Vulnerability · Published Oct 5, 2026 · Assigned by ProgressSoftware
CVE-2026-77802HTTP Request Smuggling Vulnerability in Progress® Telerik® Fiddler® Classic
Medium 6.3
In plain English
A medium-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, HTTP request smuggling is possible in the proxy request forwarding component. Requests containing multiple Content-Length headers with conflicting values are forwarded verbatim to the origin server, while Fiddler frames the request body using only the first Content-Length value. A local threat actor with low privileges who is able to send requests through the same Fiddler proxy instance as another user can exploit this desynchronization against a non-RFC-9110-compliant origin server that keeps the connection alive to smuggle an additional request. Because Fiddler returns the server connection to its pipe pool after reading only the first response, the unread smuggled response remains buffered on the socket and is served to the next session that reuses that connection, allowing the attacker to poison responses delivered to other users and to obtain responses intended for them.
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:H/I:H/A:N