CVE-2025-15467: OpenSSL CMS Buffer Overflow Vulnerability

CVE-2025-15467 Overview

CVE-2025-15467 is a critical stack buffer overflow vulnerability in OpenSSL affecting the parsing of CMS (Cryptographic Message Syntax) AuthEnvelopedData messages. When processing messages with maliciously crafted AEAD (Authenticated Encryption with Associated Data) parameters, the Initialization Vector (IV) encoded in ASN.1 parameters is copied into a fixed-size stack buffer without proper length validation. This allows an attacker to supply an oversized IV, triggering a stack-based out-of-bounds write that occurs before any authentication or tag verification takes place.

The vulnerability is particularly dangerous because it can be exploited without valid key material—the overflow happens during parsing, prior to cryptographic verification. Applications and services parsing untrusted CMS or PKCS#7 content using AEAD ciphers (such as S/MIME AuthEnvelopedData with AES-GCM) are vulnerable to this attack.

Critical Impact

This stack buffer overflow may lead to denial of service through application crash, or potentially enable remote code execution depending on platform and toolchain mitigations. The pre-authentication nature of the vulnerability significantly increases its severity.

Affected Products

• OpenSSL 3.6
• OpenSSL 3.5
• OpenSSL 3.4
• OpenSSL 3.3
• OpenSSL 3.0

Discovery Timeline

• 2026-01-27 - CVE CVE-2025-15467 published to NVD
• 2026-01-29 - Last updated in NVD database

Technical Details for CVE-2025-15467

Vulnerability Analysis

This vulnerability exists in OpenSSL's CMS implementation when handling AuthEnvelopedData structures that utilize AEAD ciphers such as AES-GCM. The core issue stems from improper handling of the Initialization Vector during ASN.1 parameter parsing. When a CMS message is processed, the IV is extracted from the encoded ASN.1 parameters and copied directly into a fixed-size buffer allocated on the stack.

The critical flaw is the absence of bounds checking—the code does not verify that the IV length conforms to the expected size before performing the copy operation. An attacker can craft a malicious CMS message containing an IV that exceeds the buffer's capacity, resulting in adjacent stack memory being overwritten.

What makes this vulnerability particularly severe is its pre-authentication exploitation window. The buffer overflow occurs during the initial parsing phase, before any cryptographic authentication or tag verification is performed. This means an attacker does not need access to valid encryption keys or authentication material to trigger the vulnerability.

It is important to note that the FIPS modules in versions 3.6, 3.5, 3.4, 3.3, and 3.0 are not affected, as the CMS implementation falls outside the OpenSSL FIPS module boundary. Additionally, OpenSSL versions 1.1.1 and 1.0.2 are not vulnerable to this issue.

Root Cause

The root cause is classified as CWE-787 (Out-of-bounds Write). The vulnerability originates from missing input validation when copying the IV parameter from ASN.1-encoded AEAD cipher parameters into a fixed-size stack buffer. The code assumes the IV will be of a specific expected length but fails to enforce this constraint before the memory copy operation, allowing attacker-controlled data to overflow the buffer boundaries.

Attack Vector

The attack vector is network-based, requiring no authentication or user interaction. An attacker can exploit this vulnerability by sending a specially crafted CMS or PKCS#7 message to any application or service that processes untrusted cryptographic content. Common attack surfaces include:

• Email servers and clients processing S/MIME encrypted messages
• Web applications handling CMS-signed or encrypted data
• Certificate processing systems
• Any service accepting PKCS#7 or CMS content from untrusted sources

The attacker constructs a malicious CMS AuthEnvelopedData message with an oversized IV in the AEAD parameters. When the target application parses this message using vulnerable OpenSSL versions, the stack buffer overflow is triggered immediately during parsing, potentially allowing the attacker to overwrite return addresses or other critical stack data, depending on exploit sophistication and platform protections.

Detection Methods for CVE-2025-15467

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 SentinelOne's 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

How to Mitigate CVE-2025-15467

Immediate Actions Required

• 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. Multiple commits have been published to remediate the issue across affected branches. 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

For complete details, refer to the OpenSSL Security Advisory and the Openwall OSS-Security Discussion.

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)

# Verify OpenSSL version on your system
openssl version -a

# Check for vulnerable versions (3.0, 3.3, 3.4, 3.5, 3.6)
# If vulnerable, update using your package manager:

# Debian/Ubuntu
sudo apt update && sudo apt upgrade openssl libssl-dev

# RHEL/CentOS/Fedora
sudo dnf update openssl openssl-devel

# Verify the update was successful
openssl version -a