UltraVNC through version 1.8.2.2 uses a weak PRNG for generating VNC authentication challenges. The vncRandomBytes() function seeds the libc rand() function with a combination of time(0), getpid(), and rand(), resulting in a combined seed space of approximately 31 bits. Because these seed values are publicly observable or guessable, an attacker can enumerate the seed space and predict the 16-byte challenge within seconds. This predictability undermines the security of the VNC authentication mechanism, allowing potential forgery or offline brute-force attacks on authentication responses. On Windows, an alternative code path using CryptGenRandom may be present, but its reachability in shipped binaries is under investigation. No vendor advisory or patch is currently available.

The vulnerability allows an attacker who can observe the VNC authentication exchange to predict the authentication challenge bytes due to the weak PRNG seeding. This can enable forgery of authentication challenges or offline brute-force attacks against authentication responses, potentially compromising authentication integrity. The impact is limited to confidentiality and integrity with no reported impact on availability. The CVSS score of 4.8 reflects a medium severity with network attack vector, high attack complexity, no privileges required, and no user interaction needed.

Patch status is not yet confirmed — check the vendor advisory for current remediation guidance. Until an official fix is available, users should consider alternative secure VNC solutions or additional network-level protections such as VPNs or strong access controls to mitigate exposure. Avoid relying solely on UltraVNC authentication in environments where an attacker can observe authentication exchanges.