Hacking JSON Web Tokens (JWTs)

And how attackers forge tokens and log in as someone else

Vickie Li
Vickie Li
Oct 27, 2019 · 7 min read

JSON web tokens are a type of access tokens that are widely used in commercial applications. They are based on the JSON format and includes a token signature to ensure the integrity of the token.

Today, we are going to talk about the security implications of using JSON web tokens (and signature-based tokens in general), and how they can be exploited by attackers to bypass access control.

How do JSON Web Tokens work?

A JSON web token consists of three components: a header, a payload, and a signature.

The header section of a JSON web token identifies the algorithm used to generate the signature. It is a base64url encoded string of a JSON blob like this:

(Base64url encoding is a modified version of base64 for the URL format. It is similar to base64, but uses different non-alphanumeric characters and omits padding. )

The most common algorithms used are HMAC and RSA algorithms.

The payload section contains the information that is actually used for access control. This section, too, is base64url encoded before being used in the token.

The signature is the part that is used to validate that the token has not been tampered with. It is calculated by concatenating the header with the payload, then signing with the algorithm specified in the header.

For this specific token, the string “eyBhbGcgOiBIUzI1NiwgdHlwIDogSldUIH0K.eyB1c2VyX25hbWUgOiBhZG1pbiB9Cg” is signed with the HS256 algorithm with the secret key “key”. The resulting string is 4Hb/6ibbViPOzq9SJflsNGPWSk6B8F6EqVrkNjpXh7M.

You get the complete token by concatenating each section (header, payload, and signature) with a “.” in between each section.


Ways to bypass JSON Web Token controls

When implemented correctly, JSON web tokens provide a secure way to identify the user since the data contained in the payload section cannot be tampered with. (Since the user does not have access to the secret key, she cannot sign the token herself.)

But if implemented incorrectly, there are ways that an attacker can bypass the security mechanism and forge arbitrary tokens.

One of the ways that attackers can forge their own tokens is by tampering with the alg field of the header. If the application does not restrict the algorithm type used in the JWT, an attacker can specify which algorithm to use, which could compromise the security of the token.

  1. None algorithm

JWT supports a “none” algorithm. If the alg field is set to “none”, any token would be considered valid if their signature section is set to empty. For example, the following token would be considered valid:


It is simply the base64url encoded versions of these two blobs, and no signature is present.

This feature was originally used for debugging purposes. But if not turned off in a production environment, it would allow attackers to forge any token they want by setting the alg field to “none”. They can then impersonate anyone on the site by using the forged tokens.

2. HMAC algorithm

The two most common types of algorithms used for JWTs are HMAC and RSA. With HMAC, the token would be signed with a key, then later verified with the same key. As for RSA, the token would first be created with a private key, then verified with the corresponding public key.

It is critical that the secret key for HMAC tokens and the private key for RSA tokens are kept a secret since they are used to sign the tokens.

Now let’s say that there is an application that was originally designed to use RSA tokens. The tokens are signed with a private key A, which is kept a secret from the public. Then the tokens are verified with public key B, which is available to anyone. This is okay as long as the tokens are always treated as RSA tokens.

Now if the attacker changes the alg to HMAC, she might be able to create valid tokens by signing the forged tokens with the RSA public key B.

This is because originally when the token is signed with RSA, the program verifies it with the RSA public key B. When the signing algorithm is switched to HMAC, the token is still verified with the RSA public key B, but this time, the token can be signed with the same public key B (since it’s using HMAC).

It is also possible that the signature of the token is never verified after it arrives at the application. This way an attacker could simply bypass the security mechanism by providing an invalid signature.

It could also be possible to brute force the key used to sign a JWT.

The attacker has a lot of information to start with: she knows the algorithm used to sign the token, the payload that was signed and the resulting signature. If the key used to sign the token is not complex enough, she might be able to brute force it easily.

If an attacker is not able to brute force the key, she might try leaking the secret key instead. If another vulnerability (like a directory traversal, XXE, SSRF) exists that allows the attacker to read the file where the key value is stored, the attacker can steal the key and sign arbitrary tokens.

KID stands for “Key ID”. It is an optional header field in JWTs, and it allows developers to specify the key to be used for verifying the token. The proper usage of a KID parameter looks like this:

Since this field is controlled by the user, it can be manipulated by attackers and lead to dangerous consequences.

  1. Directory traversal

Since the KID is often used to retrieve a key file from the file system, if it is not sanitized before use, it can lead to a directory traversal attack. When this is the case, the attacker would be able to specify any file in the file system as the key to be used to verify the token.

For example, the attacker can force the application into using a publicly available file as the key, and sign an HMAC token using that file.

2. SQL injection

The KID could also be used to retrieve the key from a database. In this case, it might be possible to utilize SQL injection to bypass JWT signing.

If SQL injection is possible on the KID parameter, the attacker can use this injection to return any value she wants.

For example, this above injection will cause the application to return the string “key” (since the key named “aaaaaaa” doesn’t exist in the database). The token will then be verified with the string “key” as the secret key.

In addition to a key ID, JSON web token standards also provide developers with the ability to specify keys via a URL.

  1. JKU header parameter

JKU stands for “JWK Set URL”. It is an optional header field used to specify a URL that points to a set of keys that are used to verify the token. If this field is allowed and not properly restricted, an attacker could host their own key file and specify that the application uses it to verify tokens.

2. JWK header parameter

The optional JWK (JSON Web Key) header parameter allows attackers to embed the key used to verify the token directly in the token.

3. X5U, X5C URL manipulation

Similar to the jku and jwk headers, the x5u and x5c header parameters allow attackers to specify the public key certificate or certificate chain used to verify the token. x5u specifies the information in the URI form while x5c allows the certificate values to be embedded in the token.

Other JWT security issues

There are also other JWT issues that arise when they are not correctly implemented. These are not very common, but definitely worth looking out for:

Information leak

Since JSON web tokens are used for access control, they often contain information about the user.

If the token is not encrypted, anyone can base64 decode the token and read the token’s payload. So if the token contains sensitive information, it might become a source of information leaks. A properly implemented signature section of the JSON web token provides data integrity, not confidentiality.

Command injection

Sometimes when the KID parameter is passed directly into an insecure file read operation, it is possible to inject commands into the code flow.

One of the functions that could allow this type of attack is the Ruby open() function. This function allows attackers to execute system commands by simply tacking the command onto the input after the KID file name:

This is just one example. Theoretically, vulnerabilities like this can happen whenever an application passes any of the header parameters unsanitized into any function resembling system(), exec(), etc.

Ultimately, JSON web tokens are just another form of user input. They should always be handled with skepticism and sanitized rigorously.


If you are interested in the topic or are working on a JWT related project, read about the current best practices here, and some secure implementations here!

Disclaimer: This post was written to bring attention to JSON Web Token vulnerabilities and help developers recognize common pitfalls. Please do not use this information to attack sites or impersonate others.

Trying this on systems where you don’t have permission to test is illegal. If you’ve found a vulnerability, please disclose it responsibly to the vendor. Help make our Internet a safer place.

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Vickie Li

Written by

Vickie Li

Professional investigator of nerdy stuff. Hacks and secures. Creates god awful infographics. https://twitter.com/vickieli7

The Startup

Medium's largest active publication, followed by +720K people. Follow to join our community.

Vickie Li

Written by

Vickie Li

Professional investigator of nerdy stuff. Hacks and secures. Creates god awful infographics. https://twitter.com/vickieli7

The Startup

Medium's largest active publication, followed by +720K people. Follow to join our community.

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