Buffer overflow in Uvnc Ultravnc
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/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.
Vulnerability class: Buffer Overflow
Published · last modified .
CVSS v3 metric
CVSS v3 base score 9.8 (Critical). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H.
EPSS exploit prediction
EPSS: 0.026 (85.0th percentile), scored .
Low probability of exploitation in the next 30 days, but still tracked. 85th percentile — 85.0% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.026 · newest: 0.026 · change: 0.000
Affected products
- Uvnc Ultravnc — versions 0
Weakness classification (CWE)
References
- 33c584b5-0579-4c06-b2a0-8d8329fcab9c (Product, vendor-advisory)
- 33c584b5-0579-4c06-b2a0-8d8329fcab9c (Product, product)
- 33c584b5-0579-4c06-b2a0-8d8329fcab9c
Frequently asked questions
- What is CVE-2026-7840?
- CVE-2026-7840 is a critical-severity vulnerability in Uvnc Ultravnc, classified under Out-of-bounds Write. CVSS score: 9.8/10. Published 2026-07-01.
- How severe is CVE-2026-7840?
- Critical severity. CVSS v3 base score is 9.8 out of 10.