CVE-2026-7831
HighCVSS 7.6Exploitation Probability (EPSS)
Low risk41th percentile - higher than 41% of all known CVEs
Summary
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.
Risk Assessment
A malicious VNC server can exploit this vulnerability to crash the client (denial of service) or potentially execute arbitrary code if stack canaries are disabled. The attack requires user interaction by connecting to the malicious server.
Recommendation
Immediately update UltraVNC to a version newer than 1.8.2.2 once available. Until then, avoid connecting to untrusted VNC servers.
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-2026-7830High
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.
- CVE-2026-44040Medium
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.
Original NVD description (English source)
UltraVNC viewer through 1.8.2.2 contains an off-by-one stack buffer overflow in the RFB ServerInit message handler. In vncviewer/ClientConnection.cpp, when the server-supplied nameLength equals exactly 2024 the code declares a 2024-byte stack buffer _dn[2024] and calls ReadString(_dn, 2024). ReadString writes the NUL terminator at buf[length], i.e., _dn[2024], one byte past the end of the stack buffer. A malicious VNC server can trigger this condition by advertising a desktop name of length 2024 in its ServerInit message. On release builds without stack canaries the single-byte NUL overwrite adjacent stack data. On builds with /GS stack protection the canary is corrupted and the process terminates, resulting in denial of service. User interaction (connecting the viewer to the malicious server) is required.