JSON Web Tokens (JWTs) are quickly becoming a popular way to implement information exchange and authorisation in single sign-on scenarios.
As with many things, this technology can be either quite secure or very insecure at the same time and a lot is dependent on the implementation. This opens a number of possibilities for attackers to exploit vulnerabilities if this standard is poorly implemented or outdated libraries are sued.
Here are some of the possible attack scenarios:
- A attackers can modify the token and hashing algorithm to indicate, through the ‘none’ keyword, that the integrity of the token has already been verified, fooling the server into accepting it as a valid token
- Attackers can change the algorithm from ‘RS256’ to ‘HS256’ and use the public key to generate a HMAC signature for the token, as server trusts the data inside the header of a JWT and doesn’t validate the algorithm it used to issue a token. The server will now treat this token as one generated with ‘HS256’ algorithm and use its public key to decode and verify it
- JWTs signed with HS256 algorithm could be susceptible to private key disclosure when weak keys are used. Attackers can conduct offline brute-force or dictionary attacks against the token, since a client does not need to interact with the server to check the validity of the private key after a token has been issued by the server
- Sensitive information (e.g. internal IP addresses) can be revealed, as all the information inside the JWT payload is stored in plain text
I recommend the following steps to address the concerns above:
- Reject tokens set with ‘none’ algorithm when a private key was used to issue them
- Use appropriate key length (e.g. 256 bit) to protect against brute force attacks
- Adjust the JWT token validation time depending on required security level (e.g. from few minutes up to an hour). For extra security, consider using reference tokens if there’s a need to be able to revoke/invalidate them
- Use HTTPS/SSL to ensure JWTs are encrypted during client-server communication, reducing the risk of the man-in-the-middle attack
- Overall, follow the best practices for implementing them, only use up-to-date and secure libraries and choose the right algorithm for requirements
OWASP have more detailed recommendations with Java code samples alongside other noteworthy material for common vulnerabilities and secure coding practices, so I encourage you to check it out if you need more information.
Cyber Security Leadership 