CVE-2026-77803

Front-end Desynchronization Vulnerability in Progress® Telerik® Fiddler® Classic

Published Oct 5, 2026 CVSS 3.1 · 3.6 Low EPSS < 1% KEV · Not listed
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.

Remediation

Vendor Workaround

Disable client connection reuse in Fiddler Classic: open Tools > Options > Connections and uncheck the "Reuse client connections" checkbox. Disabling client pipe reuse prevents the excess data of a malformed request from being parsed as a pipelined request.

Tracking

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Advisories

No advisories available
History
Type Values Removed Values Added
Metrics ssvc

{'options': {'Automatable': 'no', 'Exploitation': 'none', 'Technical Impact': 'partial'}, 'version': '2.0.3'}

Type Values Removed Values Added
First Time appeared Progress Software
Progress Software progress Telerik Fiddler Classic
Vendors & Products Progress Software
Progress Software progress Telerik Fiddler Classic
Type Values Removed Values Added
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.
Title Front-end Desynchronization Vulnerability in Progress® Telerik® Fiddler® Classic
Weaknesses CWE-444
References
Metrics cvssV3_1

{'score': 3.6, 'vector': 'CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:N'}

Weaknesses

Sources

Status
PUBLISHED
Assigner
ProgressSoftware
Published
Updated
Oct 6, 2026, 5:50 PM
Reserved
Aug 21, 2026, 1:37 PM
Status
Awaiting Analysis
Published
Oct 5, 2026, 1:16 PM
Modified
Oct 6, 2026, 6:16 PM