CVE-2026-55961: wolfSSL PKCS#7 Signature Verification Bypass

Technical summary

The vulnerability exists in wolfSSL's OpenSSL compatibility layer, specifically in the PKCS7_verify() function. PKCS#7 can legitimately contain certs-only objects with no signerInfos, meaning no actual cryptographic signatures are present. The vulnerable code path accepts these degenerate objects as verified, failing to enforce the requirement that at least one valid signature must be cryptographically validated before returning success. The PKCS7_NOVERIFY flag, which was intended only to skip certificate chain validation, was inadvertently being treated as a blanket waiver of signature verification itself. The fix now mandates that a signature has actually been verified, rejecting degenerate bundles regardless of flag settings.

Business impact

Any system using wolfSSL's PKCS#7 compatibility APIs for signature validation—including code-signing infrastructure, document authentication, email gateway signing checks, or supply-chain security—becomes vulnerable to accepting fraudulent or unsigned content. This breaks the integrity guarantees these systems depend on. An attacker could inject malicious code, forge signed documents, or bypass authentication checks. Organizations relying on PKCS#7 for trust decisions face direct risk of data tampering and unauthorized execution.

Affected systems

This vulnerability affects only OpenSSL-compatible builds of wolfSSL that use the PKCS7_verify() API. It does not impact wolfSSL's native APIs or non-OpenSSL-compatible configurations. Applications linking against wolfSSL in OpenSSL compatibility mode and calling PKCS7_verify() on untrusted PKCS#7 bundles are at risk. Verify your build configuration and which APIs your application uses.

Exploitability

Exploitability is straightforward from the attacker's perspective: constructing a certs-only PKCS#7 object requires no cryptographic material or special privileges. The attack is network-accessible, requires no user interaction, and no authentication. Any network-facing application that processes PKCS#7 structures (email servers, signature validators, document processors) can be targeted. The primary obstacle is discovering which systems use wolfSSL's PKCS#7 APIs; once identified, exploitation is trivial.

Remediation

Update wolfSSL to a patched version. Verify the specific patch version against wolfSSL's security advisory and release notes. Organizations should audit code paths that call PKCS7_verify() to understand trust dependencies. If wolfSSL cannot be updated immediately, implement additional validation: never rely solely on PKCS7_verify() for security decisions; verify that signerInfos exist and contain validated signature data before trusting the result. Consider using native APIs if OpenSSL compatibility is not required.

Patch guidance

Contact wolfSSL or consult their official security advisories for patched version numbers and availability timelines. Patches should be validated in your development environment before production rollout. Prioritize systems processing untrusted PKCS#7 data, such as email servers or code-signing validators. Regression testing should confirm that legitimately signed PKCS#7 objects continue to verify correctly after patching.

Detection guidance

Monitor application logs for PKCS7_verify() calls on certs-only or degenerate PKCS#7 structures. If you can instrument the application, log signerInfos count and signature validation outcomes before accepting the result. Network-based detection is difficult without payload inspection; focus on runtime monitoring and code-level instrumentation. Check for unexpected PKCS#7 bundles lacking signature blocks in email gateways or document processors.

Why prioritize this

This is a HIGH-severity integrity bypass in a cryptographic validation function. The attack requires no special access, privileges, or user interaction, and the impact—bypassing signature verification—directly enables data tampering and code execution in downstream systems. Although not yet on the KEV catalog, the straightforward exploitability and severity warrant immediate attention, especially for any organization using PKCS#7 for trust decisions.

Risk score, explained

CVSS 3.1 score of 7.5 (HIGH) reflects network accessibility, low attack complexity, no privilege or interaction requirements, and high integrity impact. The attack vector and success conditions are uncomplicated, allowing widespread exploitation. The lack of availability or confidentiality impact prevents a critical rating, but the integrity bypass directly enables secondary attacks on dependent systems.

Frequently asked questions