# CVE-2026-77804: Race condition in Progress® Telerik® Fiddler® Classic

> A local attacker can exploit a race condition during root certificate installation to inject a malicious certificate, enabling TLS traffic interception and modification.

- **Severity:** Medium 6.6 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:R/S:U/C:H/I:H/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Local or same network only
- **Victim action needed:** A user has to click or open something
- **Fix available:** No
- **Published:** 5 Oct 2026
- **Updated:** 6 Oct 2026
- **Also known as:** GHSA-rx7m-qprj-p54h

## What is CVE-2026-77804?

CVE-2026-77804 is a medium-severity vulnerability in Progress® Telerik® Fiddler® Classic, published on 5 Oct 2026. It can only be exploited from the same machine or local network, and only if a user opens a file or follows a link.

### Official 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.

Weakness: CWE-367 (Time-of-check Time-of-use (TOCTOU) Race Condition)

## Which versions are affected by CVE-2026-77804?


## How do I fix CVE-2026-77804?

No fix has been published for CVE-2026-77804 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-77804 being exploited?

There is no known exploitation of CVE-2026-77804 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-77804 and fix it if it is affected.

What it is: CVE-2026-77804 is a medium-severity vulnerability in Progress® Telerik® Fiddler® Classic, published on 5 Oct 2026. It can only be exploited from the same machine or local network, and only if a user opens a file or follows a link.
Exploitation: There is no known exploitation of CVE-2026-77804 in the wild and no public exploit code.

Affected product: Progress® Telerik® Fiddler® Classic. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-77804/ (Markdown: https://insights.sbomify.com/CVE-2026-77804/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-77804
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-77804
- OSV: https://osv.dev/vulnerability/CVE-2026-77804

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-77804/
Summaries are generated and may contain mistakes; the official description above is the record.