Vulnerability in Progress Software Progress® Telerik® Fiddler® Classic
CVE-2026-77804
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.
Vulnerability class: TOCTOU (Time-of-Check to Time-of-Use)
Published .
CVSS v3 metric
CVSS v3 base score 6.6 (Medium). Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:R/S:U/C:H/I:H/A:N.
EPSS exploit prediction
No EPSS score from FIRST.org yet. New CVEs usually get one within a few days; no score means no signal yet, not low risk. How EPSS works.
Affected products
Weakness classification (CWE)
References
Frequently asked questions
- What is CVE-2026-77804?
- CVE-2026-77804 is a medium-severity vulnerability in Progress Software Progress® Telerik® Fiddler® Classic, classified under Time-of-check Time-of-use (TOCTOU) Race Condition. CVSS score: 6.6/10. Published 2026-10-05.
- How severe is CVE-2026-77804?
- Medium severity. CVSS v3 base score is 6.6 out of 10.