CVE-2026-44040
MEDIUMDescription
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.
How to fix
No published remediation has been found for this vulnerability's affected products yet.
Mitigation guidance may be in the linked vendor advisories in the References section below.
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CVSS v3 Vector
Exploitability
Impact
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N
Exploit Intelligence
Low risk: more likely to be exploited than 33% of all known CVEs.
References
Related Vulnerabilities
Other CWE-338 vulnerabilities, ordered by exploit likelihood. View all
| CVE | Severity | CVSS | EPSS | Exploited | Fix |
|---|---|---|---|---|---|
| CVE-2008-0166 | High | 7.5 | 71% | - | Fix |
| CVE-2009-2367 | Critical | 9.8 | 23% | - | - |
| CVE-2024-29868 | Critical | 9.1 | 6.0% | - | - |
| CVE-2024-31497 | Medium | 5.9 | 5.8% | - | Fix |
| CVE-2021-43799 | High | 8.6 | 5.4% | - | Fix |
| CVE-2008-3280 | Medium | 5.9 | 4.0% | - | - |
Common questions
How do I fix CVE-2026-44040?
No published fix for CVE-2026-44040 has been found yet. Check the linked vendor advisories for guidance as it becomes available.
Is CVE-2026-44040 being actively exploited?
Not that we know of. CVE-2026-44040 is not in the CISA Known Exploited Vulnerabilities catalog. Its EPSS score of 0.40% is the estimated probability that it will be exploited in the next 30 days. That is higher than 33% of all scored CVEs.
How severe is CVE-2026-44040?
CVE-2026-44040 has a CVSS v3 base score of 4.8, rated medium. CVSS rates the technical impact if the vulnerability is exploited, not how likely that is, so weigh it alongside the exploit-prediction score when you decide what to patch first.
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This product uses NVD data but is not endorsed or certified by the NVD. EPSS scores courtesy of FIRST.org (https://www.first.org/epss). Source: CISA KEV Catalog. Data as of 2026-07-09.