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    "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"
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                "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"
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            ],
            "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.",
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                            "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"
                    ]
                }
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                    "format": "CVSS",
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                            "lang": "en",
                            "value": "GENERAL"
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                    "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"
                    }
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            ],
            "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.",
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                            "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>"
                        }
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                }
            ],
            "credits": [
                {
                    "lang": "en",
                    "value": "NATO Cyber Security Centre (NCSC)",
                    "type": "finder"
                }
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            "source": {
                "discovery": "EXTERNAL"
            },
            "x_generator": {
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            {
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                            "content": {
                                "timestamp": "2026-10-06T17:50:20.748709Z",
                                "id": "CVE-2026-77803",
                                "options": [
                                    {
                                        "Exploitation": "none"
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                                        "Automatable": "no"
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                                        "Technical Impact": "partial"
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                                "role": "CISA Coordinator",
                                "version": "2.0.3"
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                "title": "CISA ADP Vulnrichment",
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                    "shortName": "CISA-ADP",
                    "dateUpdated": "2026-10-06T17:50:34.935Z"
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