CVE-2026-5466: wolfSSL ECCSI Auth Bypass Vulnerability

CVE-2026-5466 Overview

A critical cryptographic vulnerability exists in wolfSSL's ECCSI (Elliptic Curve-Based Certificateless Signatures for Identity-Based Encryption) signature verification implementation. The wc_VerifyEccsiHash function decodes the r and s scalars from the signature blob via mp_read_unsigned_bin without validating that they lie within the valid range of [1, q-1]. This improper verification of cryptographic signature (CWE-347) allows an attacker to craft forged signatures that verify against any message for any identity using only publicly-known constants.

Critical Impact

Complete signature verification bypass enabling universal signature forgery across all ECCSI-protected communications and identities.

Affected Products

• wolfSSL library (versions utilizing ECCSI signature verification)
• Applications and services implementing wolfSSL ECCSI cryptographic functions
• Identity-based encryption systems relying on wolfSSL's wc_VerifyEccsiHash function

Discovery Timeline

• 2026-04-10 - CVE CVE-2026-5466 published to NVD
• 2026-04-13 - Last updated in NVD database

Technical Details for CVE-2026-5466

Vulnerability Analysis

The vulnerability resides in wolfSSL's ECCSI signature verification routine, specifically within the wc_VerifyEccsiHash function. ECCSI is an identity-based signature scheme where the r and s components are critical scalar values that must conform to strict mathematical constraints defined by the elliptic curve parameters.

In a properly implemented ECCSI verification, the r and s values extracted from a signature must be validated to ensure they fall within the range [1, q-1], where q is the order of the elliptic curve's base point. Without this boundary check, the verification algorithm may accept mathematically invalid signatures that should be rejected.

The flaw enables attackers to construct specially crafted signatures using publicly available curve constants that will pass verification for arbitrary messages and identities. This fundamentally breaks the authentication guarantees that ECCSI is designed to provide, as the verification no longer proves that the signature was created by a legitimate identity holder.

Root Cause

The root cause is a missing bounds validation in the wc_VerifyEccsiHash function. When the function calls mp_read_unsigned_bin to decode the r and s scalars from the signature blob, it fails to subsequently verify that these values satisfy the mathematical requirement of being in the range [1, q-1]. This is a classic case of improper verification of cryptographic signature (CWE-347), where critical validation steps are omitted from the signature verification process.

Attack Vector

The attack requires adjacent network access to exploit. An attacker with knowledge of the ECCSI public parameters (which are by design publicly available) can craft a malicious signature blob containing carefully chosen r and s values that fall outside the valid range but still cause the verification to succeed.

The attack methodology involves:

1. Obtaining the public ECCSI parameters for the target system
2. Crafting a signature blob with r and/or s values that exploit the missing boundary checks
3. Submitting the forged signature along with any arbitrary message
4. The vulnerable wc_VerifyEccsiHash function accepts the invalid signature as authentic

This attack does not require prior authentication or user interaction, making it particularly dangerous for systems relying on ECCSI for identity verification.

Detection Methods for CVE-2026-5466

Indicators of Compromise

• Unusual ECCSI signature verification patterns where signatures verify for unexpected identities
• Log entries showing signature verifications with anomalous r or s scalar values (if detailed logging is enabled)
• Authentication events where the claimed identity does not match expected patterns
• Unexpected successful authentications in ECCSI-protected services

Detection Strategies

• Implement application-layer logging to capture ECCSI signature verification attempts and their parameters
• Monitor for authentication anomalies where verified identities do not match expected user behavior
• Deploy network monitoring to detect unusual patterns in ECCSI-based authentication traffic
• Use SentinelOne Singularity to monitor for exploitation attempts targeting wolfSSL-based applications

Monitoring Recommendations

• Enable verbose logging on applications using wolfSSL ECCSI functionality to capture signature parameters
• Establish baseline authentication patterns and alert on deviations that may indicate signature forgery
• Monitor for rapid successive signature verifications that could indicate attack reconnaissance

How to Mitigate CVE-2026-5466

Immediate Actions Required

• Review all applications and services using wolfSSL's ECCSI signature verification functionality
• Apply the latest wolfSSL patches that address the scalar bounds checking issue
• Consider temporarily disabling ECCSI-based authentication until patches can be applied
• Audit recent authentication logs for any suspicious verification patterns

Patch Information

The wolfSSL development team has addressed this vulnerability. The fix adds proper bounds checking to validate that r and s scalars fall within the valid range [1, q-1] before proceeding with signature verification. For technical details on the patch implementation, refer to the wolfSSL GitHub Pull Request #10102.

Organizations should update to the patched version of wolfSSL as soon as it becomes available through their distribution channels.

Workarounds

• Implement application-level validation of ECCSI signature parameters before calling wc_VerifyEccsiHash
• Consider using alternative signature schemes supported by wolfSSL that are not affected by this vulnerability
• Deploy additional authentication layers to reduce reliance on ECCSI as a single point of trust
• Restrict network access to ECCSI-protected services to trusted adjacent networks only

# Verify wolfSSL version and rebuild with patched source
# Clone the patched wolfSSL repository
git clone https://github.com/wolfssl/wolfssl.git
cd wolfssl
git checkout main  # Ensure you have the latest patched version

# Rebuild wolfSSL with ECCSI support
./autogen.sh
./configure --enable-eccsi
make
sudo make install

# Verify the installation
wolfssl-config --version