Information Disclosure in Axum with Jwt Tokens

Information disclosure in an Axum service using JWT tokens typically occurs when token contents or validation logic are exposed through logs, error messages, or API responses. Axum is a Rust web framework that does not manage token creation or validation itself; it relies on middleware or custom extractors that parse and verify JWTs. If the JWT implementation logs the raw token, the signing key, or detailed failure reasons, attackers can harvest information that aids further attacks.

Consider a common pattern where a JWT is extracted from the Authorization header and validated via a library such as jsonwebtoken. If validation fails (e.g., due to an invalid signature or expired token), a verbose error can be returned to the client, revealing which step failed and potentially the token’s structure. In Axum, this can happen when extractors or handlers propagate errors without sanitizing them. For example, returning a plain string error like "Invalid token signature" leaks semantic details that can be used to refine brute-force or substitution attacks.

Another vector involves logging. If the application logs the full decoded claims or the raw JWT string, tokens containing sensitive information—such as user identifiers, roles, or scopes—may be persisted in logs or monitoring systems. In distributed systems, logs are often aggregated; an exposed JWT in a log line can be replayed or analyzed later. A typical unsafe handler might look like this, where a failure results in a detailed response that should be avoided:

use axum::{routing::get, Router};
use jsonwebtoken::{decode, Algorithm, DecodingKey, Validation, TokenData};
use serde::{Deserialize, Serialize};

#[derive(Debug, Deserialize, Serialize)]
struct Claims {
    sub: String,
    exp: usize,
}

async fn handler(
    auth_header: Option,
) -> Result<String, (axum::http::StatusCode, String)> {
    let token = auth_header.ok_or((axum::http::StatusCode::UNAUTHORIZED, "Missing token".to_string()))?;
    let token_str = token.to_str().map_err(|_| (axum::http::StatusCode::UNAUTHORIZED, "Invalid header".to_string()))?;
    let _data = decode:: Router {
    Router::new().route("/protected", get(handler))
}

In this example, the error message "Token error: ..." can disclose details about token validation, such as whether the signature failed or the token was expired. An attacker can use these distinctions to probe the system. Additionally, if the token’s claims are accidentally echoed in responses or logs—such as returning user identifiers in error details—this constitutes direct information disclosure.

SSRF and unsafe consumption patterns can also amplify information disclosure risks when JWTs are handled. For example, if an Axum service accepts URLs or inputs that influence token introspection endpoints, an attacker may coerce the service into making internal requests that leak metadata. Even without SSRF, insecure deserialization or weak validation of token metadata can expose internal logic. To mitigate these risks, Axum applications must ensure that JWT validation errors are generic, that tokens are never logged in full, and that any introspection or parsing does not expose internal state.

Remediation focuses on minimizing information leakage and ensuring JWT handling does not expose validation details or sensitive claims. In Axum, this involves tightening error handling, avoiding logging of tokens or claims, and standardizing responses.

First, use a uniform error response for all token-related failures. Instead of returning the underlying error from the JWT library, map errors to a generic message and HTTP status code. This prevents attackers from distinguishing between missing tokens, malformed tokens, signature failures, or expiration issues. Here is a safer handler pattern:

use axum::{routing::get, Router, http::StatusCode};
use jsonwebtoken::{decode, Algorithm, DecodingKey, Validation, errors::Error as JwtError};
use serde::{Deserialize, Serialize};

#[derive(Debug, Deserialize, Serialize)]
struct Claims {
    sub: String,
    exp: usize,
}

async fn handler(
    auth_header: Option,
) -> Result<String, (StatusCode, String)> {
    let token = auth_header.ok_or((StatusCode::UNAUTHORIZED, "Unauthorized".to_string()))?;
    let token_str = token.to_str().map_err(|_| (StatusCode::UNAUTHORIZED, "Unauthorized".to_string()))?;
    decode:: Router {
    Router::new().route("/protected", get(handler))
}

Second, prevent logging of raw tokens or decoded claims. If logging is necessary for debugging, redact sensitive parts and never persist full tokens. For instance, log only the outcome of validation, not the token string or claims payload.

Third, enforce strict validation settings to avoid accepting insecure tokens. Explicitly set the algorithm and avoid "none" or weak keys. If your application supports key rotation, ensure the decoding key is retrieved securely and is not hard-coded as in the example above. In production, use environment variables or a secure configuration provider to supply the secret or public key.

Finally, apply the same discipline to any downstream calls made while handling tokens. Ensure that SSRF-prone parameters are validated and that token introspection endpoints do not leak information through timing differences or error messages. By standardizing errors and guarding logs, Axum services can use JWT tokens while reducing the risk of information disclosure.