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🛡️ CVE-2026-7840 — ultravnc

🔴 CVSS 9.8 — Critical ✅ No Known Exploit CWE-787 NVD
9.8
CVSS Score
0 Low4 Medium7 High9 Critical10

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.

How this vulnerability can be exploited

This issue can be reached over the network, attack complexity is low, an attacker needs no privileges on the target. No user interaction is required. The scope is unchanged, so the impact stays within the vulnerable component. Rated impact: confidentiality high, integrity high, availability high.

CVSS metrics in full

The score comes from this vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

  • Attack vector: Network — reachable from anywhere that can route to the service.
  • Attack complexity: Low — the attack works reliably, with no preparation.
  • Privileges required: None — an unauthenticated stranger can try it.
  • User interaction: None — nobody has to be tricked into anything.
  • Scope: Unchanged — the damage stays inside the vulnerable component.
  • Confidentiality impact: High — total loss, or loss the attacker controls.
  • Integrity impact: High — total loss, or loss the attacker controls.
  • Availability impact: High — total loss, or loss the attacker controls.

Weakness class

CVE-2026-7840 is classified as CWE-787: Out-of-bounds Write. Data is written past the end or before the start of a buffer, corrupting whatever is stored there.

Affected software

CVE-2026-7840 is recorded against 1 package.

  • ultravnc (fixed in 1.8.2.2)

Timeline and source

Published on 1 July 2026 and last revised on 9 July 2026. No public exploit is currently recorded for this entry. Record sourced from NVD.

References

github.com
uvnc.com
www.securin.io

Other advisories for this package

ultravnc has other advisories on record. If you are patching this one, these are worth checking on the same host:

Same weakness in other software

These advisories are the same class of weakness (CWE-787: Out-of-bounds Write) in other software:

Details

Severity CRITICAL
CVSS Score 9.8
CVSS Vector CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
CWE CWE-787
Public Exploit ✅ No
Source NVD
Published 2026-07-01
Updated 2026-08-20
Modified 2026-07-09
Fix URL N/A

Affected Packages

Software From version Fixed in
ultravnc — 1.8.2.2

Similar Threats

Exploit Protection

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ⓘ Data Notice: The information presented above has been compiled from publicly available internet sources. Boteraser aggregates this data solely for informational purposes and does not independently classify, evaluate, or endorse any findings about the vulnerabilities listed. The accuracy and completeness of this information is the sole responsibility of the original publishers. Boteraser and its operators accept no liability for any decisions made based on this data.

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