CVE-2026-5500: wolfSSL AES-GCM Tag Truncation Vulnerability

CVE-2026-5500 Overview

A cryptographic validation vulnerability exists in wolfSSL's wc_PKCS7_DecodeAuthEnvelopedData() function that fails to properly sanitize the AES-GCM authentication tag length. The function lacks a lower bounds check on the authentication tag length received from untrusted sources, allowing attackers to drastically reduce cryptographic security guarantees.

Critical Impact

A man-in-the-middle attacker can truncate the MAC field from 16 bytes to 1 byte, reducing authentication tag verification from 2⁻¹²⁸ to 2⁻⁸, making brute-force attacks trivially feasible.

Affected Products

• wolfSSL library versions prior to the security fix
• Applications using wc_PKCS7_DecodeAuthEnvelopedData() for PKCS#7 AuthEnvelopedData decryption
• Systems relying on wolfSSL for AES-GCM authenticated encryption

Discovery Timeline

• 2026-04-10 - CVE CVE-2026-5500 published to NVD
• 2026-04-13 - Last updated in NVD database

Technical Details for CVE-2026-5500

Vulnerability Analysis

This vulnerability stems from improper input validation (CWE-20) in wolfSSL's PKCS#7 AuthEnvelopedData decoding implementation. The wc_PKCS7_DecodeAuthEnvelopedData() function processes AES-GCM encrypted content but does not enforce minimum length requirements for the authentication tag parameter.

AES-GCM is an authenticated encryption mode that provides both confidentiality and integrity through a cryptographic tag appended to ciphertext. The standard specifies tag lengths of 128, 120, 112, 104, 96, 64, or 32 bits, with 128 bits (16 bytes) recommended for maximum security. By accepting arbitrary tag lengths without validation, the implementation permits attackers to specify dangerously short tags.

The practical impact is severe: an authentication tag reduced to 8 bits provides only 256 possible values, meaning an attacker can forge valid-looking encrypted messages with a success probability of approximately 1 in 256 attempts—a trivially brute-forceable collision space.

Root Cause

The root cause is missing input validation in the wc_PKCS7_DecodeAuthEnvelopedData() function. The code path that parses the authentication tag length from ASN.1-encoded PKCS#7 structures does not implement a lower bounds check to ensure the tag meets minimum cryptographic requirements. This allows external input to control a security-critical parameter without proper sanitization.

Attack Vector

The vulnerability is exploitable via a network-based man-in-the-middle attack. An attacker positioned between a sender and receiver can intercept PKCS#7 AuthEnvelopedData messages and modify the authentication tag length field in the ASN.1 structure. By truncating the expected 16-byte tag to a single byte, the attacker reduces the security margin from 2¹²⁸ combinations to just 2⁸ (256) possibilities.

This enables several attack scenarios:

The attacker can forge messages that pass authentication checks with high probability through trivial brute-force attempts. Additionally, modified ciphertext blocks may be accepted if the truncated tag happens to match, compromising message integrity. The attack requires no knowledge of cryptographic keys—only the ability to modify in-transit data.

Detection Methods for CVE-2026-5500

Indicators of Compromise

• Unusual or truncated authentication tag lengths in PKCS#7 AuthEnvelopedData structures (tags shorter than 12 bytes)
• Multiple authentication failures followed by successful decryption with the same session
• Anomalous ASN.1 structure sizes in encrypted message traffic
• Evidence of man-in-the-middle positioning such as ARP spoofing or certificate warnings

Detection Strategies

• Implement network monitoring for PKCS#7 traffic and flag messages with authentication tags shorter than the expected 16 bytes
• Deploy intrusion detection rules to identify potential MITM activity targeting TLS or encrypted communications
• Enable enhanced logging in applications using wolfSSL to capture authentication tag lengths during decryption operations
• Utilize SentinelOne's behavioral analysis to detect applications processing malformed cryptographic structures

Monitoring Recommendations

• Monitor wolfSSL-dependent applications for unusual decryption patterns or repeated authentication attempts
• Implement alerting on network segments where encrypted PKCS#7 data is transmitted
• Review TLS session logs for evidence of downgrade attacks or certificate manipulation that could enable MITM positioning
• Track wolfSSL library versions across the environment to identify vulnerable deployments

How to Mitigate CVE-2026-5500

Immediate Actions Required

• Update wolfSSL to the latest patched version that enforces authentication tag length validation
• Audit applications using wc_PKCS7_DecodeAuthEnvelopedData() and prioritize updates for internet-facing services
• Implement network-level protections against MITM attacks including certificate pinning and HSTS
• Consider implementing additional application-layer integrity checks independent of the PKCS#7 authentication tag

Patch Information

The vulnerability has been addressed in the wolfSSL project. The fix implements proper lower bounds validation for the AES-GCM authentication tag length parameter. Technical details and the patch implementation are available in the wolfSSL Pull Request #10102.

Organizations should update to the patched version of wolfSSL and recompile any applications statically linked against the library.

Workarounds

• Deploy network segmentation to limit exposure of vulnerable systems to potential MITM attacks
• Implement certificate pinning in client applications to prevent interception of encrypted communications
• Add application-layer message authentication (such as HMAC) independent of PKCS#7 structures as defense in depth
• Monitor and alert on any incoming PKCS#7 AuthEnvelopedData with non-standard tag lengths at the network perimeter

# Verify wolfSSL version and check for vulnerable function usage
# Check installed wolfSSL version
wolfssl-config --version

# Search codebase for affected function calls
grep -r "wc_PKCS7_DecodeAuthEnvelopedData" /path/to/application/source

# After patching, verify the library has been updated
ldd /path/to/application | grep wolfssl