CVE-2026-44040
UltraVNC vncauth.c uses time-seeded libc rand() to generate VNC authentication challenge bytes
Description
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.
INFO
Published Date :
July 1, 2026, 3:33 a.m.
Last Modified :
July 1, 2026, 3:33 a.m.
Remotely Exploit :
Yes !
Source :
securin
CVSS Scores
| Score | Version | Severity | Vector | Exploitability Score | Impact Score | Source |
|---|---|---|---|---|---|---|
| CVSS 3.1 | MEDIUM | 33c584b5-0579-4c06-b2a0-8d8329fcab9c |
Solution
- Update UltraVNC to a patched version.
- Verify secure random number generation is used.
- If possible, migrate to a more secure authentication method.
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