CVE-2026-44040

MEDIUM

UltraVNC vncauth.c uses time-seeded libc rand() to generate VNC authentication challenge bytes

Title source: cna
STIX 2.1

Exploitation Summary

EIP tracks 1 public exploit for CVE-2026-44040. PoCs published by HermesNA-1.

AI-analyzed exploit summary This repository contains an auto-generated stub module for CVE-2026-44040, describing a cryptographically weak pseudo-random number generator in UltraVNC through 1.8.2.2. The code includes placeholder methods (`check` and `run`) with no actual exploit implementation, only warnings to replace with real PoC code.

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.

Exploits (1)

github STUB 1 stars
by HermesNA-1 · pythonpoc
https://github.com/HermesNA-1/SnakeSploit/tree/main/data/modules_generated/cve-2026-44040_ultravnc_through_1822.py

This repository contains an auto-generated stub module for CVE-2026-44040, describing a cryptographically weak pseudo-random number generator in UltraVNC through 1.8.2.2. The code includes placeholder methods (`check` and `run`) with no actual exploit implementation, only warnings to replace with real PoC code.

Classification
Stub 99%
Attack Type
Auth Bypass
Complexity
Moderate
Reliability
Theoretical
Target: UltraVNC through 1.8.2.2
No auth needed
Prerequisites: Network access to UltraVNC server (default port 5900)
mistral-large-3 · analyzed Jul 09, 2026 Full analysis →

References (3)

Core 3
Core References
Vendor Advisory vendor-advisory
UltraVNC project page
https://uvnc.com/
Product product
UltraVNC source repository
https://github.com/ultravnc/UltraVNC

Scores

CVSS v3 4.8
EPSS 0.0028
EPSS Percentile 20.6%
Attack Vector NETWORK
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N

CISA SSVC

Vulnrichment
Exploitation none
Automatable no
Technical Impact partial

Details

CWE
CWE-338
Status published
Products (2)
uvnc/UltraVNC < 1.8.2.2
uvnc/ultravnc < 1.8.2.2
Published Jul 01, 2026
Tracked Since Jul 01, 2026