CVE-2026-7840: critical vulnerability in uvnc UltraVNC
UltraVNC repeater HTTP server global buffer overflow via long URI (pre-auth RCE)
Published · Updated
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28Vexday Risk Score
No sign of exploitation. No public exploitation artifact known so far.
ssvc Trackcvss 9.3epss 2.6%
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/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N
Affected products
uvnc · UltraVNCRelated CVEs — uvnc UltraVNC
In the same product, most dangerous first.
CVE-2026-7838HIGHUltraVNC viewer heap buffer overflow via integer overflow in RFB connection-failure reason lengthEPSS 1.9%CVE-2026-7828MEDIUMUltraVNC repeater integer overflow in win_log malloc leading to heap overflowEPSS 1.4%CVE-2026-7829HIGHUltraVNC repeater authenticated out-of-bounds write in rule parser via oversized tokenEPSS 0.9%CVE-2026-7831HIGHUltraVNC viewer off-by-one stack overflow in ServerInit desktop name parsingEPSS 0.7%CVE-2026-7839CRITICALUltraVNC repeater ships hardcoded default admin password allowing unauthenticated admin accessEPSS 0.7%CVE-2026-44042LOWUltraVNC repeater wi_uudecode off-by-one in base64 decode boundary checkEPSS 0.5%