CVE-2026-44040

Description (CNA)

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.

Affected (CNA)

CWE

CWE-338

References

• https://uvnc.com/
• https://github.com/ultravnc/UltraVNC
• https://www.securin.io/zero-days/cve-2026-44040-libc-rand-weak-rng-vnc-auth-challenge-ultravnc

ADP enrichment