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Improper Input Validation


Severity Medium
Score 5.9/10


fast-jwt provides fast JSON Web Token (JWT) implementation. Versions prior to 3.3.2, the fast-jwt library does not properly prevent JWT algorithm confusion for all public key types. The "publicKeyPemMatcher" in "fast-jwt/src/crypto.js" does not properly match all common PEM formats for public keys. To exploit this vulnerability, an attacker needs to craft a malicious JWT token containing the HS256 algorithm, signed with the public RSA key of the victim application. This attack will only work if the victim application utilizes a public key containing the "BEGIN RSA PUBLIC KEY" header. Applications using the RS256 algorithm, a public key with a "BEGIN RSA PUBLIC KEY" header, and calling the verify function without explicitly providing an algorithm, are vulnerable to this algorithm confusion attack which allows attackers to sign arbitrary payloads which will be accepted by the verifier. As a workaround, change line 29 of "blob/master/src/crypto.js" to include a regular expression.

  • HIGH
  • HIGH
  • NONE
  • NONE
  • NONE
  • NONE

CWE-20 - Improper Input Validation

The product receives input or data, but it does not validate or incorrectly validates that the input has the properties that are required to process the data safely and correctly.


Advisory Timeline

  • Published