Progress Telerik Fiddler Classic prior 6.0.20262.10021 Proxy Request Forwarding request smuggling

Summaryinfo

A vulnerability was found in Progress Telerik Fiddler Classic and classified as problematic. The affected element is an unknown function of the component Proxy Request Forwarding. Such manipulation leads to request smuggling. This vulnerability is documented as CVE-2026-77802. The attack can be executed remotely. There is not any exploit available. It is suggested to upgrade the affected component.

Detailsinfo

A vulnerability, which was classified as problematic, has been found in Progress Telerik Fiddler Classic. Affected by this issue is an unknown code of the component Proxy Request Forwarding. The manipulation with an unknown input leads to a request smuggling vulnerability. Using CWE to declare the problem leads to CWE-444. The product acts as an intermediary HTTP agent (such as a proxy or firewall) in the data flow between two entities such as a client and server, but it does not interpret malformed HTTP requests or responses in ways that are consistent with how the messages will be processed by those entities that are at the ultimate destination. Impacted is integrity. CVE summarizes:

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.

The advisory is available at docs.telerik.com. This vulnerability is handled as CVE-2026-77802 since 08/21/2026. The exploitation is known to be easy. The attack may be launched remotely. No form of authentication is required for exploitation. The technical details are unknown and an exploit is not available.

Upgrading to version 6.0.20262.10021 eliminates this vulnerability.

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Productinfo

Vendor

• Progress

Name

• Telerik Fiddler Classic

License

• commercial

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CPE 2.2info

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