What’s Vulnerable › Vulnerabilities
CVE-2026-77803: Front-end Desynchronization Vulnerability in Progress® Telerik® Fiddler® Classic
- Severity: low (CVSS 3.6 v3.1)
- Exploited in the wild: not listed in CISA KEV
- EPSS: 0.1% probability of exploitation in the next 30 days
- Public exploit or detection template: none known
- Published: 2026-10-05 · updated 2026-10-06
- Weakness: CWE-444
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
- Progress Software Progress® Telerik® Fiddler® Classic: from 1.0.0 before 6.0.20262.10021 — fixed in 6.0.20262.10021
How to fix
Upgrade to a fixed version: Progress Software Progress® Telerik® Fiddler® Classic 6.0.20262.10021. Check the vendor advisory for mitigations.
References
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