---
id: CVE-2026-77803
title: >-
  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
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 that contains both a Content-Length
  and a Transf…
severity: low
cvss: 3.6
cvssVector: 'CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:N'
cwe:
  - CWE-444
vendor: Progress Software
product: Progress® Telerik® Fiddler® Classic
affected:
  - progress_telerik_fiddler_classic >= 1.0.0 < 6.0.20262.10021
published: '2026-10-05'
updated: '2026-10-05'
sourceUpdated: '2026-10-05T13:16:54.453'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-77803'
references:
  - url: >-
      https://docs.telerik.com/fiddler/knowledge-base/kb-security-front-end-desynchronization-cve-2026-77803
    label: security@progress.com
tags:
  - nvd
  - cve.org
ingestedAt: '2026-10-05T13:20:09.439Z'
---

## Overview

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

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.