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Weak Executable Signature Verification in Progress Telerik Fiddler Classic
CVE-2026-77805 affects Progress Telerik Fiddler Classic for Windows versions prior to 6.0.20262.10021. Before launching external helper tools, the application validates their Authenticode signatures and matches certificate subject names against a broad allow list of publisher-name fragments, but does not verify that each binary is the specific executable shipped with the installed product version. A low-privileged local attacker who can replace a helper executable with another validly signed binary from an allow-listed publisher can cause unintended code execution. Helpers that request elevation can execute the substituted binary with Administrator privileges if a user launches the tool and approves its elevation prompt.
What it means and what to do now: patch path, interim mitigations, and the assume-compromise checklist.
What an attacker gets, and what they’ve been doing with it.
Successful exploitation causes Fiddler Classic to execute a substituted helper binary. For helpers that request elevation, user approval of the elevation prompt can result in execution with Administrator privileges and local privilege escalation.
If you can’t patch tonight, do this now.
Until an upgrade is possible, avoid launching external tools from Fiddler Classic, including Windows 8 Loopback Exemptions and the machine store trust tool. If an external tool must be launched, scrutinize its UAC prompt, verify that it identifies the expected executable, and reject unexpected elevation requests.
Patch, then assume compromise.
Upgrade Progress Telerik Fiddler Classic for Windows to version 6.0.20262.10021 or later, which addresses the vulnerability.
No public exploits tracked yet. Mallory keeps watching.
Products and vendors Mallory has correlated with this vulnerability. Open in Mallory to drill down to specific CPE configurations and version ranges.
8 sources tracked across advisories, community write-ups, and news. New activity surfaces here as Mallory finds it.