Medium · CVSS 6.6CVE-2026-77804

Time-of-check Time-of-use (TOCTOU) Race Condition in Root Certificate Installation in Progress® Telerik® Fiddler® ClassicCVE-2026-77804

Answer in brief

CVE-2026-77804 records a Medium severity (CVSS 6.6) vulnerability in Time-of-check Time-of-use (TOCTOU) Race Condition in Root Certificate Installation in Progress® Telerik® Fiddler® Classic. The current sources do not mark it as known exploited. The current feed maps Progress Software/Progress® Telerik® Fiddler® Classic (generic). Check affected ranges and fixed versions before updating.

Analysis pending evidence review

HOL Guard separates source facts from reviewed analysis. See the methodology.

Published Oct 5, 2026Updated Oct 5, 2026Source checked Oct 5, 2026First seen by HOL Oct 5, 2026Material review Oct 5, 2026

Upstream Advisory

Key facts

Risk: Medium · CVSS 6.6
Exploitation: Not marked as known exploited
Affected software: 1 mapped package or product
Fix availability: Available

Why this deserves its current priority

CVSS is 6.6. The current sources do not mark it as known exploited. Treat this as a source-backed prioritization signal, not a statement about your environment.

Analysis status

Analysis pending evidence review

Factual feed record only; HOL analysis is not approved for indexing. Read the methodology.

Affected scope and exposure questions

The current feed maps Progress Software/Progress® Telerik® Fiddler® Classic (generic). Check affected ranges and fixed versions before updating.

Mapped affected packages and fixed versions
Package	Affected range	Fixed version
Progress Software/Progress® Telerik® Fiddler® Classicgeneric	>=1.0.0 <6.0.20262.10021	6.0.20262.10021

Recommended response

Check inventory. Check lockfiles and deployed manifests for Progress Software/Progress® Telerik® Fiddler® Classic.
Review the reported fix. Source-reported workaround: Trust the Fiddler Classic certificate only in the Current User certificate store. When the application asks to install the certificate in the Local Computer (local machine) store, choose No. If a machine-wide trust is required, export the certificate to a file and import that file into the Local Computer Trusted Root Certification Authorities store manually.

Evidence timeline and material changes

Published upstream
Oct 5, 2026
Evidence: source:cvelist:source_dates:source-dates:record
Source modified
Oct 5, 2026
Evidence: source:cvelist:source_dates:source-dates:record
First seen by HOL
Oct 5, 2026

Sources and claim methodology

Source referencedocs.telerik.com

Upstream source description

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.

Quoted source text, attributed separately from HOL analysis.