# CVE-2026-7840: UltraVNC Repeater global buffer overflow in HTTP administration server

Source: https://junglewise.ai/threats/cve-2026-7840-ultravnc-repeater-global-buffer-overflow-in-http-administration  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-07-01. Updated: 2026-07-01  
License: CC BY 4.0 with a link back

CVE-2026-7840 · Severity: critical · CVSS 9.8 · Published 2026-07-01

UltraVNC Repeater, a tool used to facilitate remote desktop connections through firewalls, contains a critical security flaw in its web-based administration interface. An unauthenticated attacker can send a specially crafted web request to take complete control of the server running the software. This could lead to a total compromise of the system, unauthorized access to remote sessions, and a foothold for further attacks within the corporate network.

Technologies: [UltraVNC Repeater](https://junglewise.ai/threats/technologies/repeater). Vendors: [UltraVNC](https://junglewise.ai/threats/vendors/ultravnc).

## Quick answers

**Is CVE-2026-7840 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-7840 being exploited in the wild.

**How severe is CVE-2026-7840?** Critical severity, with a CVSS score of 9.8, published 1 July 2026.

**Which products and versions are affected?** UltraVNC Repeater through 1.8.2.2.

**Is there a fix for CVE-2026-7840?** No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.

**How is CVE-2026-7840 attacked?** It is attacked over the network, in a network appliance.

**Am I affected by CVE-2026-7840?** Check whether you run UltraVNC Repeater at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## Technical details

A global buffer overflow exists in the embedded HTTP administration server of UltraVNC Repeater through version 1.8.2.2. The vulnerability is located in the wi_senderr() and wi_replyhdr() functions within repeater/webgui/webutils.c, which use unchecked sprintf calls to write the HTTP request URI into a fixed 1000-byte global buffer (hdrbuf). While the buffer is small, the HTTP receive buffer accepts URIs up to 150 KB, allowing an unauthenticated remote attacker to overflow the buffer by sending a URI longer than 1500 bytes. This results in the corruption of adjacent .bss-segment globals and can be leveraged to achieve arbitrary code execution. The flaw is reachable on the default TCP port 80 before any authentication checks occur.

## Affected products

- UltraVNC UltraVNC Repeater through 1.8.2.2

## Timeline

- 2026-07-01: advisory: NVD publication date

## References

- [https://github.com/ultravnc/UltraVNC](https://github.com/ultravnc/UltraVNC)
- [https://uvnc.com/](https://uvnc.com/)