What’s Vulnerable › Vulnerabilities
CVE-2026-77804: Time-of-check Time-of-use (TOCTOU) Race Condition in Root Certificate Installation in Progress® Telerik® Fiddler® Classic
- Severity: medium (CVSS 6.6 v3.1)
- Exploited in the wild: not listed in CISA KEV
- EPSS: 0.1% probability of exploitation in the next 30 days
- Public exploit or detection template: none known
- Published: 2026-10-05 · updated 2026-10-06
- Weakness: CWE-367
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.
Affected products
- Progress Software Progress® Telerik® Fiddler® Classic: from 1.0.0 before 6.0.20262.10021 — fixed in 6.0.20262.10021
How to fix
Upgrade to a fixed version: Progress Software Progress® Telerik® Fiddler® Classic 6.0.20262.10021. Check the vendor advisory for mitigations.
References
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