UltraVNC versions up to 1.8.2.2 employ a two-party authentication scheme that relies on a 64-bit Diffie-Hellman exchange and a pseudo-random private key generated from three libc rand() calls seeded with the current time. The 64-bit modulus can be factored in under one second with Pollard's rho, and the nearly 31-bit internal state of the rand() sequence can be recovered by a passive observer in less than a minute. An attacker who can observe the MS-Logon II handshaking traffic—whether by sniffing, recording or a man-in-the-middle attack—can therefore derive the shared session key and decrypt the encapsulated username and password, resulting in full credential disclosure. The weakness is categorized as inadequate cryptography (CWE-326) and weak random number generation (CWE-338).

The vulnerability affects the UltraVNC product line, specifically all releases through 1.8.2.2 that support the legacy MS-Logon II authentication protocol. Versions employing the newer X25519 + AES-256-GCM scheme (MS-Logon III) are not impacted.

The CVSS score of 7.4 indicates a high-severity flaw, and although an EPSS score is not available, the attack requires only passive network monitoring—something a threat actor can accomplish on any open network segment. The vulnerability is not listed in the CISA KEV catalog, but its mathematical weakness means a determined attacker can exploit it in minutes. Unless mitigation steps are applied, exposed services are susceptible to credential theft.