Lab: JWT authentication bypass via flawed signature verification
Summary: PortSwigger's lab demonstrates a JWT authentication bypass in which the server is insecurely configured to accept unsigned tokens. By changing the sub claim to administrator, setting the alg parameter to none, and removing the signature while leaving the trailing dot, a logged-in user reaches the /admin panel and deletes the user carlos.
What you should know: A single misconfiguration - accepting alg: none - turns any authenticated session into an administrator session, and the only step needed to exploit it is editing a token in the browser.
Read the original at PortSwigger Research ↗
- Topics
- Vulnerabilities & Patching
- Related
- Attackers Exploit UnrealIRCd Flaw That Enables Server-Level Command Injection Cyber Risks · Nov 14, 2026U.S. CISA adds Check Point and UnrealIRCd flaws to its Known Exploited Vulnerabilities catalog Cyber Risks · Nov 14, 2026PostgreSQL Patches 52 Flaws, Including 22-Year-Old Bug Enabling Authenticated RCE Cyber Risks · Nov 13, 2026