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CVE-2026-7840

9.8 · CRITICAL

UltraVNC repeater through 1.8.2.2 contains a global buffer overflow in its embedded HTTP administration server.

Published Jul 1, 2026CWE-787EPSS 2.63%

Overview

CVE-2026-7840 is a critical-severity vulnerability. It was published on July 1, 2026 and has a CVSS 3.1 base score of 9.8 (CRITICAL).

This vulnerability has a CVSS 3.1 base score of 9.8, rated CRITICAL. It can be exploited remotely over the network. No authentication or special privileges are required for exploitation.

Technical 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.

Remediation

Check the references section for vendor advisories, patches, and mitigation guidance. If immediate patching is not possible, review the CVSS vector to understand the attack surface and apply compensating controls such as network segmentation or access restrictions.

References

GitHubgithub.com/ultravnc/UltraVNCRefuvnc.com/Refwww.securin.io/zero-days/cve-2026-7840-pre-auth-global-buffer-overflow-repeater-http-admin-ultravnc

How CyberStrike helps

Frequently asked questions

What is CVE-2026-7840?

CVE-2026-7840 is a critical-severity vulnerability. It was published on July 1, 2026 and has a CVSS 3.1 base score of 9.8 (CRITICAL).

How severe is CVE-2026-7840?

This vulnerability has a CVSS 3.1 base score of 9.8, rated CRITICAL. It can be exploited remotely over the network. No authentication or special privileges are required for exploitation.

How do I fix or remediate CVE-2026-7840?

Check the references section for vendor advisories, patches, and mitigation guidance. If immediate patching is not possible, review the CVSS vector to understand the attack surface and apply compensating controls such as network segmentation or access restrictions.

How can CyberStrike help with CVE-2026-7840?

CyberStrike is an open-source, AI-augmented pentesting engine. Rather than flag a version number, it tests whether an issue like CVE-2026-7840 is actually reachable in your own applications and APIs, then verifies each finding before reporting it. That tells you if the weakness is a real, exploitable risk in your environment or a false positive you can safely deprioritize.

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