Heap Overflow in Actix with Jwt Tokens

A heap overflow in an Actix web service that processes JWT tokens typically occurs when token parsing or validation logic performs unchecked copies into fixed-size heap buffers. In Rust, this is less common than in languages without bounds-checking, but it can still arise through unsafe blocks, FFI calls, or misuse of serialization/deserialization crates that write into pre-allocated buffers. When a JWT token is large or malformed, an attacker can supply a token crafted to trigger excessive writes beyond intended buffer boundaries. Because Actix routes often decode the token early in the request lifecycle, the overflow may occur before authorization checks, allowing an unauthenticated attacker to influence memory layout.

The combination is risky because JWT tokens carry elevated privileges; a token that is accepted without sufficient validation may be processed with higher trust. If the token parsing code uses fixed-size arrays or relies on unchecked length assumptions (for example, assuming a header or payload segment fits within a predefined buffer), an oversized segment can corrupt adjacent heap metadata. This can lead to information disclosure or control-flow manipulation. Because middleBrick scans the unauthenticated attack surface, it can detect symptoms such as inconsistent responses to oversized tokens or missing length checks, even when the runtime does not crash.

Real-world patterns include using the jsonwebtoken crate with custom validation logic that manually inspects claims, or integrating with Actix extractors that deserialize the token body into a fixed-size structure. A vulnerable extractor might allocate a small buffer and then copy token segments into it without verifying the segment lengths against the buffer size. middleBrick’s input validation and unsafe consumption checks can surface these issues by sending tokens with extreme lengths or unusual encodings and observing whether the service behaves inconsistently.

Remediation centers on removing fixed-size buffers for JWT processing and using safe, length-checked abstractions. Avoid manual copying of token segments into fixed arrays; instead, parse the token as a string or a structured value and validate lengths before use. Prefer high-level crates that handle memory safely and enforce bounds automatically.

Example 1: Safe JWT validation with jsonwebtoken and Actix extractor

use actix_web::{web, HttpResponse, Result};
use jsonwebtoken::{decode, Algorithm, DecodingKey, Validation, TokenData};
use serde::{Deserialize, Serialize};

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

async fn validate_token_jwt(auth_header: web::Header<String>) -> Result<HttpResponse> {
    let token = auth_header.into_inner();
    let token = token.strip_prefix("Bearer ").unwrap_or(&token);
    if token.is_empty() {
        return Ok(HttpResponse::BadRequest().body("Missing token"));
    }
    // Use a reasonable maximum token length to prevent resource exhaustion
    if token.len() > 4096 {
        return Ok(HttpResponse::Payload_Too_Large().body("Token too large"));
    }
    let validation = Validation::new(Algorithm::HS256);
    let key = DecodingKey::from_secret(b"secret");
    let token_data: TokenData<Claims> = decode(token, &key, &validation)
        .map_err(|_| actix_web::error::ErrorUnauthorized("Invalid token"))?;
    Ok(HttpResponse::Ok().json(token_data.claims))
}

Example 2: Actix middleware that enforces token size limits before parsing

use actix_web::{dev::ServiceRequest, Error, middleware::Next};
use actix_web::body::BoxBody;
use actix_web::http::header::HeaderValue;

async fn jwt_size_limiter(
    req: ServiceRequest,
    next: Next<BoxBody>
) -> Result<actix_web::dev::ServiceResponse<BoxBody>, Error> {
    if let Some(header) = req.headers().get("authorization") {
        if let Ok(auth) = header.to_str() {
            let token = auth.strip_prefix("Bearer ").unwrap_or(auth);
            // Reject tokens larger than 4096 bytes early
            if token.len() > 4096 {
                return Err(actix_web::error::ErrorPayloadTooLarge("Token too large"));
            }
        }
    }
    next.call(req).await
}

Example 3: Avoiding fixed-size buffers in custom parsing

// Instead of this potentially unsafe pattern:
// let mut buffer = [0u8; 256];
// buffer.copy_from_slice(token_bytes);
// Use a Vec or String with length checks:
fn safe_token_store(token_bytes: &[u8) -> Result<Vec<u8>, &'static str> {
    const MAX_TOKEN_BYTES: usize = 4096;
    if token_bytes.len() > MAX_TOKEN_BYTES {
        return Err("Token exceeds maximum size");
    }
    let mut safe_buffer = Vec::with_capacity(token_bytes.len());
    safe_buffer.extend_from_slice(token_bytes);
    Ok(safe_buffer)
}

These examples emphasize length validation, rejection of oversized tokens before processing, and using safe abstractions that prevent unchecked heap writes. middleBrick can validate these controls by testing with long tokens and observing whether the service rejects them gracefully rather than exhibiting unstable behavior.