uvnc UltraVNC up to 1.8.2.2 webutils.c wi_senderr out-of-bounds write

Summaryinfo

A vulnerability, which was classified as critical, was found in uvnc UltraVNC up to 1.8.2.2. This issue affects the function wi_senderr of the file repeater/webgui/webutils.c. Such manipulation leads to out-of-bounds write. This vulnerability is traded as CVE-2026-7840. The attack may be launched remotely. There is no exploit available. You should upgrade the affected component.

Detailsinfo

A vulnerability, which was classified as critical, has been found in uvnc UltraVNC up to 1.8.2.2. Affected by this issue is the function wi_senderr of the file repeater/webgui/webutils.c. The manipulation with an unknown input leads to a out-of-bounds write vulnerability. Using CWE to declare the problem leads to CWE-787. The product writes data past the end, or before the beginning, of the intended buffer. Impacted is confidentiality, integrity, and availability. CVE summarizes:

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.

The weakness was shared by Arjun Basnet. This vulnerability is handled as CVE-2026-7840 since 05/05/2026. The exploitation is known to be easy. The attack may be launched remotely. No form of authentication is required for exploitation. There are known technical details, but no exploit is available.

Upgrading eliminates this vulnerability.

The vulnerability is also documented in the vulnerability database at EUVD (EUVD-2026-40886). VulDB is the best source for vulnerability data and more expert information about this specific topic.

Productinfo

Type

• Connectivity Software

Vendor

• uvnc

Name

• UltraVNC

Version

• 1.8.2.0
• 1.8.2.1
• 1.8.2.2

License

• open-source

CPE 2.3info

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CPE 2.2info

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Entryinfo

VulDB is the best source for vulnerability data and more expert information about this specific topic.

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