CVE-2026-7840

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.

CVSS v3 9.8 CRITICAL

9.8^/10

CVSS v3 : CRITICAL

Vector :

Exploitability : 3.9 / Impact : 5.9

Attack Vector NETWORK

Attack Complexity LOW

Privileges Required NONE

User Interaction NONE

Confidentiality Impact HIGH

Integrity Impact HIGH

Availability Impact HIGH

Scope UNCHANGED

References

Link	Resource
https://github.com/ultravnc/UltraVNC	Product
https://uvnc.com/	Product
https://www.securin.io/zero-days/cve-2026-7840-pre-auth-global-buffer-overflow-repeater-http-admin-ultravnc

Configurations

Configuration 1 (hide)

cpe:2.3:a:uvnc:ultravnc:*:*:*:*:*:*:*:*

History

No history.

Information

Published : 2026-07-01 05:16

Updated : 2026-07-09 06:16

NVD link : CVE-2026-7840

Mitre link : CVE-2026-7840

CVE.ORG link : CVE-2026-7840

JSON object : View

Products Affected

uvnc

ultravnc

CWE

CWE-787

Out-of-bounds Write

{"id": "CVE-2026-7840", "cveTags": [], "metrics": {"ssvcV203": [{"source": "134c704f-9b21-4f2e-91b3-4a467353bcc0", "ssvcData": {"id": "CVE-2026-7840", "role": "CISA Coordinator", "options": [{"exploitation": "none"}, {"automatable": "yes"}, {"technicalImpact": "total"}], "version": "2.0.3", "timestamp": "2026-07-01T12:33:40.320112Z"}}], "cvssMetricV31": [{"type": "Secondary", "source": "33c584b5-0579-4c06-b2a0-8d8329fcab9c", "cvssData": {"scope": "UNCHANGED", "version": "3.1", "baseScore": 9.8, "attackVector": "NETWORK", "baseSeverity": "CRITICAL", "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H", "integrityImpact": "HIGH", "userInteraction": "NONE", "attackComplexity": "LOW", "availabilityImpact": "HIGH", "privilegesRequired": "NONE", "confidentialityImpact": "HIGH"}, "impactScore": 5.9, "exploitabilityScore": 3.9}], "cvssMetricV40": [{"type": "Secondary", "source": "33c584b5-0579-4c06-b2a0-8d8329fcab9c", "cvssData": {"Safety": "NOT_DEFINED", "version": "4.0", "Recovery": "NOT_DEFINED", "baseScore": 9.3, "Automatable": "NOT_DEFINED", "attackVector": "NETWORK", "baseSeverity": "CRITICAL", "valueDensity": "NOT_DEFINED", "vectorString": "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", "exploitMaturity": "NOT_DEFINED", "providerUrgency": "NOT_DEFINED", "userInteraction": "NONE", "attackComplexity": "LOW", "attackRequirements": "NONE", "privilegesRequired": "NONE", "subIntegrityImpact": "NONE", "vulnIntegrityImpact": "HIGH", "integrityRequirement": "NOT_DEFINED", "modifiedAttackVector": "NOT_DEFINED", "subAvailabilityImpact": "NONE", "vulnAvailabilityImpact": "HIGH", "availabilityRequirement": "NOT_DEFINED", "modifiedUserInteraction": "NOT_DEFINED", "modifiedAttackComplexity": "NOT_DEFINED", "subConfidentialityImpact": "NONE", "vulnConfidentialityImpact": "HIGH", "confidentialityRequirement": "NOT_DEFINED", "modifiedAttackRequirements": "NOT_DEFINED", "modifiedPrivilegesRequired": "NOT_DEFINED", "modifiedSubIntegrityImpact": "NOT_DEFINED", "modifiedVulnIntegrityImpact": "NOT_DEFINED", "vulnerabilityResponseEffort": "NOT_DEFINED", "modifiedSubAvailabilityImpact": "NOT_DEFINED", "modifiedVulnAvailabilityImpact": "NOT_DEFINED", "modifiedSubConfidentialityImpact": "NOT_DEFINED", "modifiedVulnConfidentialityImpact": "NOT_DEFINED"}}]}, "affected": [{"source": "33c584b5-0579-4c06-b2a0-8d8329fcab9c", "affectedData": [{"vendor": "uvnc", "modules": ["repeater"], "product": "UltraVNC", "versions": [{"status": "affected", "version": "0", "versionType": "custom", "lessThanOrEqual": "1.8.2.2"}], "defaultStatus": "unaffected"}]}], "published": "2026-07-01T05:16:24.820", "references": [{"url": "https://github.com/ultravnc/UltraVNC", "tags": ["Product"], "source": "33c584b5-0579-4c06-b2a0-8d8329fcab9c"}, {"url": "https://uvnc.com/", "tags": ["Product"], "source": "33c584b5-0579-4c06-b2a0-8d8329fcab9c"}, {"url": "https://www.securin.io/zero-days/cve-2026-7840-pre-auth-global-buffer-overflow-repeater-http-admin-ultravnc", "source": "33c584b5-0579-4c06-b2a0-8d8329fcab9c"}], "vulnStatus": "Modified", "weaknesses": [{"type": "Secondary", "source": "33c584b5-0579-4c06-b2a0-8d8329fcab9c", "description": [{"lang": "en", "value": "CWE-787"}]}], "descriptions": [{"lang": "en", "value": "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."}], "lastModified": "2026-07-09T06:16:22.340", "configurations": [{"nodes": [{"negate": false, "cpeMatch": [{"criteria": "cpe:2.3:a:uvnc:ultravnc:*:*:*:*:*:*:*:*", "vulnerable": true, "matchCriteriaId": "593F7EC1-0922-4652-817A-7D57A0F1210D", "versionEndIncluding": "1.8.2.2"}], "operator": "OR"}]}], "sourceIdentifier": "33c584b5-0579-4c06-b2a0-8d8329fcab9c"}