GHSA-h395-gr6q-cpjc

Summary:

It has been discovered that there is a Type Confusion vulnerability in jsonwebtoken, specifically, in its claim validation logic.

When a standard claim (such as nbf or exp) is provided with an incorrect JSON type (Like a String instead of a Number), the library’s internal parsing mechanism marks the claim as “FailedToParse”. Crucially, the validation logic treats this “FailedToParse” state identically to “NotPresent”.

This means that if a check is enabled (like: validate_nbf = true), but the claim is not explicitly marked as required in required_spec_claims, the library will skip the validation check entirely for the malformed claim, treating it as if it were not there. This allows attackers to bypass critical time-based security restrictions (like “Not Before” checks) and commit potential authentication and authorization bypasses.

Details:

The vulnerability stems from the interaction between the TryParse enum and the validate function in src/validation.rs.

1. The TryParse Enum: The library uses a custom TryParse enum to handle claim deserialization:

enum TryParse<T> {
    Parsed(T),
    FailedToParse, // Set when deserialization fails (e.g. type mismatch)
    NotPresent,
}

If a user sends {“nbf”: “99999999999”} (legacy/string format), serde fails to parse it as u64, and it results in TryParse::FailedToParse.

1. The Validation Logic Flaw (src/validation.rs): In Validation::validate, the code checks for exp and nbf like this:

// L288-291
if matches!(claims.nbf, TryParse::Parsed(nbf) if options.validate_nbf && nbf > now + options.leeway) {
    return Err(new_error(ErrorKind::ImmatureSignature));
}

This matches! macro explicitly looks for TryParse::Parsed(nbf).

• If claims.nbf is FailedToParse, the match returns false. • The if block is skipped. • No error is returned.

1. The “Required Claims” Gap: The only fallback mechanism is the “Required Claims”...