CVE-2026-7840: 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/webu

criticalCVSS 9.8CVE-2026-7840

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.

Details

Source: NVD Recent CVEs (US · database · site)
Severity: critical — CVSS 9.8
Published: 2026-07-01
Last updated: 2026-07-01
Exploitation: Not in CISA KEV at last sync

Original advisory: https://nvd.nist.gov/vuln/detail/CVE-2026-7840

Exploitation outlook

EPSS (FIRST.org) estimates each CVE’s probability of exploitation in the next 30 days — here is the CSIRTS.com read on those numbers.

Moderate exploitation riskCVE-2026-78401.4% 30-day exploitation probability. Patch within normal cadence, watch for KEV listing. Riskier than 70% of all scored CVEs.

Referenced CVEs

CVE	CSIRTS overview	External
CVE-2026-7840	coverage & exploitation status	NVD · CVE.org

Same CVEs, other sources

How other CERTs, PSIRTs and databases cover the vulnerabilities in this advisory.

high[NEW] [high] UltraVNC: Multiple vulnerabilitiescert-bund

Recent advisories for UltraVNC repeater

A cluster of recent advisories against the same product widens the attack surface — attackers routinely chain freshly published CVEs on one product, so review these together.

criticalCVE-2026-7839: UltraVNC repeater through 1.8.2.2 initializes the HTTP administration server with a hardcoded d…nvd · 2026-07-01
highCVE-2026-7829: UltraVNC repeater through 1.8.2.2 contains a post-authentication out-of-bounds write in the all…nvd · 2026-07-01
mediumCVE-2026-7828: UltraVNC repeater through 1.8.2.2 contains an integer overflow in the HTTP request logging path…nvd · 2026-07-01
lowCVE-2026-44042: UltraVNC repeater through 1.8.2.2 contains an off-by-one error in the Base64 decode helper use…nvd · 2026-07-01