Cryptographic Failures in Axum with Mongodb

When an Axum service stores or transmits data that requires confidentiality or integrity and interacts with MongoDB, cryptographic failures often arise from mismatched expectations about encryption at rest, in transit, and in application logic. Axum, a Rust web framework, does not implicitly encrypt data before it reaches MongoDB; if the application does not explicitly encrypt fields before serialization, sensitive values may be persisted in plaintext or with weak protections.

In this stack, a common vulnerability pattern is relying on MongoDB transport encryption (TLS) alone while leaving sensitive fields such as personal identifiers, authentication tokens, or financial details unencrypted at the application layer. If the MongoDB deployment is misconfigured (e.g., TLS disabled or using outdated protocols), data in transit can be exposed. Additionally, if the application uses predictable or static keys, or stores keys alongside data (e.g., in environment variables without protection), an attacker who gains access to the database or the application runtime can decrypt sensitive records.

Another specific risk arises from deserialization and schema handling in Axum extractors. If an API endpoint accepts raw BSON or JSON and maps it directly to MongoDB documents without validating and encrypting sensitive properties, it may inadvertently store secrets in reversible form. This becomes critical in compliance contexts mapped to OWASP API Top 10 A02:2023 (Cryptographic Failures) and can intersect with findings from middleBrick checks such as Data Exposure and Encryption, which flag unencrypted sensitive payloads.

Consider a concrete scenario: an Axum handler receives a JSON payload containing a user’s email and password, hashes the password with a weak scheme (or stores it as-is), and inserts the document into MongoDB without field-level encryption. middleBrick’s Data Exposure check may detect that the payload or stored record contains credentials in recoverable form, while the Encryption check highlights missing protections for data at rest. If the API also exposes an OpenAPI spec, inconsistent documentation about encryption requirements can further mislead consumers, increasing the risk of insecure integrations.

LLM/AI Security probes from middleBrick can surface additional risks when AI-assisted coding generates API scaffolding that inadvertently omits encryption steps or uses insecure defaults. For example, a generated route might serialize a struct with sensitive fields and insert it into MongoDB without encryption, exposing secrets through logs or backups. The scanner’s System Prompt Leakage and Output Scanning capabilities help detect such patterns by identifying insecure code generation behaviors and ensuring that cryptographic best practices are not bypassed by automated tooling.

To address cryptographic failures in Axum with MongoDB, implement explicit encryption at the application layer before data reaches the database, enforce strong transport settings, and validate deserialization paths. Below are concrete, idiomatic Rust examples using the MongoDB Rust driver and common cryptographic crates.

First, define a data model where sensitive fields are encrypted before serialization. Use serde with a custom wrapper that handles encryption and decryption, and ensure the MongoDB collection stores only ciphertext for protected fields.

use mongodb::bson::{doc, Bson, Document};
use mongodb::options::ClientOptions;
use mongodb::Client;
use serde::{Deserialize, Serialize};
use aes_gcm::{Aes256Gcm, Key, Nonce, aead::{Aead, KeyInit, generic_array::GenericArray}};
use base64::{Engine as _, engine::general_purpose};

#[derive(Serialize, Deserialize, Debug)]
struct UserRecord {
    #[serde(rename = "_id")]
    id: String,
    #[serde(serialize_with = "encrypt_field", deserialize_with = "decrypt_field")]
    email: String,
    #[serde(serialize_with = "encrypt_field", deserialize_with = "decrypt_field")]
    password_hash: String,
}

fn encrypt_field(value: &str, serializer: S) -> Result
where
    S: serde::Serializer,
{
    let key = Key::from_slice(include_bytes!("./keys/aes_key_32")); // 32 bytes key stored securely
    let cipher = Aes256Gcm::new(key);
    let nonce = Nonce::from_slice(b"unique nonce"); // in practice, generate per value and store with ciphertext
    let ciphertext = cipher.encrypt(nonce, value.as_bytes())
        .map_err(|_| serde::ser::Error::custom("encryption failed"))?;
    let combined = [nonce.to_vec(), ciphertext].concat();
    serializer.serialize_str(&general_purpose::STANDARD.encode(combined))
}

fn decrypt_field<'de, D>(deserializer: D) -> Result
where
    D: serde::Deserializer<'de>,
{
    let encoded = String::deserialize(deserializer)?;
    let bytes = general_purpose::STANDARD.decode(&encoded).map_err(serde::de::Error::custom)?;
    let key = Key::from_slice(include_bytes!("./keys/aes_key_32"));
    let cipher = Aes256Gcm::new(key);
    let (nonce_bytes, ciphertext) = bytes.split_at(12);
    let nonce = Nonce::from_slice(nonce_bytes);
    let plaintext = cipher.decrypt(nonce, ciphertext.as_ref())
        .map_err(serde::de::Error::custom("decryption failed"))?;
    String::from_utf8(plaintext).map_err(serde::de::Error::custom("invalid utf-8"))
}

async fn insert_user(client: &Client, user: UserRecord) -> mongodb::error::Result<()> {
    let coll = client.database("appdb").collection("users");
    let doc = mongodb::bson::to_document(&user)?;
    coll.insert_one(doc, None).await?;
    Ok(())
}

use mongodb::options::ClientOptions;

async fn build_secure_client(uri: &str) -> mongodb::error::Result {
    let mut client_options = ClientOptions::parse(uri).await?;
    client_options.tls = Some(mongodb::options::TlsOptions {
        enabled: true,
        allow_invalid_hostnames: false,
        allow_invalid_certificates: false,
        ..Default::default()
    });
    Client::with_options(client_options)
}

use axum::extract::Json;
use validator::Validate;

#[derive(Validate, Serialize, Deserialize)]
struct CreateUser {
    #[validate(email)]
    email: String,
    #[validate(length(min = 12))]
    password: String,
}

async fn create_user(Json(payload): Json) -> Result {
    payload.validate().map_err(|e| (StatusCode::BAD_REQUEST, e.to_string()))?;
    // proceed to encrypt and insert
    Ok(axum::http::StatusCode::CREATED)
}