Insecure Deserialization in Axum with Jwt Tokens

Insecure deserialization occurs when an application processes untrusted data without sufficient validation, allowing attackers to manipulate object graphs or execute unintended code. In Axum, this risk can intersect with JWT token handling when tokens are deserialized and mapped into application structures with insufficient checks.

JWTs are typically represented as compact, cryptographically signed strings. In Axum, developers often decode the token payload into claims structures (for example, a Claims struct) and then use those claims for authorization or session management. If the deserialization logic is overly permissive—such as using generic map-based access or frameworks that allow dynamic field resolution—attackers may supply specially crafted tokens that trigger unsafe deserialization paths when additional, non-standard fields are processed by downstream libraries.

Consider an Axum handler that decodes a JWT and then passes the claims payload into a component that performs further deserialization (e.g., reading nested JSON fields into complex Rust structs using Serde). An attacker who can control or predict parts of the token payload might inject serialized objects that, when deserialized by those components, lead to logic manipulation, privilege escalation, or unauthorized state changes. While JWT signature verification should prevent tampering, implementation gaps—such as accepting unsigned tokens, using weak algorithms, or skipping audience/issuer validation—can undermine the integrity of the token before claims ever reach deserialization code.

Another angle involves token introspection or revocation lists stored as serialized objects. If Axum routes or middleware deserialize those objects without strict type checks or schema validation, maliciously crafted tokens can exploit gadget chains or type confusion to alter behavior. Because JWTs often carry user roles or scopes, insecure deserialization of related data structures can lead to privilege escalation (BOLA/IDOR-like outcomes) even when the token itself is valid.

Real-world patterns to watch for include using serde_json::from_value on untyped token claims, or relying on dynamic formats that interpret attacker-supplied type indicators. These patterns can map to OWASP API Top 10 categories such as broken object level authorization and excessive data exposure when combined with weak JWT validation.

Remediation centers on strict validation of JWTs and careful handling of claims to avoid unsafe deserialization. Use a well-maintained JWT library, validate all required claims, and avoid generic or dynamic deserialization of token payloads.

Example: Validate and decode a JWT with fixed claims in Axum middleware.

use axum::{extract::Request, middleware::Next, response::Response, Extension};
use jsonwebtoken::{decode, Algorithm, DecodingKey, Validation, TokenData};
use serde::{Deserialize, Serialize};

#[derive(Debug, Serialize, Deserialize)]
struct Claims {
    sub: String,
    role: String,
    exp: usize,
    // Do not include generic `serde_json::Value` fields unless strictly necessary
}

async fn jwt_middleware(
    request: Request,
    next: Next,
) -> Response {
    let token = match extract_jwt_from_header(&request) {
        Some(t) => t,
        None => return Response::builder()
            .status(401)
            .body("Missing token".into())
            .unwrap(),
    };

    let validation = Validation::new(Algorithm::RS256);
    let mut validation = validation;
    validation.validate_exp = true;
    validation.set_audience(&["my-api-audience"]);
    validation.set_issuer(&["https://auth.example.com"]);

    let token_data: TokenData<Claims> = match decode::(
        &token,
        &DecodingKey::from_rsa_pem(include_bytes!("public_key.pem")).unwrap(),
        &validation,
    ) {
        Ok(data) => data,
        Err(_) => return Response::builder()
            .status(401)
            .body("Invalid token".into())
            .unwrap(),
    };

    // Claims are now strongly typed; avoid further deserialization of raw payload
    let Extension(state) = request.extensions().get().unwrap();
    let response = next.run(request).await;
    response
}

fn extract_jwt_from_header(request: &Request) -> Option<String> {
    request.headers()
        .get("authorization")
        .and_then(|v| v.to_str().ok())
        .map(|s| s.trim_start_matches("Bearer ").to_string())
        .filter(|s| !s.is_empty())
}

Key practices:

• Always specify concrete claim types instead of using serde_json::Value or map-like deserialization for token payloads.
• Enforce algorithm restrictions (e.g., RS256/ES256) and disable unsafe_skip_validate_signature in production.
• Validate audience, issuer, and expiration rigorously; do not accept tokens with missing or mismatched fields.
• Avoid passing raw token payloads into downstream deserializers; if you must process nested data, use strongly typed structs with strict schemas.
• Use short token lifetimes and rotate signing keys; store public keys securely and pin them where possible.

If your application requires processing opaque blobs or custom claims, define explicit structs and use serde’s derive macros to enforce schema conformance rather than relying on dynamic deserialization.