CVE-2026-77804: Progress Software Progress Telerik Fiddler Classic

Medium severity, CVSS 6.6. EPSS: 0.1% chance of exploitation in the next 30 days.

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.

Affected products

• Progress Software Progress Telerik Fiddler Classic: from 1.0.0, before 6.0.20262.10021 (fixed in 6.0.20262.10021)

Published 2026-10-05. Last modified 2026-10-06.