CVE-2026-44040

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.

CVSS v3 4.8 MEDIUM

4.8^/10

CVSS v3 : MEDIUM

Vector :

Exploitability : 2.2 / Impact : 2.5

Attack Vector NETWORK

Attack Complexity HIGH

Privileges Required NONE

User Interaction NONE

Confidentiality Impact LOW

Integrity Impact LOW

Availability Impact NONE

Scope UNCHANGED

References

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

Configurations

Configuration 1 (hide)

cpe:2.3:a:uvnc:ultravnc:*:*:*:*:*:*:*:*

History

09 Jul 2026, 06:16

Type	Values Removed	Values Added
References		() https://www.securin.io/zero-days/cve-2026-44040-libc-rand-weak-rng-vnc-auth-challenge-ultravnc -

02 Jul 2026, 14:58

Type	Values Removed	Values Added
CPE		cpe:2.3:a:uvnc:ultravnc::::::::
References	~~() https://github.com/ultravnc/UltraVNC -~~	() https://github.com/ultravnc/UltraVNC - Product
References	~~() https://uvnc.com/ -~~	() https://uvnc.com/ - Product, Release Notes
First Time		Uvnc Uvnc ultravnc

01 Jul 2026, 05:16

Type	Values Removed	Values Added
New CVE

Information

Published : 2026-07-01 05:16

Updated : 2026-07-09 06:16

NVD link : CVE-2026-44040

Mitre link : CVE-2026-44040

CVE.ORG link : CVE-2026-44040

JSON object : View

Products Affected

uvnc

ultravnc

CWE

CWE-338

Use of Cryptographically Weak Pseudo-Random Number Generator (PRNG)

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