Description
UltraVNC repeater through 1.8.2.2 contains a post-authentication out-of-bounds write in the allow/deny rule parser. In repeater/webgui/settings.c:225-272, after strncpy_s copies a rule token into temp1[rule1] (25-byte destination) or temp2/temp3 (16-byte destination), the code unconditionally writes a NUL terminator at temp1[rule1][len] = 0 without clamping len to the destination size. When an authenticated administrator saves a rule with a token length equal to or greater than the destination size, the NUL byte is written one or more bytes past the end of the stack-allocated array, corrupting adjacent stack data. An attacker who has obtained admin credentials (including via CVE-2026-7839 default password) can trigger this to gain code execution on the repeater host.
Problem types
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/...ob-nul-write-repeater-rule-parser-ultravnc