UltraVNC through version 1.8.2.2 employs a weak PRNG in the vncRandomBytes() function for generating 16-byte VNC authentication challenges. The function seeds the libc rand() with a combination of time(0), getpid(), and rand(), producing a seed space of approximately 31 bits based on publicly observable values. This predictability allows an attacker who can observe the authentication exchange to enumerate the seed space and predict the challenge bytes within seconds, facilitating forgery or offline brute-force of authentication responses. On Windows, an alternative code path using CryptGenRandom may be present, but its reachability in shipped binaries is under investigation. There is no current official fix or patch disclosed.

The vulnerability allows attackers who can observe the authentication exchange to predict the VNC authentication challenge due to the weak PRNG seeding. This can lead to forgery of authentication challenges or enable offline brute-force attacks against authentication responses, potentially compromising authentication integrity. The impact is rated as medium with limited confidentiality and integrity impact and no availability impact.

Patch status is not yet confirmed — check the vendor advisory for current remediation guidance. Until an official fix is available, users should consider additional protective measures such as restricting network access to UltraVNC services and monitoring for suspicious authentication attempts. No vendor advisory or patch links are currently provided.