CVE-2026-6329: wolfSSL PKCS#12 MAC Verification Bypass

Technical summary

The vulnerability resides in wolfSSL's PKCS#12 MAC verification routine. During verification, the implementation compares a locally computed HMAC against the MAC value extracted from the PKCS#12 structure; however, the comparison uses the attacker-controlled length field parsed from the input file rather than enforcing the correct output length of the configured hash algorithm. This allows an attacker to craft a PKCS#12 file with a truncated (or empty) MAC that will pass the integrity check. The root cause is a missing validation step: the code should verify that the parsed MAC length matches the digest output length of the algorithm in use before performing the comparison. The impact is classified as CWE-347 (Improper Verification of Cryptographic Signature), since the MAC serves as a cryptographic integrity mechanism.

Business impact

PKCS#12 files are widely used to distribute and store private keys, certificates, and certificate chains—often in enterprise certificate management, VPN provisioning, and code-signing workflows. If an attacker can forge or tamper with a PKCS#12 bundle and have it accepted as valid, they could inject malicious certificates, replace legitimate keys, or inject compromised credentials into systems that consume these files. In high-trust scenarios (e.g., an attacker with network or file-system access intercepting key distribution), this could lead to undetected credential compromise, unauthorized code signing, or lateral movement. The CVSS score of 6.5 reflects the need for authenticated access (PR:L) and the confidentiality impact being none, but the integrity impact is high, making this a medium-severity issue suitable for prioritized patching in environments that rely on PKCS#12 for certificate or key distribution.

Affected systems

wolfSSL versions using the vulnerable PKCS#12 MAC verification code are affected. Organizations should check whether their deployments use wolfSSL directly (embedded in applications, IoT devices, or TLS libraries) or indirectly (via third-party software that bundles wolfSSL). Particularly at-risk are applications that parse, validate, or import PKCS#12 files received from external sources, such as enterprise certificate management systems, VPN clients, or HSM integration tools. Verify the specific affected versions against wolfSSL's official advisory and determine which deployed versions require patching.

Exploitability

Exploitation requires an attacker to have authenticated access and the ability to modify a PKCS#12 file in transit or at rest (or to convince a user to import a malicious PKCS#12 bundle). The Network Vector (AV:N) and Low Complexity (AC:L) indicate that the attack surface is broad once an attacker can supply or intercept a PKCS#12 file. However, the Privileges Required (PR:L) constraint means the attacker must have some level of authenticated access to the system or must trick an authorized user into importing a crafted file. There is no public KEV entry as of the publication date, suggesting active exploitation is not yet widespread, but the straightforward nature of the flaw—accepting any MAC length—means a proof-of-concept could be developed easily once the vulnerability is widely known.

Remediation

Apply a patch from wolfSSL that corrects the MAC verification logic to enforce that the parsed MAC length must match the digest output length of the configured algorithm before performing any comparison. Specifically, the code must validate that the attacker-supplied MAC length equals the expected length of the hash function in use (e.g., 32 bytes for SHA-256, 20 bytes for SHA-1) and reject any PKCS#12 structure with a mismatched length. Verify the patch version against wolfSSL's official security advisory. For organizations unable to patch immediately, consider restricting the import and processing of PKCS#12 files to trusted, pre-validated sources and implement out-of-band verification of PKCS#12 integrity using alternate cryptographic checks if available.

Patch guidance

Monitor wolfSSL's security advisories and release notes for a patched version. The fix will involve modifying the PKCS#12 MAC verification code to validate that the MAC length field from the input matches the expected digest length before performing the HMAC comparison. Apply the patch in a controlled manner: first test in a non-production environment with your existing PKCS#12 files to ensure backward compatibility and correct behavior. If you distribute or rely on PKCS#12 files in your supply chain, refresh all such artifacts after patching to ensure they are validated under the corrected logic. Refer to wolfSSL's official advisory for precise version information, compatibility notes, and any configuration changes needed.

Detection guidance

Monitor for attempts to import or parse PKCS#12 files, especially if they originate from untrusted sources. Log and alert on any PKCS#12 parsing operations that occur outside normal certificate-management windows. If possible, implement secondary validation of PKCS#12 file integrity using an external tool or a patched version of wolfSSL in a sandboxed verification flow. Inspect PKCS#12 files offline using utilities that display the MAC length and hash algorithm used; any mismatch between the declared MAC length and the algorithm's digest size is a red flag. Additionally, monitor systems for unusual certificate imports or key-installation operations that might indicate a successful exploit.

Why prioritize this

Although this is a MEDIUM-severity vulnerability with a CVSS score of 6.5 and no active KEV listing, it should be prioritized based on how your organization uses PKCS#12 files. If PKCS#12 is a core part of your certificate distribution, key provisioning, or supply-chain security, patch promptly. The flaw is straightforward to understand and likely to be exploited in targeted supply-chain attacks once more widely known. Organizations in regulated industries (finance, healthcare, defense) that rely on certificate integrity for compliance should treat this as high priority. For organizations that rarely handle PKCS#12 files or only accept them from highly trusted, controlled sources, remediation can be scheduled in a normal maintenance window—but it should not be deferred indefinitely.

Risk score, explained

The CVSS 3.1 score of 6.5 (MEDIUM) reflects: Network-based attack vector (AV:N), low complexity (AC:L), and requirement for authenticated access (PR:L). The Integrity impact is High (I:H) because a successful attack allows an attacker to tamper with the contents of a PKCS#12 bundle without detection. The Confidentiality and Availability impacts are None (C:N, A:N), lowering the overall score. The Scope is Unchanged (S:U). The medium rating appropriately captures that this is not a remote unauthenticated code execution flaw, but it is a serious cryptographic bypass that undermines trust in a widely-used certificate-distribution mechanism. Risk should be elevated within your organization if PKCS#12 is central to your security architecture.

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