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

MITRE NVD CVE.ORG JSON API

Summary

CVECVE-2026-77804
StatePUBLISHED
AssignerProgressSoftware
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-10-05 13:16:54 UTC
Updated2026-10-06 18:16:56 UTC
DescriptionIn 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.

Risk And Classification

Primary CVSS: v3.1 6.6 MEDIUM from [email protected]

CVSS:3.1/AV:L/AC:L/PR:L/UI:R/S:U/C:H/I:H/A:N

EPSS: 0.000580000 probability, percentile 0.000040000 (date 2026-10-06)

Problem Types: CWE-367 | CWE-367 CWE-367: Time-of-check Time-of-use (TOCTOU) Race Condition

VersionSourceTypeScoreSeverityVector
3.1[email protected]Secondary6.6MEDIUMCVSS:3.1/AV:L/AC:L/PR:L/UI:R/S:U/C:H/I:H/A:N
3.1CNACVSS6.6MEDIUMCVSS:3.1/AV:L/AC:L/PR:L/UI:R/S:U/C:H/I:H/A:N

CVSS v3.1 Breakdown

Attack Vector
Local
Attack Complexity
Low
Privileges Required
Low
User Interaction
Required
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
None

CVSS:3.1/AV:L/AC:L/PR:L/UI:R/S:U/C:H/I:H/A:N

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA Progress Software Progress Telerik Fiddler Classic affected 1.0.0 6.0.20262.10021 custom Windows

References

ReferenceSourceLinkTags
docs.telerik.com/fiddler/knowledge-base/kb-security-root-certificate-installat... [email protected] docs.telerik.com
CVE Program record CVE.ORG www.cve.org canonical
NVD vulnerability detail NVD nvd.nist.gov canonical, analysis

Vendor Comments And Credit

Discovery Credit

CNA: NATO Cyber Security Centre (NCSC) (en)

Additional Advisory Data

Workarounds

CNA: 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.

© CVE.report 2026

Use of this information constitutes acceptance for use in an AS IS condition. There are NO warranties, implied or otherwise, with regard to this information or its use. Any use of this information is at the user's risk. It is the responsibility of user to evaluate the accuracy, completeness or usefulness of any information, opinion, advice or other content. EACH USER WILL BE SOLELY RESPONSIBLE FOR ANY consequences of his or her direct or indirect use of this web site. ALL WARRANTIES OF ANY KIND ARE EXPRESSLY DISCLAIMED. This site will NOT BE LIABLE FOR ANY DIRECT, INDIRECT or any other kind of loss.

CVE, CWE, and OVAL are registred trademarks of The MITRE Corporation and the authoritative source of CVE content is MITRE's CVE web site. This site includes MITRE data granted under the following license.

Free CVE JSON API cve.report/api

CVE.report and Source URL Uptime Status status.cve.report