CVE-2026-86158: Missing Authentication in the local .NET backend of Progress Telerik Fiddler Everywhere

Severity: high (CVSS 7.7 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: 2026-09-29 · updated 2026-09-30
Weakness: CWE-306

Description

Missing authentication in the local .NET backend (Fiddler.WebUi) of Progress Software Fiddler Everywhere 8.0.2 allows a local unauthenticated attacker to mint OAuth tokens and read the machine-in-the-middle root certificate through an unauthenticated localhost HTTP and SignalR RPC channel.

Affected products

Progress Software Progress® Telerik® Fiddler® Everywhere: from 1.0.0 before 8.2.0 — fixed in 8.2.0

How to fix

Upgrade to a fixed version: Progress Software Progress® Telerik® Fiddler® Everywhere 8.2.0. Check the vendor advisory for mitigations.

References

https://www.telerik.com/fiddler/fiddler-everywhere/documentation/knowledge-base/kb-security-missing-authentication-for-critical-function-cve-2026-86158

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