{"dataType": "CVE_RECORD", "dataVersion": "5.2", "cveMetadata": {"cveId": "CVE-2026-77802", "assignerOrgId": "f9fea0b6-671e-4eea-8fde-31911902ae05", "state": "PUBLISHED", "assignerShortName": "ProgressSoftware", "dateReserved": "2026-08-21T13:37:28.618Z", "datePublished": "2026-10-05T12:38:58.912Z", "dateUpdated": "2026-10-05T12:38:58.912Z"}, "containers": {"cna": {"providerMetadata": {"orgId": "f9fea0b6-671e-4eea-8fde-31911902ae05", "shortName": "ProgressSoftware", "dateUpdated": "2026-10-05T12:38:58.912Z"}, "title": "HTTP Request Smuggling 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, HTTP request smuggling is possible in the proxy request forwarding component. Requests containing multiple Content-Length headers with conflicting values are forwarded verbatim to the origin server, while Fiddler frames the request body using only the first Content-Length value. A local threat actor with low privileges who is able to send requests through the same Fiddler proxy instance as another user can exploit this desynchronization against a non-RFC-9110-compliant origin server that keeps the connection alive to smuggle an additional request. Because Fiddler returns the server connection to its pipe pool after reading only the first response, the unread smuggled response remains buffered on the socket and is served to the next session that reuses that connection, allowing the attacker to poison responses delivered to other users and to obtain responses intended for them.", "supportingMedia": [{"type": "text/html", "base64": false, "value": "In Progress\u00ae Telerik\u00ae Fiddler\u00ae Classic for Windows, versions prior to v6.0.20262.10021, HTTP request smuggling is possible in the proxy request forwarding component. Requests containing multiple Content-Length headers with conflicting values are forwarded verbatim to the origin server, while Fiddler frames the request body using only the first Content-Length value. A local threat actor with low privileges who is able to send requests through the same Fiddler proxy instance as another user can exploit this desynchronization against a non-RFC-9110-compliant origin server that keeps the connection alive to smuggle an additional request. Because Fiddler returns the server connection to its pipe pool after reading only the first response, the unread smuggled response remains buffered on the socket and is served to the next session that reuses that connection, allowing the attacker to poison responses delivered to other users and to obtain responses intended for them."}]}], "references": [{"url": "https://docs.telerik.com/fiddler/knowledge-base/kb-security-http-request-smuggling-cve-2026-77802", "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": "HIGH", "integrityImpact": "HIGH", "availabilityImpact": "NONE", "baseSeverity": "MEDIUM", "baseScore": 6.3, "vectorString": "CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:N"}}], "workarounds": [{"lang": "en", "value": "Disable server connection reuse in Fiddler Classic: open Tools > Options > Connections and uncheck the \"Reuse server connections\" checkbox. Disabling server pipe reuse prevents a poisoned server connection from being handed to another session.", "supportingMedia": [{"type": "text/html", "base64": false, "value": "<div>Disable server connection reuse in Fiddler Classic: open <b>Tools &gt; Options &gt; Connections</b> and uncheck the <b>\"Reuse server connections\"</b> checkbox. Disabling server pipe reuse prevents a poisoned server connection from being handed to another session.</div>"}]}], "credits": [{"lang": "en", "value": "NATO Cyber Security Centre (NCSC)", "type": "finder"}], "source": {"discovery": "EXTERNAL"}, "x_generator": {"engine": "Vulnogram 0.2.0"}}}}

{}

{"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": "[email protected]"}], "cveTags": [], "descriptions": [{"lang": "en", "value": "In Progress\u00ae Telerik\u00ae Fiddler\u00ae Classic for Windows, versions prior to v6.0.20262.10021, HTTP request smuggling is possible in the proxy request forwarding component. Requests containing multiple Content-Length headers with conflicting values are forwarded verbatim to the origin server, while Fiddler frames the request body using only the first Content-Length value. A local threat actor with low privileges who is able to send requests through the same Fiddler proxy instance as another user can exploit this desynchronization against a non-RFC-9110-compliant origin server that keeps the connection alive to smuggle an additional request. Because Fiddler returns the server connection to its pipe pool after reading only the first response, the unread smuggled response remains buffered on the socket and is served to the next session that reuses that connection, allowing the attacker to poison responses delivered to other users and to obtain responses intended for them."}], "id": "CVE-2026-77802", "lastModified": "2026-10-05T13:16:54.293", "metrics": {"cvssMetricV31": [{"cvssData": {"attackComplexity": "HIGH", "attackVector": "LOCAL", "availabilityImpact": "NONE", "baseScore": 6.3, "baseSeverity": "MEDIUM", "confidentialityImpact": "HIGH", "integrityImpact": "HIGH", "privilegesRequired": "LOW", "scope": "UNCHANGED", "userInteraction": "NONE", "vectorString": "CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:N", "version": "3.1"}, "exploitabilityScore": 1.0, "impactScore": 5.2, "source": "[email protected]", "type": "Secondary"}]}, "published": "2026-10-05T13:16:54.293", "references": [{"source": "[email protected]", "url": "https://docs.telerik.com/fiddler/knowledge-base/kb-security-http-request-smuggling-cve-2026-77802"}], "sourceIdentifier": "[email protected]", "vulnStatus": "Received", "weaknesses": [{"description": [{"lang": "en", "value": "CWE-444"}], "source": "[email protected]", "type": "Primary"}]}

{}

{"affected": [{"configurations": [{"platform": ["windows"], "status": "affected", "versions": {"scheme": "generic", "value": "[1.0.0,6.0.20262.10021)"}}], "enrichment": {"confidence": 95.0, "confidence_source": "inferred", "scores": [{"score": 95.0, "source": "inferred"}, {"score": 100.0, "source": "matching"}]}, "original": {"product": "Progress\u00ae Telerik\u00ae Fiddler\u00ae Classic", "source": "cna", "vendor": "Progress Software"}, "product": "telerik_fiddler_classic", "vendor": "progress"}], "created": "2026-10-05T14:45:19.477821+00:00", "updated": "2026-10-05T15:15:08.678913+00:00", "vendors": ["progress", "progress$PRODUCT$telerik_fiddler_classic"]}