CVE-2026-42766

What it actually does

This vulnerability is a NULL pointer dereference in OpenSSL's password-based CMS decryption handling. A password-encrypted CMS message can omit the optional ASN.1 field PasswordRecipientInfo.keyDerivationAlgorithm; affected OpenSSL CMS code dereferences that missing field without validating it exists. If an application accepts attacker-controlled CMS messages and attempts password-based CMS decryption, the process can crash, causing denial of service. The flaw affects availability only; it does not directly enable code execution, privilege escalation, data theft, or integrity modification. The OpenSSL FIPS modules in 4.0, 3.6, 3.5, 3.4, and 3.0 are not affected because the vulnerable code is outside the FIPS module boundary.

Why it matters to the business

Services, gateways, mail-processing systems, document workflows, APIs, or backend jobs that decrypt password-encrypted CMS content using OpenSSL may be remotely crashable when they process a malicious message. The practical consequence is service interruption, failed message processing, job restarts, queue buildup, or repeated application outages if an attacker can continuously submit malformed CMS objects. The CVSS score is moderate at 5.9, EPSS is low and flat at approximately 1% over 30 days, it is not listed in CISA KEV, and no ransomware use is reported, so broad emergency response is not indicated; however, exposed CMS-processing interfaces should still be prioritized because exploitation can be unauthenticated and can impact availability.

How an attack unfolds

An attacker first identifies an application path that accepts CMS or S/MIME-like encrypted content and causes the server-side application to perform password-based CMS decryption with OpenSSL. The attacker then crafts a password-encrypted CMS message whose PasswordRecipientInfo structure omits the optional keyDerivationAlgorithm field. When the target application passes this malformed CMS object into the vulnerable OpenSSL CMS decryption logic, OpenSSL attempts to dereference the absent field. This results in a NULL pointer dereference and typically terminates the application process with a segmentation fault or similar crash. If the service automatically restarts, the attacker can repeatedly submit the crafted message to create recurring denial of service. There are no named threat actors, public exploit maturity, KEV listing, or ransomware campaigns reported for this issue in the provided intelligence.

How to fix it

Upgrade OpenSSL to a vendor-provided fixed release as soon as one is available for the affected product line or operating system package. No exact fixed versions were provided in the input, so security teams should track OpenSSL and downstream OS/package advisories for CVE-2026-42766 and apply the first fixed package applicable to their distribution. Where immediate patching is not possible, reduce exposure by disabling password-based CMS decryption for untrusted inputs, rejecting password-encrypted CMS content at trust boundaries, adding ASN.1/CMS validation that rejects PasswordRecipientInfo objects lacking keyDerivationAlgorithm before invoking OpenSSL decryption, and isolating CMS processing in a supervised worker process with restart limits, queue controls, and resource isolation. Ensure that service-level retry behavior does not repeatedly reprocess the same crashing payload. The OpenSSL FIPS modules in 4.0, 3.6, 3.5, 3.4, and 3.0 are not affected, but applications may still be vulnerable if they call the non-FIPS CMS implementation outside the module boundary.

How to confirm the fix worked

Confirm exposure by inventorying applications and libraries that link to OpenSSL and use CMS password-based decryption functions on externally supplied content. Validate remediation by checking that OpenSSL or downstream packages have been updated to a vendor release that explicitly includes the fix for this issue. In a non-production test environment, use a controlled malformed password-encrypted CMS sample with PasswordRecipientInfo.keyDerivationAlgorithm omitted and verify that the application rejects the input cleanly without crashing. Review application behavior for graceful error handling, worker isolation, and poison-message handling so the same CMS object is not retried indefinitely. Confirm logs show handled parse/decryption failures rather than SIGSEGV, segmentation fault, core dump, or process restart events during malformed CMS testing.

Typical attack path

1. Identify a service, API, mail processor, file-processing workflow, or backend job that accepts CMS encrypted messages from untrusted sources.
2. Confirm or infer that the application performs password-based CMS decryption using OpenSSL.
3. Create a password-encrypted CMS message with PasswordRecipientInfo.keyDerivationAlgorithm omitted from the ASN.1 structure.
4. Submit the crafted CMS message to the target processing path.
5. Trigger OpenSSL CMS decryption of the crafted message.
6. Cause a NULL pointer dereference and application crash.
7. Repeat submissions to sustain denial of service if the service restarts automatically.

How to detect it

To determine whether exploitation already occurred, focus on application crash telemetry, process supervisor logs, operating system logs, core dumps, and message-processing logs from systems that decrypt password-encrypted CMS content. Look for segmentation faults, SIGSEGV, access violations, abrupt process exits, container restarts, systemd restarts, Kubernetes CrashLoopBackOff events, or worker crashes occurring immediately after receiving or processing CMS, S/MIME, PKCS#7, or password-encrypted payloads. Where core dumps are available, inspect stack traces for OpenSSL CMS decryption routines and NULL pointer dereference behavior around PasswordRecipientInfo or key derivation handling. Review inbound message stores, API request bodies, attachment quarantine areas, and dead-letter queues for CMS objects that repeatedly cause worker failure; preserve suspected samples for offline ASN.1 inspection. Network and application logs may not expose the malformed ASN.1 field directly, so detection is most reliable when correlating incoming encrypted CMS content with crashes or restarts. No named threat actors, KEV activity, or ransomware campaigns are reported in the provided data.

• PasswordRecipientInfo
• keyDerivationAlgorithm
• CMS_decrypt
• CMS_RecipientInfo_decrypt
• CMS_EncryptedData_decrypt
• SIGSEGV
• Segmentation fault
• core dumped
• CrashLoopBackOff
• systemd-coredump
• openssl cms
• openssl smime
• application/pkcs7-mime
• application/x-pkcs7-mime
• application/pkcs7-signature
• .p7m
• .p7c

What your security team should do

• Prioritize internet-facing or partner-facing services that accept encrypted CMS, S/MIME, PKCS#7, or similar message formats and perform password-based CMS decryption.
• Inventory OpenSSL versions and application code paths using CMS password-based decryption, including embedded, statically linked, containerized, and appliance-provided OpenSSL copies.
• Apply vendor OpenSSL or operating system package updates when a fixed version is available; track downstream advisories because exact fixed versions were not provided.
• Until patched, block or quarantine password-encrypted CMS messages from untrusted sources where business processes allow it.
• Add input validation or format filtering to reject CMS PasswordRecipientInfo objects missing keyDerivationAlgorithm before invoking OpenSSL decryption.
• Run CMS processing in isolated workers with crash containment, restart throttling, queue dead-lettering, and monitoring for repeated crashes from the same source or object.
• Search historical logs and crash telemetry for segmentation faults or core dumps in services that process CMS content.
• Create alerting for spikes in CMS decryption failures, application restarts, SIGSEGV events, or repeated processing of the same encrypted message.
• Treat urgency as moderate unless the affected CMS-processing interface is exposed to unauthenticated or high-volume untrusted input.