CVE-2026-44040

[CVE catalog]

Severity

Medium · CVSS 4.8CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N

Published

Jun 30, 2026, 10:00:00 PM UTC

Source

[NVD] · [CVE record]

EPSS

0.40% · Oct 25, 2026

UltraVNC through 1.8.2.2 uses a cryptographically weak pseudo-random number generator to produce VNC authentication challenge bytes. In rfb/vncauth.c:119-129, the vncRandomBytes() function seeds libc rand() with time(0) + getpid() + rand() and generates a 16-byte challenge. The combined seed space is approximately 31 bits (libc rand() internal state) and is entirely determined by publicly-observable values (wall-clock time and process ID). An attacker who can observe the authentication exchange can enumerate the seed space and predict the challenge within seconds, enabling forgery or offline brute-forcing of responses. Note: on Windows, the active code path may use vncEncryptBytes2.cpp which calls CryptGenRandom; reachability on shipped Windows binaries requires compile-graph verification and is under investigation.