CVE-2026-7828
MediumCVSS 5.3Exploitation Probability (EPSS)
Elevated risk60th percentile - higher than 60% of all known CVEs
Summary
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.
Risk Assessment
The risk for the organization is a remote unauthenticated attack that may cause service instability or potentially lead to privilege escalation if further exploited.
Recommendation
Immediately update UltraVNC repeater to a version newer than 1.8.2.2 once a patch is released. Until then, restrict access to the repeater HTTP port to trusted networks only.
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-2026-44041Medium
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 repeater through 1.8.2.2 contains an integer overflow in the HTTP request logging path. In repeater/webgui/settings.c:336, the win_log() function allocates list nodes via malloc(sizeof(struct LIST) + strlen(line)), where line is derived from HTTP request URIs. If strlen(line) is sufficiently large, the addition overflows to a value smaller than sizeof(struct LIST), causing a heap allocation smaller than required. The subsequent strcpy of the full string into the undersized allocation produces a heap buffer overflow. In the current implementation this overflow is bounded by the HTTP receive buffer size (WI_RXBUFSIZE = 153600 bytes, well below SIZE_MAX on 32-bit builds), limiting practical exploitability to a partial heap write. A remote unauthenticated attacker can trigger the theoretical overflow path by sending a maximally-sized URI in an HTTP request to the repeater HTTP port.