Description
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.
Problem types
Integer Overflow or Wraparound
Product status
Any version
Timeline
| 2026-06-02: | Vulnerability discovered during security audit |
| 2026-06-17: | Reported to vendor (coordinated disclosure) |
| 2026-09-15: | Planned public disclosure (90-day window) |
Credits
Arjun Basnet, Securin (arjun.basnet@securin.io)
References
uvnc.com/ (UltraVNC project page)
github.com/ultravnc/UltraVNC (UltraVNC source repository)
www.securin.io/...-overflow-win-log-malloc-ultravnc-repeater