# CVE-2026-44040

## Summary

- **CVE ID:** CVE-2026-44040
- **Severity:** MEDIUM
- **CVSS Score:** 4.8 (CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N)
- **CWE:** CWE-338
- **Published:** Jul 1, 2026
- **Last Modified:** Jul 9, 2026

## 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.

## Affected Products

- uvnc — UltraVNC (0)

## References

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

## Exploitation Prediction (EPSS)

- **EPSS Score:** 0.40%
- **EPSS Percentile:** 33.2

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_Exported from OnDuty AI Vulnerability Intelligence on 2026-09-12._