Cache Poisoning in Rocket with Hmac Signatures

Cache poisoning occurs when an attacker causes a cache (e.g., CDN, reverse proxy, or in-memory cache) to store malicious responses that are served to other users. In Rocket, if you use Hmac Signatures only for selected routes while relying on caching for unauthenticated or partially authenticated paths, you can inadvertently cache responses that should be personalized or protected, leading to data leakage or privilege confusion.

Consider a Rocket endpoint that returns user-specific data and uses Hmac Signatures to validate a tamper-proof token included in the request. If the caching layer does not include the Hmac payload or signature in the cache key, two different users with distinct tokens may receive the same cached response. For example, User A’s sensitive profile could be cached and then served to User B if the cache key ignores the Hmac parameter or the signed claims that differentiate users.

Another scenario involves query parameters used for cache differentiation that are not validated by the Hmac scheme. An attacker could manipulate unauthenticated query parameters to poison the cache with crafted content, while the Hmac signature on other parts of the request fails to protect the full request context. Because Rocket’s routing can bind parameters independently of signature validation, a misconfigured cache key that excludes signed headers or body content enables the poisoned cache entry to persist.

These issues are particularly relevant when OpenAPI/Swagger spec analysis is used to generate client integrations without ensuring that cache-related headers and signature inputs are consistently considered. If runtime findings from unauthenticated scans highlight mismatches between expected and actual cache behavior, the exposure becomes clearer: cached responses may reflect one user’s Hmac-signed data while being served to others who did not provide a valid signature.

To identify such risks, scans that test the unauthenticated attack surface can expose endpoints where Hmac Signatures are partially implemented but cache controls are inconsistent. This does not imply automatic fixing; rather, it highlights the need to align cache key construction with the security boundaries enforced by Hmac validation in Rocket.

Remediation focuses on ensuring that the cache key incorporates all inputs that the Hmac scheme protects, and that responses are cached only when the signature and claims provide sufficient context to prevent cross-user contamination.

Below are concrete Rocket code examples in Rust that demonstrate how to structure request handling and cache key generation alongside Hmac verification.

use rocket::serde::json::Json;
use rocket::Request;
use rocket::response::Responder;
use std::collections::HashMap;

// Example Hmac signature verification helper
fn verify_hmac_signature(
    body: &str,
    headers: &rocket::http::HeaderMap,
    expected_key: &[u8],
) -> bool {
    // In practice, use a crate like `hmac` and `sha2`
    // This is a simplified illustration
    if let Some(sig_header) = headers.get("X-API-Signature") {
        if let Ok(sig_str) = sig_header.to_str() {
            // Compute Hmac over body + selected headers
            let computed = hmac::Hash::hmac(expected_key, body.as_bytes());
            return format!("{:x}", computed) == sig_str;
        }
    }
    false
}

// Rocket route with cache-aware Hmac handling
#[rocket::post("/user/profile", data = <data>)]
fn user_profile(
    data: Json<serde_json::Value>,
    request: &Request<'_>,
) -> Result<Json<serde_json::Value>, &'static str> {
    let body = data.to_string();
    let headers = request.headers();

    // Verify Hmac signature before using or caching the response
    if !verify_hmac_signature(&body, headers, include_bytes!("../hmac_key.der")) {
        return Err("Invalid signature");
    }

    // Construct a cache key that includes signed claims and user identifier
    let user_id = match data.get("user_id") {
        Some(id) => id.as_str().unwrap_or("unknown"),
        None => return Err("Missing user_id"),
    };
    let cache_key = format!(
        "profile:{}:sig:{}",
        user_id,
        headers.get("X-API-Signature")
            .and_then(|v| v.to_str().ok())
            .unwrap_or("none")
    );

    // Pseudo-code: store/retrieve from cache using cache_key
    // if let Some(cached) = cache.get(&cache_key) { return cached; }
    // let response = build_profile_response(user_id);
    // cache.insert(cache_key, response.clone());
    // Ok(Json(response))

    // Placeholder response
    Ok(Json(serde_json::json!({
        "user_id": user_id,
        "status": "ok"
    })))
}

Key points in the example:

• The Hmac signature is verified using the request body and selected headers, ensuring that tampering is detected before any cache lookup or response generation.
• The cache key explicitly includes the user identifier and the signature value, preventing one user’s cached response from being served to another.
• If your Rocket application uses OpenAPI/Swagger spec analysis, ensure that the spec documents which headers and query parameters participate in Hmac signing so that generated clients and cache rules remain aligned with runtime behavior.

For broader protection, apply similar logic to all endpoints where Hmac Signatures guard sensitive operations, and validate that cache invalidation rules respect signature rotation and key expiration.