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CVE-2026-7840 – Pre-Auth RCE via Global Buffer Overflow – UltraVNC Repeater ≤ 1.8.2.2

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Summary

CVE-2026-7840 is a critical global buffer overflow in the embedded HTTP administration server of the UltraVNC Repeater, affecting all versions through 1.8.2.2. An unauthenticated remote attacker can send a single crafted HTTP request to overflow a fixed internal buffer and achieve arbitrary code execution on the host running the repeater. The vulnerability carries a CVSS 3.1 score of 9.8 (Critical) and a CVSS 4.0 score of 9.3 (Critical).

Technical details

  • Root cause: The functions wi_senderr() and wi_replyhdr() in repeater/webgui/webutils.c write attacker-supplied HTTP request URIs into a fixed 1000-byte global buffer (hdrbuf) via unchecked sprintf calls, with no bounds validation (CWE-787: Out-of-bounds Write).
  • Trigger conditions: The HTTP receive buffer (WI_RXBUFSIZE) accepts URIs up to approximately 150 KB. Sending a single HTTP request with a URI of 1,500 bytes or longer overflows hdrbuf by at least 500 bytes, corrupting adjacent .bss-segment globals.
  • Attack vector: Network-reachable, requires no privileges and no user interaction. The overflow is triggered before any authentication check executes. The repeater’s HTTP administration interface listens on TCP port 80 by default and is architecturally designed to be publicly accessible on a routable IP address.
  • Impact: Corruption of adjacent global memory in the .bss segment enables arbitrary remote code execution on the host running the repeater.

Affected software

  • UltraVNC Repeater all versions through 1.8.2.2

Severity

  • CVSS 3.1: 9.8 Critical — CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
  • CVSS 4.0: 9.3 Critical — CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H

Mitigation and recommended actions

  • Immediate: Upgrade to UltraVNC 1.8.2.3 or later. Version 1.8.2.3 (June 2026) introduced dedicated repeater security fixes; the latest available release is 1.8.2.4.
  • If immediate patching is not feasible: Restrict network access to the repeater’s HTTP administration port (default TCP 80) using firewall rules or network-level access controls, blocking connections from any untrusted or unanticipated sources until patching can be completed.

IONIX Status

The IONIX research team is tracking ongoing exploitation attempts and recommends immediate patching. Potentially affected assets are outlined in this post.

References

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