{"id":"CVE-2026-44040","url":"https://www.cyber-defence.io/tools/cve/CVE-2026-44040","generatedAt":"2026-10-28T02:13:52.106Z","title":"UltraVNC through 1.8.2.2 uses a cryptographically weak pseudo-random number generator to produce VNC authentication challenge bytes.","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.","published":"2026-07-01T05:16:21.000Z","lastModified":"2026-10-25T17:10:00.000Z","status":"Modified","sourceIdentifier":"33c584b5-0579-4c06-b2a0-8d8329fcab9c","cvss":{"version":"3.1","score":6.5,"severity":"MEDIUM","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N"},"cwe":["CWE-338"],"affected":["uvnc/ultravnc"],"epss":{"score":0.00397,"percentile":0.33669,"date":"2026-10-26","history":[]},"kev":{"listed":false},"exploits":{"count":0,"verified":false,"firstPublished":null,"source":"Exploit-DB (https://gitlab.com/exploit-database/exploitdb)","entries":[]},"verdict":{"level":"low","text":"Lower severity and a low EPSS score (0.40%). Track it; it rarely justifies an emergency change on its own."},"changes":[],"references":[{"url":"https://github.com/ultravnc/UltraVNC","tags":["Product"],"source":"33c584b5-0579-4c06-b2a0-8d8329fcab9c"},{"url":"https://uvnc.com/","tags":["Product","Release Notes"],"source":"33c584b5-0579-4c06-b2a0-8d8329fcab9c"},{"url":"https://www.securin.io/zero-days/cve-2026-44040-libc-rand-weak-rng-vnc-auth-challenge-ultravnc","source":"33c584b5-0579-4c06-b2a0-8d8329fcab9c"}],"sources":{"nvd":"https://nvd.nist.gov/vuln/detail/CVE-2026-44040","epss":"https://www.first.org/epss/","kev":"https://www.cisa.gov/known-exploited-vulnerabilities-catalog","exploitdb":"https://www.exploit-db.com/"},"licence":"CC BY 4.0 — link back to the CVE Explorer if you publish the results; upstream data remains subject to its own terms."}