{"id":"CVE-2026-77803","title":"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","summary":"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 Transf…","severity":"low","cvss":3.6,"cvssVector":"CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:N","cwe":["CWE-444"],"vendor":"Progress Software","product":"Progress® Telerik® Fiddler® Classic","affected":["progress_telerik_fiddler_classic >= 1.0.0 < 6.0.20262.10021"],"published":"2026-10-05","updated":"2026-10-06","sourceUpdated":"2026-10-06T18:16:56.270","source":"NVD","sourceUrl":"https://nvd.nist.gov/vuln/detail/CVE-2026-77803","references":[{"url":"https://docs.telerik.com/fiddler/knowledge-base/kb-security-front-end-desynchronization-cve-2026-77803","label":"security@progress.com"}],"tags":["nvd","cve.org"],"ssvc":{"exploitation":"none","automatable":"no","technicalImpact":"partial","timestamp":"2026-10-06T17:50:20.748709Z"},"ingestedAt":"2026-10-05T13:20:09.439Z","epss":0.00097,"epssPercentile":0.00717,"slug":"CVE-2026-77803","body":"## Overview\n\nIn 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.\n\n## Remediation\n\nRefer to the linked advisories for vendor-supplied fixes and affected version ranges.","depth":"sunlit","depthScore":20,"depthScoreParts":{"impact":19.8,"likelihood":0,"exploitation":0,"ransomware":0},"changes":[]}