CVE-2026-28389: 
OpenSSL vulnerability analysis and mitigation

CVE-2026-28389 is a NULL pointer dereference vulnerability in OpenSSL's CMS (Cryptographic Message Syntax) processing that can cause a Denial of Service. It affects applications that call CMS_decrypt() on untrusted input, such as S/MIME processing or CMS-based protocols. The vulnerability was disclosed on April 7, 2026, and affects OpenSSL versions 1.0.2 (before 1.0.2zp), 1.1.1 (before 1.1.1zg), 3.0.x (before 3.0.20), 3.3.x (before 3.3.7), 3.4.x (before 3.4.5), 3.5.x (before 3.5.6), and 3.6.x (before 3.6.2). It carries a CVSS v3.1 base score of 7.5 (High) (OpenSSL Advisory, GitHub Advisory).

The root cause is a NULL pointer dereference (CWE-476) in the dh_cms_set_shared_info() and ecdh_cms_set_shared_info() functions within crypto/cms/cms_dh.c and crypto/cms/cms_ec.c. When processing a CMS EnvelopedData message that uses KeyAgreeRecipientInfo, the code accesses alg->parameter directly without first checking whether the optional parameters field of KeyEncryptionAlgorithmIdentifier is present (i.e., non-NULL). An attacker can craft a CMS EnvelopedData message that omits this optional field, causing a SIGSEGV before any authentication or cryptographic operations occur. The fix, applied via X509_ALGOR_get0(), properly extracts the parameter type and value with a NULL-safe check before dereferencing (OpenSSL Commit f80f83b, OpenSSL Commit 16cea41). The FIPS modules in versions 3.0, 3.3, 3.4, 3.5, and 3.6 are not affected, as the vulnerable code lies outside the FIPS module boundary (OpenSSL Advisory).

Successful exploitation results in a process crash (SIGSEGV) in any application that calls CMS_decrypt() on attacker-controlled input, causing a Denial of Service. The crash occurs before authentication or cryptographic operations complete, meaning services such as S/MIME email gateways, CMS-based protocol handlers, or any application processing untrusted encrypted messages could be rendered unavailable. There is no evidence of confidentiality or integrity impact; the vulnerability is limited to availability (OpenSSL Advisory, GitHub Advisory).

1. Identify target: Locate services that process untrusted CMS or S/MIME data using a vulnerable OpenSSL version (1.0.2 < 1.0.2zp, 1.1.1 < 1.1.1zg, 3.0.x < 3.0.20, 3.3.x < 3.3.7, 3.4.x < 3.4.5, 3.5.x < 3.5.6, 3.6.x < 3.6.2), such as email gateways, mail servers with S/MIME support, or custom CMS-based protocol endpoints.
2. Craft malicious CMS message: Construct a CMS EnvelopedData message that uses KeyAgreeRecipientInfo (DH or ECDH key agreement) and deliberately omits the optional parameters field from the KeyEncryptionAlgorithmIdentifier.
3. Deliver the payload: Submit the crafted CMS message to the target application — for example, by sending a specially crafted S/MIME email to a mail server, or by supplying the message to any API endpoint that calls CMS_decrypt() on untrusted input.
4. Trigger crash: The vulnerable dh_cms_set_shared_info() or ecdh_cms_set_shared_info() function dereferences the NULL alg->parameter pointer, causing a SIGSEGV and crashing the process, resulting in Denial of Service (OpenSSL Commit f80f83b, OpenSSL Advisory).

• Logs: Application crash logs or core dumps referencing a SIGSEGV in OpenSSL CMS processing functions (dh_cms_set_shared_info, ecdh_cms_set_shared_info); repeated unexpected process restarts in S/MIME or CMS-handling services.
• Network: Unusual or malformed CMS/S/MIME messages received by mail servers or CMS endpoints, particularly those using KeyAgreeRecipientInfo with missing algorithm parameters; repeated connection attempts followed by service crashes.
• Process: Unexpected termination (SIGSEGV/signal 11) of processes linked to OpenSSL that handle encrypted message decryption; crash dumps from mail transfer agents or CMS-processing daemons.

Update OpenSSL to a patched version: 1.0.2zp or later, 1.1.1zg or later, 3.0.20 or later, 3.3.7 or later, 3.4.5 or later, 3.5.6 or later, or 3.6.2 or later (OpenSSL Advisory). Downstream distributions including Ubuntu (USN-8155-1/2), Debian, SUSE, openSUSE, Slackware, Amazon Linux, and FreeBSD have released updated packages. As a temporary workaround where patching is not immediately possible, consider implementing input validation and filtering for CMS messages from untrusted sources, or disabling S/MIME and CMS-based protocol processing until patches are applied. FIPS-mode deployments using the FIPS modules in versions 3.0, 3.3, 3.4, 3.5, and 3.6 are not affected by this issue.

The OpenSSL project published a formal security advisory on April 7, 2026, rating the issue as moderate severity and providing patched releases across all supported branches (OpenSSL Advisory). Microsoft also acknowledged the vulnerability in their security update guide (Microsoft Advisory). Security news outlets including Help Net Security, CyberSecurityNews, and 9to5Linux covered the OpenSSL 3.6.2 release and its security fixes. Multiple Linux distributions (Ubuntu, Debian, SUSE, openSUSE, Slackware, Amazon Linux, FreeBSD) promptly issued updated packages, reflecting broad ecosystem awareness and rapid response.