CVE-2026-44040 json
UltraVNC vncauth.c uses time-seeded libc rand() to generate VNC authentication challenge bytes
published 2026-07-01 4 mo ago modified 2026-10-25 14 d ago also EUVD-2026-40879
- cvss
- 4.8
- version 3.1, CNA
- cisa kev
- Not listed
- not in CISA's catalog
- epss
- 0.4%
- EPSS 0.4% chance of exploitation in 30 days, percentile 34%
- public exploits
- 0
- none known
Summary
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.
CISA SSVC
- Exploitation
- none
- Automatable
- no
- Technical impact
- partial
- Assessed
- 4 mo ago
Exploit prediction
0.40% chance of exploitation in the next 30 days, higher than 34% of all scored CVEs (FIRST EPSS, 2026-11-08).
CVSS scores
| assigned by | version | score | vector |
|---|---|---|---|
| CNA | 3.1 | 4.8 | CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N |
| NVD | 3.1 | 6.5 | CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N |
Affected products
- uvnc UltraVNC
Classification
- Weakness
-
- CWE-338 Use of Cryptographically Weak Pseudo-Random Number Generator (PRNG)
References
-
uvnc.com
/
vendor-advisory, product, release-notes
-
github.com
/ultravnc/UltraVNC
product
- securin.io /zero-days/cve-2026-44040-libc-rand-weak-rng-vnc-auth-challenge-ultravnc
Sources
- CVE List CVE-2026-44040
- NVD CVE-2026-44040
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