Use After Free in Axum with Jwt Tokens

Use After Free (UAF) occurs when memory is deallocated but references to it remain in use, leading to unpredictable behavior or potential code execution. In an Axum application handling JWT tokens, UAF can arise when token payloads or validation contexts are improperly managed across asynchronous boundaries or cloned references. Axum is a web framework built on Tower and Hyper, where handlers often capture state such as JSON Web Key (JWK) sets or token validation contexts. If these captures involve references to objects that are deallocated or moved—such as a parsed JWT claim structure reused across requests without proper ownership management—a handler may retain a pointer to freed memory.

Consider a scenario where a JWT validation middleware clones a key provider for each request. If the key provider internally caches decoded keys and those caches are updated or dropped under high concurrency, a request may continue using a key reference after it has been freed and potentially reallocated. This is particularly risky when the token processing pipeline uses unsafe Rust patterns, such as raw pointers or manual memory management, although typical Axum usage relies on safe Rust. Even with safe code, improper use of Arc or Mutex can cause logical use-after-free issues when stale clones are used after the underlying data is dropped.

In the context of JWT tokens, UAF can be triggered through: (1) deserialization of tokens into temporary claim objects that are moved into async blocks and later referenced after the original scope ends; (2) validation workflows that compare signatures using borrowed data that goes out of scope; and (3) error handling paths where token metadata is logged or inspected after deallocation. An attacker may exploit this by inducing high request concurrency, forcing memory reuse that leads to information disclosure or code execution—similar to classic CWE-416 patterns mapped in API security testing by scanners like middleBrick.

For example, if an Axum handler deserializes a JWT using jsonwebtoken and then spawns an async task that captures a reference to the decoded claims without ensuring the data outlives the task, the runtime may access invalid memory. middleBrick’s LLM/AI Security and Input Validation checks can flag such risky patterns during scans by correlating runtime behavior with OpenAPI specs and known attack patterns, though it does not perform source analysis.

Real-world parallels include CVE-2023-38545-like memory safety issues in networking stacks, where improper reference handling leads to exposure of sensitive data. In API security, this manifests as erratic behavior under load, potential crashes, or exposure of token material. The risk is elevated when JWT tokens carry elevated scopes or when middleware retains references across request lifetimes. Properly designed token handling in Axum avoids these pitfalls through strict ownership models and synchronization.

To mitigate Use After Free in Axum when working with JWT tokens, ensure that all data captured by async handlers or middleware is owned and properly synchronized. Use Arc to share immutable data safely across threads, and avoid holding references that may dangle. Always clone or deep-copy data that must persist beyond the immediate request scope, especially when spawning tasks.

Example 1: Safe JWT Validation with Owned Data

use axum::{routing::post, Router};
use jsonwebtoken::{decode, Algorithm, DecodingKey, Validation};
use serde::{Deserialize, Serialize};
use std::sync::Arc;

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

#[tokio::main]
async fn main() {
    // Load keys into an Arc to share safely across handlers
    let key_data = Arc::new(include_bytes!("keys.json").to_vec());
    let app = Router::new()
        .route("/validate", post(move |payload: String| {
            let key_data = Arc::clone(&key_data);
            async move {
                // Decode token using owned key data
                let token = payload;
                let key = DecodingKey::from_secret(&key_data);
                let validation = Validation::new(Algorithm::HS256);
                let token_data = decode::(&token, &key, &validation)?;
                Ok::<_, jsonwebtoken::errors::Error>(token_data.claims)
            }
        }));
    axum::Server::bind(&"0.0.0.0:3000".parse().unwrap()).serve(app.into_make_service()).await.unwrap();
}

Example 2: Cloning Claims for Async Tasks

use axum::{routing::post, Json};
use jsonwebtoken::{decode, DecodingKey, Validation};
use serde::Deserialize;
use std::sync::Arc;
use tokio::task;

#[derive(Debug, Clone, Deserialize)]
struct Claims {
    user_id: u64,
}

async fn handle_token(Json(payload): Json) -> String {
    // Explicitly clone data before moving into async block
    let claims = decode::(
        &payload,
        &DecodingKey::from_secret("secret".as_ref()),
        &Validation::default(),
    )
    .map(|data| data.claims)
    .unwrap_or_else(|_| Claims { user_id: 0 });

    let claims_clone = claims.clone();
    task::spawn(async move {
        // Use cloned, owned data safely
        println!("Processing user: {}", claims_clone.user_id);
    });

    format!("User ID: {}", claims.user_id)
}

// Integrate into Axum router
// let app = Router::new().route("/token", post(handle_token));

These examples emphasize ownership: the key material and claims are cloned or wrapped in Arc to ensure they live long enough. Avoid borrowing across .await points unless you are certain of lifetimes. middleBrick’s CLI can be used to scan endpoints for patterns that may indicate missing synchronization, though remediation remains a developer responsibility.

Additional best practices include: using typed wrappers around DecodingKey to enforce lifetime-free operations, validating token expiration before use, and ensuring logging does not capture dangling references. For teams using the Pro plan, continuous monitoring can detect anomalous token handling behavior in production-like environments, complementing secure coding practices.