CVE-2026-44041
MediumCVSS 4.3Exploitation Probability (EPSS)
Low risk24th percentile - higher than 24% of all known CVEs
Summary
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.
Risk Assessment
The risk includes potential information disclosure from adjacent memory regions or a process crash (denial of service) if the over-read crosses a page boundary. Under typical Win32 API usage, this requires an abnormal caller contract.
Recommendation
It is recommended to update UltraVNC to a version newer than 1.8.2.2 as soon as it becomes available. Until then, avoid using non-standard or untrusted input sources.
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-2026-44040Medium
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 through 1.8.2.2 contains an out-of-bounds read in the wide-string to multibyte conversion helper. In rfb/dh.cpp:204, the vncWc2Mb() function passes a caller-supplied WCHAR pointer to wcslen() before any bounds check. If the caller provides a wide-character buffer that is not properly NUL-terminated, wcslen() reads past the end of the buffer until it encounters a NUL wchar, resulting in an out-of-bounds read. Under typical Win32 API usage this requires an abnormal caller contract. Impact is limited to a potential information disclosure from adjacent memory regions or a process crash (denial of service) if the over-read crosses a page boundary.