Vulnerability · Published Oct 5, 2026 · Assigned by ProgressSoftware
CVE-2026-77803Front-end Desynchronization Vulnerability in Progress® Telerik® Fiddler® Classic
Low 3.6
In plain English
A low-severity security flaw in Progress® Telerik® Fiddler® Classic from Progress Software. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.1% (higher than 1% of all known vulnerabilities).
Official description
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 (as reported)
| Vendor | Product | Versions |
|---|---|---|
| Progress Software | Progress® Telerik® Fiddler® Classic | 1.0.0 – before 6.0.20262.10021 |
Weakness: CWE-444 · Vector: CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:N