CVE-2026-77803: Progress Software Progress Telerik Fiddler Classic

Low severity, CVSS 3.6. EPSS: 0.1% chance of exploitation in the next 30 days.

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.

Affected products

• Progress Software Progress Telerik Fiddler Classic: from 1.0.0, before 6.0.20262.10021 (fixed in 6.0.20262.10021)

Published 2026-10-05. Last modified 2026-10-06.