CVE-2016-5673

UltraVNC repeater through version 1.8.2.2 contains a global buffer overflow in its embedded HTTP administration server. The functions wi_senderr() and wi_replyhdr() in repeater/webgui/webutils.c write the caller-supplied HTTP request URI into a fixed 1000-byte global buffer (hdrbuf) via unchecked sprintf calls. The HTTP receive buffer accepts URIs up to approximately 150 KB (WI_RXBUFSIZE = 153600), so an unauthenticated attacker who can reach the repeater HTTP port (default TCP 80) can overflow hdrbuf by at least 500 bytes with a single HTTP request containing a URI of 1500 bytes or longer, corrupting adjacent .bss-segment globals.

UltraVNC repeater up to version 1.8.2.2 initializes the HTTP administration server with a hardcoded default password. On first run, when settings2.txt is absent, it writes the literal string "adminadmi2" as the admin password. The HTTP Basic-auth handler checks this password without rate-limiting or lockout.

In UltraVNC repeater up to version 1.8.2.2, a post-authentication out-of-bounds write vulnerability exists. In the allow/deny rule parser, after copying a token into a fixed-size buffer, the code unconditionally writes a NUL terminator at position len without checking if it exceeds the buffer size. An authenticated administrator can save a rule with a token length equal to or greater than the buffer size, overwriting adjacent stack data.

UltraVNC repeater up to version 1.8.2.2 contains an off-by-one error in the Base64 decode helper used for HTTP Basic authentication. The wi_uudecode() function in webutils.c:817 uses a strict greater-than comparison (>) instead of greater-than-or-equal (>=), leaving a latent vulnerability. Currently, the risk is limited to a one-byte write at the boundary of a 1024-byte stack buffer.