CVE-2026-5501
Published: 10 April 2026
Summary
CVE-2026-5501 is a high-severity Improper Certificate Validation (CWE-295) vulnerability in Wolfssl Wolfssl. Its CVSS base score is 8.1 (High).
Operationally, exploitation aligns with the MITRE ATT&CK technique Exploit Public-Facing Application (T1190); ranked at the 6.6th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog.
The strongest mitigations our analysis identified are NIST 800-53 SC-17 (Public Key Infrastructure Certificates) and SI-2 (Flaw Remediation).
Threat & Defense at a Glance
Threat & Defense Details
Mitigating Controls (NIST 800-53 r5)AI
Directly requires timely identification, reporting, and patching of the specific flaw in wolfSSL_X509_verify_cert that allows forged certificate chains.
Mandates establishment and management of PKI certificates with defined validation methods to prevent acceptance of malformed chains lacking leaf signature verification.
Requires verification that security functions, including certificate chain validation in the OpenSSL compatibility layer, operate as intended to mitigate improper validation flaws.
MITRE ATT&CK Enterprise TechniquesAI
Why these techniques?
Vulnerability enables remote exploitation of public-facing apps (nginx/haproxy) via malformed cert chains (T1190) and directly facilitates MITM/impersonation by bypassing X.509 validation (T1557).
NVD Description
wolfSSL_X509_verify_cert in the OpenSSL compatibility layer accepts a certificate chain in which the leaf's signature is not checked, if the attacker supplies an untrusted intermediate with Basic Constraints `CA:FALSE` that is legitimately signed by a trusted root. An attacker who…
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obtains any leaf certificate from a trusted CA (e.g. a free DV cert from Let's Encrypt) can forge a certificate for any subject name with any public key and arbitrary signature bytes, and the function returns `WOLFSSL_SUCCESS` / `X509_V_OK`. The native wolfSSL TLS handshake path (`ProcessPeerCerts`) is not susceptible and the issue is limited to applications using the OpenSSL compatibility API directly, which would include integrations of wolfSSL into nginx and haproxy.
Deeper analysisAI
CVE-2026-5501 is a high-severity vulnerability (CVSS 8.1, CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N) in the wolfSSL_X509_verify_cert function within wolfSSL's OpenSSL compatibility layer. The flaw allows acceptance of a malformed certificate chain where the leaf certificate's signature is not verified if an attacker provides an untrusted intermediate certificate with Basic Constraints CA:FALSE that is legitimately signed by a trusted root. This issue is confined to applications directly using the OpenSSL compatibility API and does not affect wolfSSL's native TLS handshake path (ProcessPeerCerts). Affected integrations include wolfSSL embedded in nginx and haproxy.
An attacker with low privileges and network access can exploit this by obtaining a legitimate leaf certificate from a trusted CA, such as a free Domain Validation certificate from Let's Encrypt. They can then forge a certificate for any subject name, embed any public key, and include arbitrary signature bytes in the chain. When processed by wolfSSL_X509_verify_cert, the function erroneously returns WOLFSSL_SUCCESS or X509_V_OK, enabling high-impact confidentiality and integrity violations, such as impersonation or man-in-the-middle attacks in compatible applications (CWE-295: Improper Certificate Validation).
Mitigation is addressed in a patch available via the wolfSSL GitHub pull request at https://github.com/wolfSSL/wolfssl/pull/10102, which security practitioners should review and apply to affected wolfSSL versions using the OpenSSL compatibility layer.
Details
- CWE(s)