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Review reviewCritical

CVE-2026-7840

uvnc UltraVNC, ultravnc

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, corrupt...

CVSS
9.3
EPSS
1.44%
70.2% percentile
CISA KEV
Not listed
Published
2026.07.01
PRIORITY ASSESSMENT

Review review

The CVSS severity warrants an early asset and exposure review.

Known exploitationNot established by KEV
Exploit probability1.44%
Technical severityCVSS 9.3

Vulnerability overview

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, corrupt...

Affected product and versions

Product
uvnc UltraVNC, ultravnc
Affected versions
<= 1.8.2.2
Fixed versions
No verified fixed-version field is available yet

Recommended response sequence

Confirm exposure before applying a vendor-supported change.

Full remediation guide
  1. 1
    Identify

    Confirm that uvnc UltraVNC, ultravnc and an affected version are present.

  2. 2
    Prioritize

    Combine exploitation signals with asset exposure and business criticality.

  3. 3
    Remediate

    Follow the vendor advisory or supported update path and preserve rollback options.

  4. 4
    Verify

    Recheck the version, service health, access paths, and relevant logs.

Technical data

CVSS vector
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
CWE
CWE-787