CVE-2025-11187: 
OpenSSL vulnerability analysis and mitigation

CVE-2025-11187 is a stack-based buffer overflow and NULL pointer dereference vulnerability in OpenSSL's PKCS#12 PBMAC1 MAC verification logic. It affects OpenSSL versions 3.4.0–3.4.3, 3.5.0–3.5.4, and 3.6.0; versions 3.3, 3.0, 1.1.1, and 1.0.2 are not affected as they do not support PBMAC1 in PKCS#12. The vulnerability was publicly disclosed on January 27, 2026, and was reported by Stanislav Fort and Petr Simecek (Aisle Research) and independently by Hamza (Metadust). It carries a CVSS v3.1 base score of 6.1 (Medium) (Red Hat Advisory, OpenSSL Advisory).

The root cause is improper parameter validation (CWE-233, CWE-787, CWE-476) in the PBMAC1_PBKDF2_HMAC function within crypto/pkcs12/p12_mutl.c. When OpenSSL verifies a PKCS#12 file using PBMAC1 for the MAC, the PBKDF2 keylength and salt parameters are consumed directly from the file without validation. If keylength exceeds 64 bytes (the size of the fixed stack buffer for the derived key), a stack-based buffer overflow occurs with attacker-controlled overflow length; if the salt parameter is not an ASN.1 OCTET STRING type, an invalid or NULL pointer dereference results. The fix adds explicit checks: the salt must be a non-NULL OCTET STRING, and keylength must be present and not exceed EVP_MAX_MD_SIZE (OpenSSL Commit, Red Hat Bugzilla). Exploitation requires an attacker to supply a maliciously crafted PKCS#12 file to a vulnerable application.

Successful exploitation can cause a crash (Denial of Service) in any application that parses untrusted PKCS#12 files using the affected OpenSSL versions. The attacker-controlled stack buffer overflow additionally carries a potential for arbitrary code execution, depending on platform-level mitigations such as stack canaries and ASLR. Confidentiality and integrity impacts are assessed as low, while availability impact is high. The FIPS modules in OpenSSL 3.4, 3.5, and 3.6 are not affected, as PKCS#12 processing falls outside the FIPS module boundary (OpenSSL Advisory, Red Hat Advisory).

1. Craft a malicious PKCS#12 file: Create a PKCS#12 file that uses PBMAC1 for the MAC algorithm. Set the PBKDF2 keylength parameter to a value greater than 64 bytes (e.g., 256 or larger) to trigger the stack buffer overflow, or set the salt field to a non-OCTET STRING ASN.1 type to trigger a NULL/invalid pointer dereference.
2. Identify a vulnerable target: Find an application that accepts and parses PKCS#12 files from user-supplied or external input and links against OpenSSL 3.4.0–3.4.3, 3.5.0–3.5.4, or 3.6.0.
3. Deliver the malicious file: Supply the crafted PKCS#12 file to the target application through any available input channel (file upload, API endpoint, command-line argument, etc.).
4. Trigger MAC verification: The application calls OpenSSL's PKCS#12 MAC verification routine, which invokes PBMAC1_PBKDF2_HMAC in crypto/pkcs12/p12_mutl.c without validating the attacker-controlled parameters.
5. Achieve DoS or code execution: The oversized keylength causes a write past the 64-byte stack buffer, potentially crashing the process (DoS) or, depending on platform mitigations, enabling code execution (Metadust PoC, OpenSSL Commit).

• Logs: Application crash logs or core dumps referencing PKCS12_R_PARSE_ERROR or stack overflow signals (SIGSEGV, SIGABRT) originating from crypto/pkcs12/p12_mutl.c or PBMAC1_PBKDF2_HMAC.
• File System: Unexpected or anomalous PKCS#12 (.p12, .pfx) files submitted to the application, particularly those with unusually large keylength values in PBMAC1 parameters.
• Process: Abnormal termination of processes that handle PKCS#12 file imports or certificate management operations, especially repeated crashes under similar input conditions.
• Network: Repeated or automated submissions of PKCS#12 files to exposed endpoints (e.g., certificate upload APIs), which may indicate fuzzing or exploitation attempts.

Upgrade OpenSSL to the patched versions: 3.4.4 or later, 3.5.5 or later, or 3.6.1 or later. Red Hat Enterprise Linux users should apply RHSA-2026:1472 (RHEL 10), RHSA-2026:1473 (RHEL 9), or RHSA-2026:1496 (RHEL 10 EUS) (Red Hat Bugzilla). As a workaround where patching is not immediately possible, restrict PKCS#12 file processing to trusted sources only, and consider sandboxing or isolating applications that parse PKCS#12 files from untrusted input. IBM products including API Connect, Cloud Pak for AIOps, Netezza Appliance, and others have also released updates addressing this vulnerability (IBM Advisory).

The vulnerability attracted notable attention due to its discovery by Aisle Research using AI-assisted methods — the team reportedly found all 12 OpenSSL vulnerabilities disclosed in the January 2026 batch, prompting coverage in Tom's Hardware, TechRadar, and LessWrong discussing the role of AI in vulnerability discovery (Aisle Research Blog). Datadog Security Labs published a technical analysis of the January 2026 OpenSSL security update covering this and related buffer overflow issues (Datadog Security Labs). The OpenSSL project and community discussed the release on the openssl-users and openssl-project mailing lists. The Canadian Centre for Cyber Security issued advisory AV26-058 covering the OpenSSL update. Community sentiment on Reddit and security forums characterized the risk as moderate given the requirement for untrusted PKCS#12 file processing.