Rate Limiting Bypass in Actix with Mongodb

Rate limiting is a control that restricts the number of requests a client can make to an endpoint within a defined time window. When an Actix service relies on application-level rate limiting without validating authorization context per request, and it uses a shared data store such as Mongodb to track request counts, attackers can bypass limits by manipulating identifiers or by exploiting unauthenticated endpoints.

Consider an Actix REST endpoint that performs user profile lookup and uses Mongodb to store request metadata. If the rate limiting logic is implemented by incrementing a counter in a Mongodb document keyed only by IP address or by an unverified query parameter, an attacker can rotate IPs (e.g., via proxy chains) or manipulate optional identifiers such as userId that are not enforced for unauthenticated calls. Because the check occurs after routing but before business logic in Actix, an attacker can send many requests that appear distinct due to changing identifiers, while the Mongodb update operations remain lightweight and do not enforce per-user throttling.

In addition, if the Actix route exposes an unauthenticated handler that queries Mongodb with user-supplied input to locate a document (e.g., by username or handle) and uses that document to decide whether to allow a request, an attacker can enumerate valid users without triggering per-user rate limits. The vulnerability is not in Mongodb itself, but in how the Actix application uses identifiers and authorization context when deciding whether to increment and check counters in Mongodb. The absence of binding the counter to an authorized subject or session enables horizontal privilege escalation and bypass of intended request caps.

During a scan, middleBrick tests such scenarios by probing unauthenticated and authenticated paths, checking whether rate limiting is applied consistently across identifier variations and whether authorization boundaries are respected before counting requests in Mongodb-backed tracking. Findings include missing user-level throttling, reliance on easily spoofed headers, and inconsistent enforcement across routes that share the same Mongodb collections.

To mitigate rate limiting bypass in Actix with Mongodb, bind rate limit counters to the authorized subject and enforce checks before performing any database operation. Use a stable identifier such as user_id from a validated authentication context rather than IP or mutable query parameters. Ensure each route that requires throttling performs the check within the Actix guard or extractor phase so that unauthorized requests are rejected before incrementing state in Mongodb.

Below is a concrete example of an Actix handler that uses Mongodb with proper user-bound rate limiting. It assumes you have validated authentication and extracted a user identifier; the example uses the mongodb Rust driver and Actix web extractors.

use actix_web::{web, HttpResponse, Result};
use mongodb::{bson::{doc, oid::ObjectId}, Client};
use std::time::{SystemTime, UNIX_EPOCH};

async fn user_profile(
    client: web::Data,
    user_id: String, // validated user identifier from auth extractor
    path: web::Path, // requested profile handle
) -> Result {
    let db = client.database("api_db");
    let coll = db.collection("rate_limits");
    let now = SystemTime::now().duration_since(UNIX_EPOCH)?.as_secs();
    let window = 60; // 60 seconds
    let max_requests = 100;

    // Use a compound key that includes the user_id to enforce per-user limits
    let key = format!("user:{}", user_id);
    let filter = doc! { "_id": &key };
    let update = doc! {
        "$set": { "_id": key.clone() },
        "$inc": { "count": 1 },
        "$min": { "window_start": &(now.saturating_sub(now % window)) },
    };
    let options = mongodb::options::UpdateOptions::builder().upsert(true).build();
    coll.update_one(filter, update, options).await?;

    // Read back to enforce limit
    let doc = coll.find_one(doc! { "_id": &key }, None).await?;
    if let Some(doc) = doc {
        let count = doc.get_i64("count").unwrap_or(0);
        if count > max_requests {
            return Ok(HttpResponse::TooManyRequests().body("Rate limit exceeded"));
        }
    }

    // Proceed with profile lookup using the validated user_id
    let profiles = db.collection("profiles");
    let filter = doc! { "handle": &path };
    let profile = profiles.find_one(filter, None).await?;
    match profile {
        Some(doc) => Ok(HttpResponse::Ok().json(doc)),
        None => Ok(HttpResponse::NotFound().body("Not found")),
    }
}