Description
UltraVNC repeater through 1.8.2.2 contains an off-by-one error in the Base64 decode helper used for HTTP Basic authentication. In repeater/webgui/webutils.c:817, the wi_uudecode() function checks whether the input length exceeds the output buffer with a strict greater-than comparison (>), while the correct check should be greater-than-or-equal (>=). When strlen(authdata) equals sizeof(decode), the decoded output length (approximately 3/4 of input) does not overflow the buffer in current practice because the outer HTTP request bounds constrain the Authorization header. However, the defective check leaves a latent off-by-one condition that could become exploitable if the buffering constraints change. The current risk is limited to a one-byte write at the boundary of a 1024-byte stack buffer under constrained conditions.
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/...ecode-off-by-one-bounded-ultravnc-repeater