Disclosure 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 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.
Source-reported weakness categories
CWE-444
Source-specific records & product guidance
Sources retain their own attribution and scoring. Follow the original record to confirm affected versions, fixed releases, and configuration conditions.
NIST National Vulnerability Database · NVD-CVE-2026-77803
Open original source · Updated Oct 06, 2026
Only CPE matches marked vulnerable=true are indexed. AND/OR platform conditions must be checked in the original NVD record.
Original records & references
PUBLISHED 2026-10-05T09:16:54-04:00
MODIFIED 2026-10-06T11:09:20-04:00
INGESTED 2026-10-06T11:45:40-04:00