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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.

PUBLISHED Reserved 2026-05-05 | Published 2026-07-01 | Updated 2026-07-09 | Assigner securin




CRITICAL: 9.3CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N
CRITICAL: 9.8CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Problem types

Out-of-bounds Write

Product status

Default status
unaffected

Any version
affected

Timeline

2026-06-02:Vulnerability discovered during security audit
2026-06-17:Reported to vendor (coordinated disclosure)
2026-09-15:Planned public disclosure (90-day window)

Credits

Arjun Basnet, Securin (arjun.basnet@securin.io) finder

References

uvnc.com/ (UltraVNC project page) vendor-advisory

github.com/ultravnc/UltraVNC (UltraVNC source repository) product

www.securin.io/...ffer-overflow-repeater-http-admin-ultravnc

cve.org (CVE-2026-7840)

nvd.nist.gov (CVE-2026-7840)

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