Double Free in Axum with Jwt Tokens

A Double Free occurs when a program attempts to free the same memory allocation more than once. In the context of Axum (a Rust web framework) using JWT tokens, this typically arises through unsafe interactions between token parsing logic and Rust's ownership system, often when integrating with C-based cryptography libraries via FFI. When JWT tokens are processed, the runtime may allocate memory for parsed claims, headers, or cryptographic keys. If Axum middleware or custom token handling code incorrectly manages lifetimes—such as cloning raw pointers, using unsafe { Box::from_raw() } on already-freed data, or improperly handling reference counts across async boundaries—the same memory can be freed twice during stack unwinding or error recovery paths.

Specifically, when JWT tokens are validated, allocations for signature verification buffers or claim deserialization can be involved. If an error occurs (e.g., invalid signature, expired token) and the error handling path redundantly drops resources, or if integration code with external C libraries (e.g., OpenSSL via openssl-sys) triggers deallocation in multiple branches, the runtime can invoke free() on the same address. This is exacerbated when Axum's extractor logic for JWT tokens uses raw pointers or when middleware manually manages token state across Future poll cycles, creating multiple opportunities for double deallocation under concurrency or panic scenarios.

The vulnerability surface is exposed when JWT token processing does not strictly adhere to Rust's ownership semantics—for example, using Rc or Arc across unsafe blocks, or failing to ensure single ownership of token-derived data. Since Axum relies on asynchronous execution and extractor composability, improper synchronization or incorrect Drop implementations for JWT-related structs can turn what should be safe token validation into a use-after-free or double-free condition, potentially leading to memory corruption or crashes.

To remediate Double Free risks when handling JWT tokens in Axum, ensure strict adherence to Rust's ownership model and avoid manual memory management. Use high-level abstractions for token parsing and validation, and ensure any integration with low-level or C-based cryptographic libraries is wrapped safely.

1. Safe JWT extraction with typed claims

Define strongly-typed claim structures and use established JWT libraries that manage memory safely. This avoids raw pointer handling and ensures single ownership.

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

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

async fn handler(
    auth_header: Option

// Safe wrapper for OpenSSL verification via high-level crate
use openssl::hash::MessageDigest;
use openssl::pkey::PKey;
use openssl::sign::Verifier;

fn verify_signature_safe(
    data: &[u8],
    signature: &[u8],
    public_key_pem: &[u8],
) -> Result<bool, openssl::error::ErrorStack> {
    let pkey = PKey::public_key_from_pem(public_key_pem)?;
    let mut verifier = Verifier::new(MessageDigest::sha256(), &pkey)?;
    verifier.update(data)?;
    Ok(verifier.verify(signature)? == 1)
}

// In Axum extractor, call the safe wrapper instead of raw FFI
async fn jwt_extractor(
    header: Option<axum::extract::Header<axum::http::header::AUTHORIZATION>>
) -> Result<Claims, (axum::http::StatusCode, String)> {
    // ... extract token ...
    // Use safe JWT crate; do not manually free buffers
    let claims = validate_jwt_token(token).await?;
    Ok(claims)
}

use std::sync::Arc;

async fn process_token(token: String) -> Result<Claims, (axum::http::StatusCode, String)> {
    let token = Arc::new(token);
    let token_clone = Arc::clone(&token);
    
    // Simulate async validation without moving references across await
    let claims = validate_async(token_clone).await?;
    Ok(claims)
}