{"id":"CVE-2026-77803","state":"PUBLISHED","title":"Front-end Desynchronization Vulnerability in Progress® Telerik® Fiddler® Classic","description":"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.","publishedAt":"2026-10-05T12:39:36.806Z","updatedAt":"2026-10-06T17:50:34.935Z","severity":{"score":3.6,"label":"Low","version":"3.1","vector":"CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:N","source":"ProgressSoftware"},"assigner":"ProgressSoftware","products":["Progress Software Progress® Telerik® Fiddler® Classic"],"affected":[{"vendor":"Progress Software","product":"Progress® Telerik® Fiddler® Classic","defaultStatus":"unaffected","versions":[{"version":"1.0.0","status":"affected","lessThan":"6.0.20262.10021","versionType":"custom"}],"platforms":["Windows"]}],"references":["https://docs.telerik.com/fiddler/knowledge-base/kb-security-front-end-desynchronization-cve-2026-77803"],"weaknesses":["CWE-444"],"ingestedAt":"2026-10-29T20:13:07.220Z","source":{"repository":"CVEProject/cvelistV5","path":"cves/2026/77xxx/CVE-2026-77803.json","commit":"556c9d3a8cd17e218bb8e452ba7cd619a384ff47"},"raw":{"dataType":"CVE_RECORD","dataVersion":"5.2","cveMetadata":{"cveId":"CVE-2026-77803","state":"PUBLISHED","datePublished":"2026-10-05T12:39:36.806Z","dateUpdated":"2026-10-06T17:50:34.935Z","assignerShortName":"ProgressSoftware","assignerOrgId":"f9fea0b6-671e-4eea-8fde-31911902ae05","dateReserved":"2026-08-21T13:37:32.974Z"},"containers":{"cna":{"title":"Front-end Desynchronization Vulnerability in Progress® Telerik® Fiddler® Classic","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."}],"affected":[{"vendor":"Progress Software","product":"Progress® Telerik® Fiddler® Classic","defaultStatus":"unaffected","versions":[{"version":"1.0.0","status":"affected","lessThan":"6.0.20262.10021","versionType":"custom"}],"platforms":["Windows"]}],"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"}}],"problemTypes":[{"descriptions":[{"cweId":"CWE-444","lang":"en","description":"CWE-444: Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling')","type":"CWE"}]}],"providerMetadata":{"shortName":"ProgressSoftware","orgId":"f9fea0b6-671e-4eea-8fde-31911902ae05","dateUpdated":"2026-10-05T12:39:36.806Z"},"impacts":[{"capecId":"CAPEC-33","descriptions":[{"lang":"en","value":"CAPEC-33 HTTP Request Smuggling"}]}],"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":[{"title":"CISA ADP Vulnrichment","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"}}}],"providerMetadata":{"shortName":"CISA-ADP","orgId":"134c704f-9b21-4f2e-91b3-4a467353bcc0","dateUpdated":"2026-10-06T17:50:34.935Z"}}]}}}