Locate a network-accessible service using OpenSSL 3.3.x–3.6.x with QUIC protocol support that calls SSL_CIPHER_find() from a client_hello_cb callback.
2. Craft malicious ClientHello: Construct a QUIC ClientHello message containing an unknown or unsupported cipher suite ID not recognized by the OpenSSL implementation.
3. Send to target: Transmit the crafted QUIC handshake packet to the target server or client endpoint.
4. Trigger NULL dereference: The server's client_hello_cb invokes SSL_CIPHER_find() with the unknown cipher ID; ssl3_get_cipher_by_char() returns NULL, and the unguarded dereference of ciph->algorithm2 causes a process crash.
5. Achieve DoS: The target process terminates abnormally, resulting in a denial of service for all connections handled by that process (OpenSSL Advisory, GitHub Commit b253963).

• Logs: Unexpected process crash logs or core dumps from OpenSSL-linked services; error messages referencing ossl_quic_get_cipher_by_char or segmentation faults in QUIC-related code paths.
• Network: Anomalous QUIC ClientHello packets containing non-standard or unrecognized cipher suite IDs sent to QUIC-enabled services.
• Process: Sudden termination (SIGSEGV/segfault) of processes using OpenSSL 3.3–3.6 with QUIC enabled, particularly those implementing client_hello_cb callbacks; repeated service restarts due to crashes.

Upgrade OpenSSL to one of the following patched versions: 3.3.6 (for 3.3.x users), 3.4.4 (for 3.4.x users), 3.5.5 (for 3.5.x users), or 3.6.1 (for 3.6.x users) (OpenSSL Advisory). Red Hat has addressed this in RHSA-2026:1472 (RHEL 10) and RHSA-2026:1473 (RHEL 9) (Red Hat Bugzilla). As a workaround where immediate patching is not possible, audit application code to ensure SSL_CIPHER_find() is not called from client_hello_cb on QUIC SSL objects with untrusted peer-supplied cipher IDs; applications not using this pattern are not affected. Note that JBoss Core Services (JBCS) and JBoss Web Server (JWS) are not affected as they do not yet support QUIC/HTTP3 (Red Hat Bugzilla).

CVE-2025-15468 was one of 12 OpenSSL vulnerabilities disclosed simultaneously in January 2026, several of which were discovered by an AI-assisted security research team (Aisle), generating notable media coverage. Tom's Hardware, TechRadar, and Heise reported on the batch disclosure, highlighting the AI team's 12-for-12 detection rate and praising the quality of their reports (Tom's Hardware, Heise). The OpenSSL project and community discussed the release on the openssl-users and openssl-project mailing lists (OpenSSL Users Group). CVE-2025-15468 itself attracted less individual attention due to its Low severity rating compared to the critical CVE-2025-15467 (stack overflow) disclosed in the same batch (SecurityOnline).