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Broken JSON Web Token

  1. Broken JSON Web Token
    1. Description
    2. Impact
    3. Scenario
    4. Prevention
    5. Testing


Broken JSON Web Token (JWT) attacks are a type of API security vulnerability that fall under the broad OWASP Top 10 Broken Authentication category of security risks. They occur when JWT authentication mechanisms fail, enabling malicious actors to craft tokens and impersonate the user of a web application.

JWT is an open internet standard representing claims to be transferred securely, with optional JSON Web Signature (JWS) and JSON Web Encryption (JWE), between parties as a JSON payload. JWTs have become ubiquitous in their use, being widely implemented in commercial applications due to their lightweight nature and digital signature enabling verification.

This widespread adoption of JWTs as a de facto session and token-based authentication standard is arguably justified if implemented properly and where appropriate (in many cases, they may be less fit for purpose than more traditional session management systems). However, they often aren’t implemented correctly and thus persist as a favoured entry point for malicious actors.


An attacker executing a successful Broken JWT attack will enjoy the same privileges on the web application as the user he / she is impersonating. Depending on the level of access and the setup of the application, an attacker could gain complete access to all data in the web application, assume administrator rights, and compromise the confidentiality, integrity, and availability of the application.


A JWT has the following form:


It comprises three parts separated by ., each one encoded in URL-safe Base64. The first part is the header that contains the algorithm and the token type:

  "alg": "HS256",
  "typ": "JWT"

The second part contains non-sensitive arbitrary data chosen by the web application, for example:

  "username": "john.doe"

The last part is the signature, that is the hashing of the first two parts using a secret as a key. For example, in the above JWT, the secret is $uper$ecret, thus:

URLSAFE_BASE64(HMACSHA256('eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJ1c2VybmFtZSI6ImpvaG4uZG9lIn0', '$uper$ecret')) => jxWz0c_9YC3INAlpUGegBtTgmDUA12Krsjtr1P2qgbw

There are numerous attack vectors malicious actors can choose from when attempting to break JWTs, including:

  • None Algorithm Attack: an attacker modifies or creates a token by setting alg (the signing algorithm) to None in the header. Since ‘alg’ is ‘None’, this is a valid JWS.
  • HMAC Algorithm Attack: an attacker creates the token by setting the signing algorithm to a symmetric HS256 instead of an asymmetric RS256, leading the API to blindly verify the token using the HS256 algorithm using the public key as a secret key. Since the public key is known, the attacker can correctly forge valid JWTs.
  • Developers may not even validate the signature at all, trusting the signature passed in the Header.

There are usually third-party libraries that implement the JWT authentication mechanism; however, developers are discouraged from crafting their own ad hoc solutions due to complexity and potential for error. Yet it is still possible to misconfigure the library and introduce vulnerabilities.

The most common mistake is to inadvertently disclose, or not choose a strong enough JWT secret.


Developers can effectively mitigate algorithm vulnerabilities by strictly adhering to several precautions:

  • Never let the header of the JWT alone drive verification.
  • Know how the algorithms are structured.
  • Ensure an appropriate, strong key size is used.
  • Use well-known JWT libraries that are not exposed to known vulnerabilities.

In cases where there is no built-in token revocation by the user, a token deny list can be implemented which mirrors the application of a traditional session management cookie.


Verify that stateless session tokens use digital signatures, encryption, and other countermeasures to protect against tampering, enveloping, replay, null cipher, and key substitution attacks.

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