Critical OpenSSL RCE Vulnerability CVE-2025-15467 Explained
TL;DR
CVE-2025-15467: Critical OpenSSL Vulnerability
A critical vulnerability, CVE-2025-15467 (CVSS 9.8), was disclosed on January 27, 2026, affecting OpenSSL versions 3.0, 3.3, 3.4, 3.5, and 3.6. This flaw enables pre-authentication remote code execution through a stack buffer overflow. If your infrastructure processes S/MIME email or untrusted CMS content, immediate patching is crucial.
Source: Orca Security
Vulnerability Details
The vulnerability lies within OpenSSL's CMS module, specifically in the parsing of encrypted messages. When handling AEAD ciphers like AES-GCM, OpenSSL extracts the Initialization Vector (IV) from the message. It expects the IV to be 12-16 bytes but copies it to a fixed 16-byte stack buffer without proper length validation. An attacker can exploit this by sending an oversized IV, causing a stack buffer overflow that corrupts adjacent memory. The vulnerable function is evp_cipher_get_asn1_aead_params() in crypto/evp/evp_lib.c according to JFrog Security Research.
The attack unfolds as follows:
- The application calls
CMS_decrypt()to process an incoming message. - OpenSSL parses the structure and identifies the use of AEAD encryption.
- It extracts the IV from ASN.1-encoded parameters.
- The IV is copied to a 16-byte stack buffer without length validation.
- An oversized IV overflows into adjacent stack memory.
The fix involves validating that the IV length is less than or equal to EVP_MAX_IV_LENGTH before copying.
Source: Orca Security
Impact and Severity
The vulnerability is particularly dangerous due to the following factors:
- Pre-authentication exploitation: The overflow occurs during initial parsing, before any cryptographic verification.
- Low attack complexity: The CVSS score is 9.8 with "Low" attack complexity and no privileges required.
- Wide deployment: OpenSSL 3.x is used in numerous mail servers, web servers, and embedded systems.
The impact includes:
- Guaranteed: Denial of service due to process crashes.
- Possible: Remote code execution, depending on platform mitigations like stack canaries and ASLR.
Red Hat notes that their Enterprise Linux builds include stack protections that mitigate the risk of code execution, although a denial-of-service condition remains possible. Ubuntu's security team also indicates that compiler hardening reduces a stack buffer overflow to a denial of service only.
Source: Orca Security
Affected Versions and Patches
The following OpenSSL versions are vulnerable:
| Vulnerable | Fixed |
|---|---|
| 3.6.0 | 3.6.1 |
| 3.5.0 – 3.5.4 | 3.5.5 |
| 3.4.0 – 3.4.3 | 3.4.4 |
| 3.3.0 – 3.3.5 | 3.3.6 |
| 3.0.0 – 3.0.18 | 3.0.19 |
OpenSSL 1.1.1 and 1.0.2 are not affected. OpenSSL 3.1 (EOL March 2025) and 3.2 (EOL November 2025) likely contain this vulnerability but will not receive patches.
Source: Orca Security
Discovery and Attribution
CVE-2025-15467 was one of 12 vulnerabilities discovered by AISLE, a security research organization that uses AI-driven vulnerability discovery. The issue was reported to OpenSSL on December 14, 2025, by Stanislav Fort, AISLE’s co-founder and chief scientist, and the fix was developed by Igor Ustinov.
Source: Orca Security
Mitigation and Remediation
Immediate Actions
- Identify all systems running vulnerable OpenSSL versions (3.0, 3.3, 3.4, 3.5, 3.6) and prioritize patching.
- Apply the security patches released by OpenSSL immediately to all affected systems.
- If immediate patching is not possible, consider temporarily disabling or restricting services that process untrusted CMS/PKCS#7 content.
- Review application architecture to identify all OpenSSL dependencies, including those in third-party libraries.
Patch Information
OpenSSL has released security patches to address this vulnerability. Organizations should apply the appropriate patch for their deployed OpenSSL version:
- OpenSSL Commit 2c8f0e5
- OpenSSL Commit 5f26d42
- OpenSSL Commit 6ced0fe
- OpenSSL Commit ce39170
- OpenSSL Commit d0071a0
Refer to the OpenSSL Security Advisory and the Openwall OSS-Security Discussion for complete details.
Workarounds
- Restrict network access to services processing CMS/PKCS#7 content to trusted sources only.
- Implement input validation at the application layer to reject CMS messages with abnormally large IV parameters before passing to OpenSSL.
- Deploy network-level filtering to block malformed CMS messages at the perimeter.
- Consider using OpenSSL 1.1.1 or 1.0.2 for critical systems if upgrading to patched 3.x versions is not immediately feasible (note: evaluate support status).
Source: CVE-2025-15467: OpenSSL CMS Buffer Overflow Vulnerability
Detection Methods
Indicators of Compromise
- Monitor for unexpected application crashes in services processing CMS/PKCS#7 content, particularly those handling S/MIME messages.
- Look for segmentation faults or access violations in processes linked against vulnerable OpenSSL versions (3.0, 3.3, 3.4, 3.5, 3.6).
- Examine logs for malformed CMS message handling errors with unusually large IV parameters.
- Check for anomalous network traffic containing oversized ASN.1 structures in encrypted message payloads.
Detection Strategies
- Deploy network intrusion detection signatures to identify CMS messages with abnormally large AEAD IV parameters.
- Implement application-level monitoring to detect parsing failures in CMS/PKCS#7 processing routines.
- Use behavioral AI to detect exploitation attempts through stack-based buffer overflow patterns.
- Enable crash dump analysis to identify exploitation attempts targeting this vulnerability.
Monitoring Recommendations
- Continuously monitor OpenSSL-dependent services for unexpected terminations or restart patterns.
- Implement file integrity monitoring on OpenSSL library files to verify patched versions are deployed.
- Configure application performance monitoring to alert on CMS parsing anomalies.
- Enable detailed logging for all services processing untrusted cryptographic content.
Source: CVE-2025-15467: OpenSSL CMS Buffer Overflow Vulnerability
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