Vulnerability Details CVE-2026-44040
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.
Exploit prediction scoring system (EPSS) score
EPSS Score 0.002
EPSS Ranking 12.6%
CVSS Severity
CVSS v3 Score 4.8
Products affected by CVE-2026-44040
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cpe:2.3:a:uvnc:ultravnc:1.0.9.6.2
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cpe:2.3:a:uvnc:ultravnc:1.1.8.9
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cpe:2.3:a:uvnc:ultravnc:1.1.9.3
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cpe:2.3:a:uvnc:ultravnc:1.1.9.6
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cpe:2.3:a:uvnc:ultravnc:1.2.0.5
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cpe:2.3:a:uvnc:ultravnc:1.2.0.9
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cpe:2.3:a:uvnc:ultravnc:1.2.1.0
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cpe:2.3:a:uvnc:ultravnc:1.2.1.2
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cpe:2.3:a:uvnc:ultravnc:1.2.1.6
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cpe:2.3:a:uvnc:ultravnc:1.2.1.7
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cpe:2.3:a:uvnc:ultravnc:1.2.2.2
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cpe:2.3:a:uvnc:ultravnc:1.2.2.3
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cpe:2.3:a:uvnc:ultravnc:1.2.2.4
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cpe:2.3:a:uvnc:ultravnc:1.2.4.0
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cpe:2.3:a:uvnc:ultravnc:1.3.2
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cpe:2.3:a:uvnc:ultravnc:1.3.4
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cpe:2.3:a:uvnc:ultravnc:1.3.4.1
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cpe:2.3:a:uvnc:ultravnc:1.3.4.2
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cpe:2.3:a:uvnc:ultravnc:1.3.6.0
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cpe:2.3:a:uvnc:ultravnc:1.3.8.0
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cpe:2.3:a:uvnc:ultravnc:1.3.8.1
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cpe:2.3:a:uvnc:ultravnc:1.4.0.5
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cpe:2.3:a:uvnc:ultravnc:1.4.0.6
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cpe:2.3:a:uvnc:ultravnc:1.4.0.9
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cpe:2.3:a:uvnc:ultravnc:1.4.2.0
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cpe:2.3:a:uvnc:ultravnc:1.4.3.0
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cpe:2.3:a:uvnc:ultravnc:1.4.3.1
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cpe:2.3:a:uvnc:ultravnc:1.4.3.5
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cpe:2.3:a:uvnc:ultravnc:1.4.3.6
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cpe:2.3:a:uvnc:ultravnc:1.5.0.0
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cpe:2.3:a:uvnc:ultravnc:1.6.0.0
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cpe:2.3:a:uvnc:ultravnc:1.6.1.0
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cpe:2.3:a:uvnc:ultravnc:1.6.4.0
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cpe:2.3:a:uvnc:ultravnc:1.8.2.2