---
id: CVE-2026-77804
title: >-
  In Progress® Telerik® Fiddler® Classic for Windows, versions prior to
  v6.0.20262.10021, a time-of-check time-of-use (TOCTOU) race condition exists
  in the installation of the HTTPS interception root certificate into the Local
  Computer cer…
summary: >-
  In Progress® Telerik® Fiddler® Classic for Windows, versions prior to
  v6.0.20262.10021, a time-of-check time-of-use (TOCTOU) race condition exists
  in the installation of the HTTPS interception root certificate into the Local
  Computer cer…
severity: medium
cvss: 6.6
cvssVector: 'CVSS:3.1/AV:L/AC:L/PR:L/UI:R/S:U/C:H/I:H/A:N'
cwe:
  - CWE-367
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: '2026-10-05T13:16:54.590'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-77804'
references:
  - url: >-
      https://docs.telerik.com/fiddler/knowledge-base/kb-security-root-certificate-installation-toctou-cve-2026-77804
    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, a time-of-check time-of-use (TOCTOU) race condition exists in the installation of the HTTPS interception root certificate into the Local Computer certificate store. Fiddler writes the certificate to a temporary file in a user-writable location and then launches the external TrustCert helper application, which elevates and imports the certificate from that file. A local threat actor with low privileges who replaces the temporary file between the time it is written and the time the elevated helper reads it can cause an attacker-supplied root certificate to be installed in the Local Computer Trusted Root Certification Authorities store, enabling subsequent interception and modification of TLS-protected traffic on the machine. Successful exploitation requires the user to initiate the certificate trust operation and approve the elevation prompt.

## Remediation

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