CVE-2026-22796: OpenSSL PKCS#7 Signature DoS Vulnerability

CVE-2026-22796 Overview

CVE-2026-22796 is a type confusion vulnerability in OpenSSL's signature verification of signed PKCS#7 data. The flaw exists in the PKCS7_digest_from_attributes() function, where an ASN1_TYPE union member is accessed without first validating the type. This improper validation can cause an invalid or NULL pointer dereference when processing malformed PKCS#7 data, resulting in a Denial of Service condition.

Applications performing signature verification of PKCS#7 data or directly calling the PKCS7_digest_from_attributes() function can be caused to crash when reading from invalid memory addresses. While the impact is limited to Denial of Service and the PKCS#7 API is considered legacy (applications should use the CMS API instead), the vulnerability affects a wide range of OpenSSL versions.

Critical Impact

Applications verifying PKCS#7 signatures can be crashed by processing attacker-supplied malformed PKCS#7 data, causing service disruption.

Affected Products

• OpenSSL 3.6
• OpenSSL 3.5
• OpenSSL 3.4
• OpenSSL 3.3
• OpenSSL 3.0
• OpenSSL 1.1.1
• OpenSSL 1.0.2

Discovery Timeline

• 2026-01-27 - CVE CVE-2026-22796 published to NVD
• 2026-01-29 - Last updated in NVD database

Technical Details for CVE-2026-22796

Vulnerability Analysis

The vulnerability is rooted in a type confusion issue (CWE-754: Improper Check for Unusual or Exceptional Conditions) within OpenSSL's PKCS#7 signature verification implementation. When the PKCS7_digest_from_attributes() function processes the message digest attribute, it accesses the value from an ASN1_TYPE union without first verifying that the type field indicates V_ASN1_OCTET_STRING.

In C, unions share memory between multiple members. The ASN1_TYPE structure uses a union to store different ASN.1 data types. When code accesses a union member assuming it contains one type while it actually contains another, the memory is interpreted incorrectly. In this case, if the type is not V_ASN1_OCTET_STRING, the code dereferences what it believes to be an octet string pointer but is actually pointing to invalid memory or NULL.

The vulnerability is network-exploitable as an attacker can craft and send malicious PKCS#7 data to any application that performs signature verification on externally-provided data. The attack requires no authentication or user interaction, making it straightforward to trigger. However, the impact is limited to crashing the application—there is no path to information disclosure or code execution.

It's worth noting that the FIPS modules in OpenSSL 3.5, 3.4, 3.3, and 3.0 are not affected by this issue, as the PKCS#7 parsing implementation falls outside the OpenSSL FIPS module boundary.

Root Cause

The root cause is improper validation of ASN.1 type fields before accessing union members. The PKCS7_digest_from_attributes() function assumes the message digest attribute value is of type V_ASN1_OCTET_STRING and directly accesses the corresponding union member without checking the actual type. When an attacker provides PKCS#7 data with a different type value, the union access results in interpreting memory incorrectly, leading to an invalid or NULL pointer dereference.

Attack Vector

The attack is network-based and can be executed by providing malformed signed PKCS#7 data to any vulnerable application that performs signature verification. The attacker crafts PKCS#7 data where the message digest attribute contains a type other than V_ASN1_OCTET_STRING. When the application calls PKCS7_digest_from_attributes() during verification, the function accesses the union member without type validation, causing the application to crash.

The vulnerability mechanism involves the ASN1_TYPE union structure being accessed without proper type checking. When processing a message digest attribute, the code reads the value assuming it is an octet string. If an attacker supplies a different ASN.1 type, the resulting pointer dereference reads from invalid memory or NULL, terminating the process. For detailed technical information, see the OpenSSL Security Advisory.

Detection Methods for CVE-2026-22796

Indicators of Compromise

• Application crashes or unexpected terminations in services performing PKCS#7 signature verification
• Core dumps or crash reports indicating segmentation faults in OpenSSL library functions
• Increased volume of malformed PKCS#7 data being submitted to verification endpoints
• Error logs showing NULL pointer dereference or memory access violations in PKCS7_digest_from_attributes()

Detection Strategies

• Monitor application logs for crashes or restarts in services that handle PKCS#7 signature verification
• Implement input validation to detect anomalous ASN.1 type fields in PKCS#7 structures before processing
• Deploy network-level inspection to identify PKCS#7 data with unexpected message digest attribute types
• Use process monitoring to detect repeated crashes that may indicate exploitation attempts

Monitoring Recommendations

• Set up alerting for service availability on applications performing PKCS#7 operations
• Monitor system logs for segmentation fault signals in processes linked against OpenSSL
• Track authentication and verification endpoint availability metrics
• Implement anomaly detection for unusual patterns in PKCS#7 submission rates or structure

How to Mitigate CVE-2026-22796

Immediate Actions Required

• Update OpenSSL to patched versions as soon as they become available for your deployment
• Review applications to identify those using the legacy PKCS#7 API and prioritize migration to the CMS API
• Implement input validation for externally-sourced PKCS#7 data before processing
• Consider deploying additional process supervision to automatically restart affected services

Patch Information

OpenSSL has released patches addressing this vulnerability. Multiple commits are available for different OpenSSL version branches:

• OpenSSL Commit 2502e7b
• OpenSSL Commit 572844b
• OpenSSL Commit 7bbca05
• OpenSSL Commit eeee3cb
• OpenSSL Commit ef2fb66

Review the OpenSSL Security Advisory for version-specific patching guidance.

Workarounds

• Migrate applications from the legacy PKCS#7 API to the recommended CMS API where possible
• Implement rate limiting on endpoints that accept PKCS#7 data to reduce DoS impact
• Deploy process supervisors (such as systemd or container orchestration) to automatically restart crashed services
• Restrict PKCS#7 signature verification to trusted internal sources where feasible

# Check OpenSSL version to determine vulnerability status
openssl version -a

# Example: Verify patched version is installed (version numbers will vary by branch)
# For OpenSSL 3.x deployments, check for security updates via your package manager
apt list --installed | grep openssl
# or for RHEL-based systems
rpm -qa | grep openssl