{"dataType":"CVE_RECORD","dataVersion":"5.2","cveMetadata":{"cveId":"CVE-2026-77803","assignerOrgId":"f9fea0b6-671e-4eea-8fde-31911902ae05","state":"PUBLISHED","assignerShortName":"ProgressSoftware","dateReserved":"2026-08-21T13:37:32.974Z","datePublished":"2026-10-05T12:39:36.806Z","dateUpdated":"2026-10-06T17:50:34.935Z"},"containers":{"cna":{"providerMetadata":{"orgId":"f9fea0b6-671e-4eea-8fde-31911902ae05","shortName":"ProgressSoftware","dateUpdated":"2026-10-05T12:39:36.806Z"},"title":"Front-end Desynchronization Vulnerability in Progress® Telerik® Fiddler® Classic","problemTypes":[{"descriptions":[{"lang":"en","cweId":"CWE-444","description":"CWE-444: Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling')","type":"CWE"}]}],"impacts":[{"capecId":"CAPEC-33","descriptions":[{"lang":"en","value":"CAPEC-33 HTTP Request Smuggling"}]}],"affected":[{"vendor":"Progress Software","product":"Progress® Telerik® Fiddler® Classic","platforms":["Windows"],"versions":[{"status":"affected","version":"1.0.0","lessThan":"6.0.20262.10021","versionType":"custom"}],"defaultStatus":"unaffected"}],"descriptions":[{"lang":"en","value":"In Progress® Telerik® Fiddler® Classic for Windows, versions prior to v6.0.20262.10021, front-end request desynchronization is possible in the proxy request forwarding component. A request that contains both a Content-Length and a Transfer-Encoding header is forwarded with both headers present, while Fiddler frames the body using Transfer-Encoding only. The remaining bytes on the reused client connection are then parsed as a separate pipelined request, so a local threat actor with low privileges can cause a single malformed request to be split into two requests forwarded to the origin server and receive an additional smuggled response, without requiring a vulnerable server.","supportingMedia":[{"type":"text/html","base64":false,"value":"In Progress® Telerik® Fiddler® Classic for Windows, versions prior to v6.0.20262.10021, front-end request desynchronization is possible in the proxy request forwarding component. A request that contains both a Content-Length and a Transfer-Encoding header is forwarded with both headers present, while Fiddler frames the body using Transfer-Encoding only. The remaining bytes on the reused client connection are then parsed as a separate pipelined request, so a local threat actor with low privileges can cause a single malformed request to be split into two requests forwarded to the origin server and receive an additional smuggled response, without requiring a vulnerable server."}]}],"references":[{"url":"https://docs.telerik.com/fiddler/knowledge-base/kb-security-front-end-desynchronization-cve-2026-77803","tags":["vendor-advisory"]}],"metrics":[{"format":"CVSS","scenarios":[{"lang":"en","value":"GENERAL"}],"cvssV3_1":{"version":"3.1","attackVector":"LOCAL","attackComplexity":"HIGH","privilegesRequired":"LOW","userInteraction":"NONE","scope":"UNCHANGED","confidentialityImpact":"LOW","integrityImpact":"LOW","availabilityImpact":"NONE","baseSeverity":"LOW","baseScore":3.6,"vectorString":"CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:N"}}],"workarounds":[{"lang":"en","value":"Disable client connection reuse in Fiddler Classic: open Tools > Options > Connections and uncheck the \"Reuse client connections\" checkbox. Disabling client pipe reuse prevents the excess data of a malformed request from being parsed as a pipelined request.","supportingMedia":[{"type":"text/html","base64":false,"value":"<div>Disable client connection reuse in Fiddler Classic: open <b>Tools &gt; Options &gt; Connections</b> and uncheck the <b>\"Reuse client connections\"</b> checkbox. Disabling client pipe reuse prevents the excess data of a malformed request from being parsed as a pipelined request.</div>"}]}],"credits":[{"lang":"en","value":"NATO Cyber Security Centre (NCSC)","type":"finder"}],"source":{"discovery":"EXTERNAL"},"x_generator":{"engine":"Vulnogram 0.2.0"}},"adp":[{"metrics":[{"other":{"type":"ssvc","content":{"timestamp":"2026-10-06T17:50:20.748709Z","id":"CVE-2026-77803","options":[{"Exploitation":"none"},{"Automatable":"no"},{"Technical Impact":"partial"}],"role":"CISA Coordinator","version":"2.0.3"}}}],"title":"CISA ADP Vulnrichment","providerMetadata":{"orgId":"134c704f-9b21-4f2e-91b3-4a467353bcc0","shortName":"CISA-ADP","dateUpdated":"2026-10-06T17:50:34.935Z"}}]}}