Mmastodon TechnologyCybersecurity first seen 9 h ago, last 9 h ago, peak #1

JWT validator skipped signature check, exposing step-by-step flaw

Original: A JWT validator that checks every claim but never the signature tells the attacker which claim to fix next. Faav's Micro

A security writeup by Faav describes a Microsoft token validation flaw where a JWT with the 'alg:none' bypass was accepted despite failing every other check. Because the validator returned granular errors — wrong tenant, wrong audience, app allowlist rejection, then 'User not found' — it effectively guided the attacker through each requirement. The writeup also mentions an AI-driven hackbot spending ten days probing email-style attack paths.

Why now: It highlights how detailed error messages in authentication systems can hand attackers a roadmap, a timely concern as AI tools automate exploitation.

FaavMicrosoftJWT

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Evidence

• A JWT validator that checks every claim but never the signature tells the attacker which claim to fix next. Faav's Microsoft Titan writeup: tenant error, audience, app allowlist, then "User not found". An alg:none token got that far. An AI hackbot spent ten days on email-style… · shellonline · 3

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