JWT Editor

JWT Editor is a Burp Suite extension and standalone application for editing, signing, verifying, encrypting and decrypting JSON Web Tokens (JWTs).

When used within Burp Suite, it provides automatic detection and in-line editing of JWTs within HTTP requests/responses, signing and encrypting of tokens and automation of several well-known attacks against JWT implementations.

Standalone mode provides the same functionality as the Burp Suite extension, but for offline JWTs which can be pasted into the tool.

A command-line option is also available to convert PEM formatted public and private keys to JWK format.

Keys View

The Keys View allows cryptographic keys to be imported/exported, generated and converted between the JWK and PEM formats.

Keys are persisted within a .jwt-editor folder within the user's home directory for Standalone mode, or within the Burp Suite user options when used as a Burp extension.

Editor View

The Editor View allows modification of the JWTs loaded into the tool via either the Entry View in Standalone mode, or Burp Suite's HTTP Request/Response view in the Proxy, History and Repeater tools.

The editor view has two layouts, JWS and JWE, which are selected depending on whether a JSON Web Signature or JSON Web Encryption is detected.

Editable Fields

A JSON text editor is provided to edit each of the JWS and JWE components that contain JSON content:

• JWS Header
• JWS Payload
• JWE Header

A hex editor is provided to alter each of the JWS and JWE fields that contain binary content:

• JWS Signature
• JWE Encrypted Key
• JWE Initialization Vector
• JWE Ciphertext
• JWE Authentication Tag

Sign

Sign presents a signing dialog that can be used to update the Signature by signing the JWS Header and Payload using a key from the Keys View that has signing capabilities

Verify

Verify will attempt to verify the Signature of a JWS Header and Payload using any key that is capable of verification from the Keys View. A dialog will be presented with the result of the verification operation.

Encrypt

Encrypt presents an encryption dialog that can be used to encrypt the JWS Header, Payload and Signature fields to produce a JWE using a key from the Keys View that is capable of encryption.

Encrypting a JWS will change the editor mode to JWE to allow modification of the JWE components after encryption.

Decrypt

Decrypt will attempt to use the keys configured in the Keys View that are capable of decryption to decrypt the content of a JWE to produce a JWS.

Decrypting a JWE will change the editor mode to JWS to allow modification of the JWS components after decryption.

Attack

The Attack option implements three well-known attacks against JSON Web Signatures:

• Embedded JWK
• 'none' Signing Algorithm
• HMAC Key Confusion

These are described in more detail below.

Format JSON

The Format JSON option on JSON fields automatically corrects the spacing and indentation of the JSON document.

Compact JSON

The handling of whitespace and newlines is important for a JSON Web Signature, as the encoded bytes of the JSON document are used to form the signature field. The Compact JSON option is used to control how the content of the JSON fields will be serialized.

When enabled, whitespace and newlines will be automatically stripped from the JSON document before serialization. When disabled, whitespace and newlines will be preserved.

This option is automatically enabled if it is detected that the original JWT did not contain whitespace or newlines.

Supported Algorithms

The following JWK types are supported:

• Octet Sequence (OCT) - AES/HMAC
• RSA
• Elliptic-Curve (ECC) - P-256, P-384, P-521
• Octet Key Pair (OKP) - x25519, x448, ed25519, ed448
• Passwords (PBES)

The following JWS/JWE algorithms are supported:

Signing

• HS256
• HS384
• HS512
• RS256
• RS512
• PS384
• PS256
• PS512
• PS384
• ES256
• ES384
• ES512
• EdDSA

Encryption

Key Encryption

• dir(ect)
• RSA1_5
• RSA-OAEP
• RSA-OAEP-256
• A128KW
• A192KW
• A256KW
• ECDH-ES
• ECDH-ES+A128KW
• ECDH-ES+A192KW
• ECDH-ES+A256KW
• A128GCMKW
• A192GCMKW
• A256GCMKW
• PBES2-HS256+A128KW
• PBES2-HS384+A192KW
• PBES2-HS512+A256KW

Content Encryption

• A128GCM
• A192GCM
• A256GCM
• A128CBC-HS256
• A192CBC-HS384
• A256CBC-HS512
• A128CBC+HS256
• A256CBC+HS512

Attacks

The JWT Editor automates three common attacks against JSON Web Signatures.

'none' Signing Algorithm

The value 'none' is defined in the JWA standard as an accepted signing algorithm for JWS. This is intended for use where an out-of-band method has been used to already verify the integrity of the JWS. However, some libraries have been found to treat this as a valid algorithm when processing a JWS.

This attack automates stripping of the signature value from a JWS.

HMAC Key Confusion

Each algorithm within JWS has a required key type (RSA, EC, OKP or oct). A vulnerability has been identified within JOSE implementations where the key type provided in a JWS header does not match that of the algorithm specified. An attacker that provides a symmetric HS256/384/512 'alg' value with a asymmetric 'kty' (EC, RSA, OKP) value may cause the validating library to use the asymmetric public key as the symmetric key input to a HMAC signature validation. As the public key is known to the attacker, the attacker can use the public key as the input to their HMAC signature and forge a signature which is accepted by the server.

The tool implements this attack using the steps outlined at https://www.nccgroup.com/ae/about-us/newsroom-and-events/blogs/2019/january/jwt-attack-walk-through/.

Embedded JWK

JWS defines a 'jwk' field within the Header which is used for the ECDH-ES algorithms as a method of transporting the public key to the recipient. However, this field has been mistakenly used by library implementations as a source of the key for signature verification. By creating a new key, embedding the key for verification within the header, and then signing the JWS Payload, an attacker is able to produce arbitrary JWT payloads.

CLI Mode

A command-line interface is provided for conversion of keys generated using other tools from PEM to JWK format.

Usage:

usage: jwt-editor.jar convert [-h] [--kid KID] key_file

positional arguments:
key_file               Public or Private Key PEM file to convert to JWK

named arguments:
-h, --help             show this help message and exit
--kid KID              JWK Key ID to be used

Example:

java -jar ./jwt-editor.jar convert key.pem --kid my-jwk

Key type is automatically detected from the PEM file.