{"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": "2026-10-05T12:39:36.806Z"}, "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\u00ae Telerik\u00ae Fiddler\u00ae 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\u00ae Telerik\u00ae Fiddler\u00ae 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\u00ae Telerik\u00ae Fiddler\u00ae 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\u00ae Telerik\u00ae Fiddler\u00ae 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"}}}}

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{"affected": [{"affectedData": [{"defaultStatus": "unaffected", "platforms": ["Windows"], "product": "Progress\u00ae Telerik\u00ae Fiddler\u00ae Classic", "vendor": "Progress Software", "versions": [{"lessThan": "6.0.20262.10021", "status": "affected", "version": "1.0.0", "versionType": "custom"}]}], "source": "security@progress.com"}], "cveTags": [], "descriptions": [{"lang": "en", "value": "In Progress\u00ae Telerik\u00ae Fiddler\u00ae 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."}], "id": "CVE-2026-77803", "lastModified": "2026-10-05T13:16:54.453", "metrics": {"cvssMetricV31": [{"cvssData": {"attackComplexity": "HIGH", "attackVector": "LOCAL", "availabilityImpact": "NONE", "baseScore": 3.6, "baseSeverity": "LOW", "confidentialityImpact": "LOW", "integrityImpact": "LOW", "privilegesRequired": "LOW", "scope": "UNCHANGED", "userInteraction": "NONE", "vectorString": "CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:N", "version": "3.1"}, "exploitabilityScore": 1.0, "impactScore": 2.5, "source": "security@progress.com", "type": "Secondary"}]}, "published": "2026-10-05T13:16:54.453", "references": [{"source": "security@progress.com", "url": "https://docs.telerik.com/fiddler/knowledge-base/kb-security-front-end-desynchronization-cve-2026-77803"}], "sourceIdentifier": "security@progress.com", "vulnStatus": "Received", "weaknesses": [{"description": [{"lang": "en", "value": "CWE-444"}], "source": "security@progress.com", "type": "Primary"}]}

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