Secrets

Overview

A high-privilege action was detected coming from an unknown source. The suspicious ticket was captured and handed over for analysis. The task is to identify the token type, extract any useful hints from the payload, crack the signing secret, and forge a new low-privilege token to prove the technique is understood.

Investigation

Identifying the Token

The provided ticket is a long base64-looking string split into three parts separated by dots. Decoding the first part reveals the header:

echo "eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJmbGFnIjoiQlRMe..." | base64 -d
{"typ":"JWT","alg":"HS256"}

This is a JWT — JSON Web Token. JWTs are used to pass authentication claims between systems and have the structure Header.Payload.Signature. The header tells us it’s signed with HS256, which means a shared secret is used to create the signature.

Reading the Payload

Pasting the full token into jwt.io shows the signature is invalid — which confirms the secret is not publicly known. But the payload is readable without knowing the secret, since the middle section is just base64:

{
  "flag": "BTL{_4_Eyes}",
  "iat": 90000000,
  "name": "GreatExp",
  "admin": true
}

The flag field stands out immediately — _4_Eyes is our hint for the secret. The admin: true field is the privilege escalation vector. Anyone holding this token would be treated as an administrator.

Cracking the Secret

With the hint in hand, hashcat can brute-force the signing secret. JWT cracking uses mode 16500:

hashcat -m 16500 hash.txt -a 3 -i '?a?a?a?a'

The -i flag enables incremental mode, trying all lengths up to 4 characters. hashcat cracks it quickly:

eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9....:bT!0

The secret is bT!0 — a short 4-character password, which is why it fell so fast.

Forging the Low-Privilege Token

Now that the secret is known, a new token can be signed. Back in jwt.io, the secret bT!0 is entered and the payload is modified to drop the admin flag:

{
  "flag": "BTL{_4_Eyes}",
  "iat": 90000000,
  "name": "GreatExp",
  "admin": false
}

jwt.io generates a new valid token with a verified signature:

yJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJmbGFnIjoiQlRMe180X0V5ZXN9IiwiaWF0Ijo5MDAwMDAwMCwibmFtZSI6IkdyZWF0RXhwIiwiYWRtaW4iOmZhbHNlfQ.nMXNFvttCvtDcpswOQA8u_LpURwv6ZrCJ-ftIXegtX4

The signature now passes — same secret, same structure, but admin is false. This demonstrates exactly why weak JWT secrets are dangerous: anyone who cracks the secret can sign whatever claims they want.