Vulnerability 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 Score
CRITICAL
Affected Products
| Vendor | Product | Versions |
|---|---|---|
| Uvnc | Ultravnc | <= 1.8.2.2 |
Related Weaknesses (CWE)
References
- https://github.com/ultravnc/UltraVNCProduct
- https://uvnc.com/Product
- https://www.securin.io/zero-days/cve-2026-7840-pre-auth-global-buffer-overflow-r
FAQ
What is CVE-2026-7840?
CVE-2026-7840 is a vulnerability with a CVSS score of 9.8 (CRITICAL). 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 cal...
How severe is CVE-2026-7840?
CVE-2026-7840 has been rated CRITICAL with a CVSS base score of 9.8/10. This is considered a critical vulnerability requiring immediate attention.
Is there a patch for CVE-2026-7840?
Check the references section above for vendor advisories and patch information. Affected products include: Uvnc Ultravnc.