CVE-2026-28390: 
Ruby vulnerability analysis and mitigation

CVE-2026-28390 is a NULL pointer dereference vulnerability in OpenSSL's CMS (Cryptographic Message Syntax) EnvelopedData processing that can cause a Denial of Service. When a CMS EnvelopedData message using KeyTransportRecipientInfo with RSA-OAEP encryption is processed, the optional pSourceFunc parameters field of the RSA-OAEP algorithm identifier is accessed without first verifying its presence, resulting in a segfault if the field is absent. Affected versions span OpenSSL 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). The vulnerability was disclosed on April 7, 2026, and 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 rsa_cms_decrypt() function within crypto/cms/cms_rsa.c. The vulnerable code directly accessed plab->parameter->type and plab->parameter->value.octet_string->data without checking whether plab->parameter (the optional parameters field of the RSA-OAEP SourceFunc algorithm identifier) was actually present. An unauthenticated remote attacker can exploit this by sending a crafted CMS EnvelopedData message with a KeyTransportRecipientInfo block that omits the optional parameters field, triggering the dereference before any authentication or cryptographic verification occurs. The fix, applied across multiple OpenSSL branches, uses X509_ALGOR_get0() to safely retrieve the algorithm OID, parameter type, and parameter value with proper NULL checks (OpenSSL Commit, OpenSSL Advisory). 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.

Successful exploitation causes the affected application to crash (segfault), resulting in a Denial of Service. The crash occurs before any authentication or cryptographic operations complete, meaning any service that calls CMS_decrypt() on attacker-supplied input — such as S/MIME email gateways, CMS-based protocol handlers, or TLS-adjacent services — can be remotely crashed without credentials. There is no known confidentiality or integrity impact; the vulnerability is limited to availability (OpenSSL Advisory, GitHub Advisory).

As of the time of disclosure, there is no public proof-of-concept exploit and no evidence of in-the-wild exploitation (GitHub Advisory). The vulnerability is not listed in the CISA Known Exploited Vulnerabilities (KEV) catalog. The EPSS score is approximately 0.029% (0.000290), indicating a very low probability of exploitation in the near term. No threat actor attribution has been reported.

1. Identify a target: Locate a service or application that processes untrusted CMS EnvelopedData messages using a vulnerable OpenSSL version (e.g., an S/MIME email processing gateway, a CMS-based decryption service, or any application calling CMS_decrypt() on external input).
2. Craft a malicious CMS message: Construct a DER- or PEM-encoded CMS EnvelopedData message that includes a KeyTransportRecipientInfo block specifying RSA-OAEP as the key encryption algorithm, but with the optional pSourceFunc field present while omitting its parameters sub-field.
3. Deliver the payload: Submit the crafted message to the target application — for example, by sending a specially crafted S/MIME email to a mail server using OpenSSL for decryption, or by submitting the CMS blob to an API endpoint that invokes CMS_decrypt().
4. Trigger the crash: The vulnerable rsa_cms_decrypt() function dereferences the NULL parameter pointer when processing the malformed pSourceFunc, causing a segfault and crashing the application process, resulting in Denial of Service (OpenSSL Commit, OpenSSL Advisory).

• Logs: Application crash logs or core dumps referencing rsa_cms_decrypt or cms_rsa.c; OpenSSL error logs showing CMS_R_UNSUPPORTED_LABEL_SOURCE or segmentation fault signals from the CMS decryption path.
• Process: Unexpected termination (SIGSEGV) of mail server processes (e.g., Postfix, Exim, Dovecot), web server worker processes, or custom CMS-processing daemons linked against a vulnerable OpenSSL version.
• Network: Repeated inbound delivery of S/MIME messages or CMS-encoded payloads from the same source IP that consistently cause service crashes; unusual patterns of connection resets following CMS message submission.
• File System: Core dump files (e.g., core, core.<pid>) in application working directories generated by the OpenSSL-linked process, potentially containing stack traces pointing to rsa_cms_decrypt().

Upgrade OpenSSL to one of the following patched versions: 1.0.2zp, 1.1.1zg, 3.0.20, 3.3.7, 3.4.5, 3.5.6, or 3.6.2 (or later). No configuration-based workaround is available; patching is the only remediation. Prioritize systems that process untrusted CMS data, including S/MIME implementations and CMS-based protocol handlers. Review and restrict which applications are permitted to process CMS EnvelopedData messages from untrusted sources as a defense-in-depth measure. Distribution-specific updates are available for Ubuntu (USN-8155-1, USN-8155-2), Debian, SUSE, Red Hat, Amazon Linux, and others (OpenSSL Advisory, GitHub Advisory).

The OpenSSL project published a formal security advisory on April 7, 2026, describing the issue and its scope (OpenSSL Advisory). Microsoft also acknowledged the vulnerability in its April 2026 security update guidance (Microsoft MSRC). Security news outlets including Help Net Security, CyberSecurityNews, and 9to5Linux covered the OpenSSL 3.6.2 release and its security fixes. Community reaction was generally measured, noting the limited impact (DoS only, no code execution) and the availability of patches across all supported branches.