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CVE-2026-7840

Full vulnerability record from the local CVENow database.

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.

Severity
CRITICAL
CVSS score
9.3 (v4.0)
Vulnerability status
Modified
Source identifier
33c584b5-0579-4c06-b2a0-8d8329fcab9c
Published
2026-07-01 05:16:24.820000
Last modified
2026-10-25 17:10:00.197000
View original CVE JSON 
{
  "id": "CVE-2026-7840",
  "sourceIdentifier": "33c584b5-0579-4c06-b2a0-8d8329fcab9c",
  "published": "2026-07-01T05:16:24.820",
  "lastModified": "2026-10-25T17:10:00.197",
  "vulnStatus": "Modified",
  "cveTags": [],
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      "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."
    },
    {
      "lang": "es",
      "value": "El repetidor UltraVNC hasta la versión 1.8.2.2 contiene un desbordamiento de búfer global en su servidor de administración HTTP incrustado. Las funciones wi_senderr() y wi_replyhdr() en repeater/webgui/webutils.c escriben la URI de solicitud HTTP proporcionada por el llamador en un búfer global fijo de 1000 bytes (hdrbuf) a través de llamadas sprintf sin verificar. El búfer de recepción HTTP acepta URIs de hasta aproximadamente 150 KB (WI_RXBUFSIZE = 153600), por lo que un atacante no autenticado que puede alcanzar el puerto HTTP del repetidor (TCP 80 por defecto) puede desbordar hdrbuf en al menos 500 bytes con una única solicitud HTTP que contenga una URI de 1500 bytes o más, corrompiendo variables globales adyacentes del segmento .bss. El desbordamiento ocurre antes de cualquier verificación de autenticación, haciéndolo alcanzable sin credenciales. Un atacante remoto no autenticado puede lograr la ejecución de código arbitrario en el host que ejecuta el repetidor."
    }
  ],
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      "affectedData": [
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          "product": "UltraVNC",
          "defaultStatus": "unaffected",
          "modules": [
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          "Safety": "NOT_DEFINED",
          "Automatable": "NOT_DEFINED",
          "Recovery": "NOT_DEFINED",
          "valueDensity": "NOT_DEFINED",
          "vulnerabilityResponseEffort": "NOT_DEFINED",
          "providerUrgency": "NOT_DEFINED"
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    "cvssMetricV31": [
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      "url": "https://github.com/ultravnc/UltraVNC",
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}