JWT Tampering

A JSON Web Token is base64url-encoded, not encrypted — its header and payload are plainly readable by anyone. Trust comes entirely from a signature the client cannot forge without the server's secret.

Header.Payload.Signature — only the signature is cryptographically protected

Editing the payload (e.g. changing "role":"user" to "role":"admin") invalidates the signature

The 'alg':'none' attack tries to convince a server to skip verification entirely

Servers must pin the expected algorithm and never trust an algorithm named in the token itself

Want this flow explained in detail as it plays?

Editable Token

..(empty)

Header

Signature

HMAC-SHA256(header.payload, server_secret) — recomputed server-side, never trusted from the client.

Edit Payload (JSON)

Defense in depth

Signature is recomputed server-side on every request and compared byte-for-byte
The verification library is pinned to an explicit allow-list of algorithms (e.g. only HS256), ignoring the alg the token itself claims
Short expiry limits the value of any token that does slip through

Vulnerable vs. Fixed

// DO NOT DO THIS
const decoded = jwt.decode(token); // no verification at all
if (decoded.role === "admin") grantAccess();