CVE-2026-3338: AWS-LC Authentication Bypass Vulnerability

CVE-2026-3338 Overview

CVE-2026-3338 is a cryptographic signature validation bypass vulnerability in the AWS Libcrypto (AWS-LC) library. The flaw exists in the PKCS7_verify() function, which improperly validates signatures when processing PKCS7 objects containing Authenticated Attributes. This vulnerability allows an unauthenticated attacker to bypass signature verification, potentially enabling the acceptance of forged or tampered cryptographic messages.

PKCS#7 is a widely-used cryptographic message syntax standard for signing and encrypting data. The improper validation in AWS-LC could allow attackers to forge signatures or bypass integrity checks in applications relying on this library for cryptographic operations.

Critical Impact

Unauthenticated attackers can bypass signature verification in PKCS7 processing, potentially allowing forged or tampered cryptographic messages to be accepted as valid. This undermines the fundamental trust guarantees of digital signatures.

Affected Products

• AWS-LC versions prior to 1.69.0
• Applications using vulnerable versions of AWS-LC for PKCS7 signature verification

Discovery Timeline

• 2026-03-02 - CVE-2026-3338 published to NVD
• 2026-03-03 - Last updated in NVD database

Technical Details for CVE-2026-3338

Vulnerability Analysis

This vulnerability is classified as CWE-347: Improper Verification of Cryptographic Signature. The flaw resides in the PKCS7_verify() function within AWS-LC, which fails to properly validate signatures when PKCS7 objects contain Authenticated Attributes.

Authenticated Attributes in PKCS#7 are additional data elements that are signed along with the content. When present, the signature should cover both the content and these attributes. The vulnerability occurs because the verification logic does not properly handle the signature validation process when these attributes are present, creating a path for signature bypass.

The network-accessible nature of this vulnerability means that any application using AWS-LC for remote signature verification could be exploited without requiring prior authentication. This is particularly concerning for applications that process signed messages from untrusted sources.

Root Cause

The root cause is improper signature validation logic in the PKCS7_verify() function when processing PKCS7 structures with Authenticated Attributes. The function fails to correctly verify that the signature covers all required components, allowing an attacker to craft malicious PKCS7 objects that pass verification despite having invalid or missing signatures.

Attack Vector

The attack vector is network-based and requires no user interaction or prior authentication. An attacker can exploit this vulnerability by sending a specially crafted PKCS7 object with manipulated Authenticated Attributes to an application using a vulnerable version of AWS-LC.

The attack flow involves:

1. Crafting a PKCS7 message with Authenticated Attributes
2. Manipulating the structure to bypass the signature verification logic
3. Sending the crafted message to a target application using AWS-LC
4. The application accepts the forged/tampered message as validly signed

The vulnerability mechanism involves improper handling of Authenticated Attributes during signature verification in the PKCS7_verify() function. For technical implementation details, see the GitHub Security Advisory GHSA-jchq-39cv-q4wj.

Detection Methods for CVE-2026-3338

Indicators of Compromise

• Unexpected acceptance of PKCS7 signed messages that should fail verification
• Log entries showing successful signature verification for known-invalid certificates
• Applications processing PKCS7 content with mismatched or tampered Authenticated Attributes
• Anomalous cryptographic operations in AWS-LC dependent applications

Detection Strategies

• Audit applications for AWS-LC library usage and verify versions are 1.69.0 or later
• Implement additional signature validation checks at the application layer as a defense-in-depth measure
• Monitor for unusual patterns in cryptographic message processing logs
• Use software composition analysis (SCA) tools to identify vulnerable AWS-LC dependencies

Monitoring Recommendations

• Enable verbose logging for cryptographic operations in applications using AWS-LC
• Monitor for failed authentication attempts that precede suspicious successful verifications
• Implement alerting for unusual PKCS7 message processing patterns
• Track dependency versions across your software supply chain for AWS-LC updates

How to Mitigate CVE-2026-3338

Immediate Actions Required

• Upgrade AWS-LC to version 1.69.0 or later immediately
• Identify all applications and services using AWS-LC for PKCS7 operations
• Review and audit any PKCS7 signature verifications performed during the vulnerable period
• Consider implementing additional application-layer validation as defense-in-depth

Patch Information

AWS has released AWS-LC version 1.69.0 which addresses this vulnerability. According to the AWS Security Bulletin 2026-005, customers of AWS services do not need to take action as AWS has already updated their services. However, applications that directly use the AWS-LC library must upgrade to version 1.69.0 or later.

The patched version is available on GitHub AWS-LC Release v1.69.0.

Workarounds

• If immediate upgrade is not possible, implement additional signature verification at the application layer
• Consider temporarily restricting PKCS7 processing to trusted sources only
• Monitor for exploitation attempts while preparing for the upgrade
• Review the GitHub Security Advisory GHSA-jchq-39cv-q4wj for additional guidance

To upgrade AWS-LC in your application, update your dependency configuration to require version 1.69.0 or later, then rebuild and redeploy affected applications. Verify the upgrade by checking the library version in your build artifacts.