CVE-2026-7840 (GCVE-0-2026-7840)

Vulnerability from cvelistv5 – Published: 2026-07-01 03:33 – Updated: 2026-07-09 04:36
VLAI
Title
UltraVNC repeater HTTP server global buffer overflow via long URI (pre-auth RCE)
Summary
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.
Severity
9.8 (Critical)
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
SSVC
Exploitation: none Automatable: yes Technical Impact: total
CISA Coordinator · CISA-ADP (v2.0.3)
Decision recorded 2026-07-01 12:33 UTC
CWE
Assigner
securin CNA under the mitre root
CNA scorecard B 80/100 over 66 records in the last 180 days details
References
Impacted products
Vendor Product Version CPE status
uvnc UltraVNC Affected: 0 , ≤ 1.8.2.2 (custom)
guessed Create a notification for this product.
Credits
Arjun Basnet, Securin (arjun.basnet@securin.io)
Show details on NVD website
To clipboard 

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