---
id: CVE-2026-77802
title: >-
  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
summary: >-
  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 va…
severity: medium
cvss: 6.3
cvssVector: 'CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/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: '2026-10-05T13:16:54.293'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-77802'
references:
  - url: >-
      https://docs.telerik.com/fiddler/knowledge-base/kb-security-http-request-smuggling-cve-2026-77802
    label: security@progress.com
tags:
  - nvd
  - cve.org
ingestedAt: '2026-10-05T13:20:09.438Z'
---

## Overview

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.

## Remediation

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