CVE Catalog

CVE-2026-77803

LowCVSS 3.6
Published: Updated: Translated: NVD NIST

Exploitation Probability (EPSS)

Low risk
0.09%

0th percentile - higher than 0% of all known CVEs

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 containing both Content-Length and Transfer-Encoding headers 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.

Risk Assessment

The risk includes the possibility of request smuggling attacks by a local user, which could lead to unauthorized data access or response manipulation. Low privileges are required, but the attack can be performed remotely.

Recommendation

It is recommended to upgrade Fiddler Classic to version v6.0.20262.10021 or later. Also restrict local access to systems running Fiddler.

Related vulnerabilities

Original NVD description (English source)

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.

Vulnerability data from NVD (NIST) · CISA KEV · EPSS