CVE-2026-7840
Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.
NVD · uneditedUltraVNC 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.
Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.
dbcve analysis · high confidenceUltraVNC repeater 1.8.2.2 and earlier contains a critical buffer overflow in its HTTP administration server. The wi_senderr() and wi_replyhdr() functions in webutils.c copy the HTTP request URI into a fixed 1000-byte global buffer (hdrbuf) using unchecked sprintf, while the server accepts URIs up to 153,600 bytes. An unauthenticated attacker sending a URI of 1500+ bytes can overflow hdrbuf, corrupt adjacent .bss globals, and achieve arbitrary code execution before any authentication occurs.
Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.
Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.
NVD · CPE data<= 1.8.2.2CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.
From the vector- Attack vector
- Network
- Complexity
- Low
- Privileges
- None
- User interaction
- None
- Scope
- Unchanged
- Confidentiality
- High
- Integrity
- High
- Availability
- High
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.
dbcve checksWork through these to decide whether this CVE applies to you.
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Confirm UltraVNC Repeater is installedCheck for UltraVNC Repeater service or executable on the system. On Windows, look in Program Files for UltraVNC or check Services for a repeater service. On Linux, check for the repeater binary in typical installation paths.Affected if UltraVNC Repeater software is present on the system
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Identify installed UltraVNC Repeater versionRight-click the UltraVNC Repeater executable, select Properties, and check the Version tab. Alternatively, check the version reported by the running service if accessible.Affected if The installed version is 1.8.2.2 or any earlier version (e.g., 1.8.2.1, 1.8.0.x, etc.)
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Verify HTTP administration server is enabledCheck if the HTTP admin interface is listening. Use netstat or equivalent to see if TCP port 80 (or configured HTTP port) is in LISTEN state associated with the UltraVNC Repeater process.Affected if The HTTP admin server is actively listening on a TCP port (default 80)
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Confirm network accessibility of HTTP admin portVerify the HTTP port is exposed to the network. Check firewall rules or attempt a local connection to the HTTP port (e.g., curl http://localhost:80) to confirm the service accepts connections.Affected if The HTTP admin port is accessible from the network (not blocked by firewall)
A system is affected if UltraVNC Repeater version 1.8.2.2 or earlier is installed AND the HTTP administration server is enabled and accessible, allowing an attacker to send a URI exceeding 1500 bytes to trigger the buffer overflow.
Generated from the published advisory. Verify against your own configuration.
Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.
From vendor dataRestrict network access to the UltraVNC repeater HTTP port (default TCP 80) via firewall rules or disable the HTTP admin interface until a patched version is available; implementing proper bounds checking in wi_senderr() and wi_replyhdr() functions is required for a permanent fix.
- Consultation3.0 h
- Implementation6.0 h
- Testing4.0 h
- Review / QA2.0 h
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Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2026-7840 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2026-7840 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
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- Verified mitigations, workarounds, and config changes
- Version or environment caveats, and links to real fixes
- No weaponised exploit code, or anything meant to cause harm
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