CVE-2026-7840 UltraVNC Repeater HTTP Buffer Overflow RCE
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.
Quick answer
UltraVNC Repeater should be reviewed and updated if it matches the affected versions. The recommended fix is to apply the vendor-supported patched version or the mitigation steps below, then retest the public website with Fixnx.
Who is affected
Affected versions
- <= 1.8.2.2
Fixed versions
- No vendor fix published yet
How to fix it
UltraVNC Repeater is affected and no public fixed version is listed in the local advisory data yet. Reduce access, disable risky features where possible, and apply the vendor fix when it is released.
- Check where UltraVNC Repeater is installed and whether it is reachable from the internet.
- Limit access to trusted users, trusted networks, or a VPN until a vendor fix is available.
- Disable the affected feature or module if the advisory points to a specific upload, login, admin, chat, or network service path.
- Watch the vendor or advisory source and apply the first supported fix as soon as it is released.
- Review logs for strange access, failed attempts, crashes, file uploads, or unexpected account changes.
Scan now. Google sign-in is only needed to unlock fix guidance.
Verify the fix
- Confirm the installed version is no longer in the affected range.
- Retest the affected page, API, upload, login, chat, container, or network flow.
- Check logs after the change for blocked attempts, crashes, or errors.
- Run a fresh Fixnx scan and save the report.
Related categories
Trusted references
FAQ
What is affected by CVE-2026-7840?
UltraVNC Repeater versions listed as affected should be reviewed: <= 1.8.2.2.
What should I fix first?
Start with internet-facing sites, admin panels, login flows, plugins, themes, modules, packages, and systems that process user-controlled input or sensitive data.
How do I confirm the fix worked?
Apply the patched version or mitigation, clear caches where relevant, retest the affected workflow, and run a new Fixnx scan to verify public website exposure signals.
How are Fixnx security risk categories chosen?
Fixnx keeps one canonical risk page and assigns only broad, relevant categories such as ecosystem, technology area, or vulnerability class.