CVE-2026-5188: Integer Underflow in wolfSSL

The integer underflow vulnerability in wolfSSL allows attackers to craft malicious X.509 certificates that can bypass certificate validation checks. By specifying an entry length in the SAN extension that exceeds the allocated buffer, the parsing process can wrap around, leading to out-of-bounds reads or writes. This could potentially allow an attacker to inject arbitrary data into the application's memory, leading to denial of service, or, in more severe cases, remote code execution. The impact is particularly concerning for applications relying on wolfSSL for secure communication, such as TLS servers and clients, VPNs, and embedded devices.

1. Reserved2026-03-30
2. Published2026-04-10
3. EPSS updated2026-04-17

The primary mitigation for CVE-2026-5188 is to upgrade to wolfSSL version 5.11.0 or later, which includes a fix for the integer underflow vulnerability. If upgrading is not immediately feasible, consider implementing stricter certificate validation checks to reject certificates with unusually large SAN entries. While not a complete fix, this can provide a temporary layer of defense. Additionally, review your application's certificate parsing logic to ensure it is robust against malformed certificates. After upgrading, confirm the fix by attempting to parse a crafted certificate with an oversized SAN entry; the parsing should fail gracefully without triggering an error or crash.