VulnerabilityModified

CVE-2026-7840

UltraVNC repeater through 1.8.2.2 contains a global buffer overflow in its embedded HTTP administration server.

CRITICAL 9.3EPSS 2.63%

Does this matter?

High impact if exploited, but EPSS currently rates exploitation as unlikely (2.63%). Schedule it in the normal patch cycle and watch for a rise in EPSS or a public exploit.

Description

UltraVNC repeater through 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. The overflow occurs before any authentication check, making it reachable without credentials. A remote, unauthenticated attacker can achieve arbitrary code execution on the host running the repeater.

CVSS 4.0: 9.3 CRITICALCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/M
EPSS: 2.63% probability · 85th percentile
CISA KEV: Not listed
Weakness: CWE-787
Affected: uvnc/ultravnc
Source: 33c584b5-0579-4c06-b2a0-8d8329fcab9c

References

Source: NVD record, EPSS from FIRST.org, KEV from CISA. Refreshed daily.