CVE-2026-44040
MediumCVSS 4.8Exploitation Probability (EPSS)
Low risk20th percentile - higher than 20% of all known CVEs
Summary
UltraVNC through 1.8.2.2 uses a cryptographically weak pseudo-random number generator to produce VNC authentication challenge bytes. The vncRandomBytes() function seeds libc rand() with time(0) + getpid() + rand(), resulting in a seed space of approximately 31 bits, entirely determined by publicly observable values. An attacker can predict the challenge within seconds, enabling forgery or offline brute-forcing of responses.
Risk Assessment
The organization risks session hijacking where an attacker observing the authentication exchange can predict the challenge and gain unauthorized access to remote desktops, leading to data breaches or system compromise.
Recommendation
Immediately upgrade UltraVNC to a version later than 1.8.2.2 that fixes the weak generator. Until then, restrict VNC server access to trusted networks and consider using VPN tunneling.
Other vulnerabilities in UltraVNC
See all- CVE-2026-7838High
In UltraVNC viewer up to version 1.8.2.2, an integer overflow leads to a heap buffer overflow in the RFB protocol failure-response parsing path. The 4-byte reasonLen field (CARD32) is passed as reasonLen+1 to CheckBufferSize(), and when set to 0xFFFFFFFF it overflows to 0, causing allocation of only 256 bytes. The subsequent ReadString() then reads the original 4 GiB into that small buffer, causing overflow.
- CVE-2026-7831High
UltraVNC viewer up to version 1.8.2.2 contains an off-by-one stack buffer overflow in the RFB ServerInit message handler. When the server supplies a desktop name of exactly 2024 bytes, the ReadString function writes a null terminator beyond the allocated buffer, potentially corrupting stack data.
- CVE-2026-7830High
UltraVNC up to version 1.8.2.2 uses weak cryptography in the MS-Logon II authentication scheme. The Diffie-Hellman key exchange relies on 64-bit parameters that can be broken in under a second, and the random number generator based on rand() with a time seed allows private key recovery within a minute.
- CVE-2026-7828Medium
UltraVNC repeater up to version 1.8.2.2 contains an integer overflow in the win_log() function during memory allocation for list nodes. An attacker can send a sufficiently long HTTP URI, causing a heap buffer overflow that allows a partial out-of-bounds write.
- CVE-2026-44041Medium
A vulnerability in UltraVNC up to version 1.8.2.2 causes an out-of-bounds read in the wide-string to multibyte conversion helper. The vncWc2Mb() function calls wcslen() on a caller-supplied buffer without prior bounds checking, potentially reading past the allocated memory.
Original NVD description (English source)
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.