JWT Authentication — Complete Deep Dive

Deep-Dive Lab Playbook — Every PortSwigger lab variant with exact payloads, bypass techniques, and zero-day extensions. 🟢 Apprentice 🟡 Practitioner 🔴 Expert

When to Use

• BSCP certification prep
• Real-world bug bounty hunting
• Building exploitation chains
• Understanding bypass techniques

Prerequisites

• Burp Suite Professional
• Burp Collaborator / interactsh
• Browser with proxy configured

Workflow

Phase 1: Reconnaissance

• Identify input vectors, parameters, and application behavior.

Phase 2: Exploitation

• Apply standard lab payloads.

Phase 3: Zero-Day Escalation

• Fuzz filters, bypass WAFs, and chain with other vulns.

Lab Playbooks

Lab 1: Unverified signature 🟢 APPRENTICE

Decode JWT, change sub to administrator, re-encode (no signing needed — server doesn't verify).

Lab 2: Flawed signature verification 🟢 APPRENTICE

Remove the signature portion entirely. Server checks signature only if present.

Lab 3: Weak signing key 🟡 PRACTITIONER

hashcat -a 0 -m 16500 JWT_TOKEN /usr/share/wordlists/rockyou.txt

Crack HS256 secret → forge any token.

Lab 4: jwk header injection 🟡 PRACTITIONER

Generate RSA key, sign token with it, embed public key in jwk header parameter. Server uses embedded key to verify.

Lab 5: jku header injection 🟡 PRACTITIONER

Host JWK Set on exploit server, set jku header to your URL. Server fetches your keys.

Lab 6: kid header path traversal 🟡 PRACTITIONER

{"kid":"../../../dev/null","alg":"HS256"}

Sign with empty string (content of /dev/null). Server uses file content as verification key.

Lab 7: Algorithm confusion 🔴 EXPERT

Get server's RSA public key → change alg to HS256 → sign with public key as HMAC secret.

openssl s_client -connect TARGET:443 | openssl x509 -pubkey -noout > public.pem
# Use public.pem as HS256 key to sign forged JWT

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Lab 8: Algorithm confusion no exposed key 🔴 EXPERT

Get two valid JWTs → derive RSA public key using rsa_sign2n tool → then do alg confusion attack.

Blue Team Detection

• Monitor access logs for anomalous payloads.
• Implement strict input validation and parameterized queries where applicable.
• Create WAF rules masking generic attack patterns.

Zero-Day Research

When standard technique fails:

1. Identify the filter/WAF
2. Fuzz with Burp Intruder custom wordlists
3. Search GitHub/Twitter for new bypasses
4. Chain with other vulns for escalation
5. Try encoding variants: URL, double-URL, unicode, hex

Key Concepts

Concept	Description
PortSwigger Vectors	Standardized approaches to vulnerability classes.
Payload Encoding	Modifying payloads to bypass basic string matching WAFs.

Output Format

Vulnerability Deep-Dive Report
==============================
Target Vector: [Endpoint]
Bypass Technique: [Explanation of bypass]
Payload Used: [Payload]
Impact Explanation: [Impact]

🔵 Blue Team

• Deploy robust WAF rules to detect anomalies.
• Monitor logs for unusual access patterns.

🛡️ Remediation & Mitigation Strategy

• Input Validation: Sanitize and strictly type-check all inputs.
• Least Privilege: Constrain component execution bounds.

📚 Shared Resources

For cross-cutting methodology applicable to all vulnerability classes, see:

• _shared/references/elite-chaining-strategy.md — Exploit chaining methodology and high-payout chain patterns
• _shared/references/elite-report-writing.md — HackerOne-optimized report writing, CWE quick reference
• _shared/references/real-world-bounties.md — Verified disclosed bounties by vulnerability class

References

• PortSwigger Labs
• PayloadsAllTheThings
• HackTricks