Bleichenbacher Attack in Axum with Cockroachdb

A Bleichenbacher attack is a cryptographic padding oracle technique that can allow an attacker to decrypt ciphertexts without knowing the key by iteratively sending specially crafted requests and observing error behavior. In an Axum application using Cockroachdb as the backend data store, the risk arises when cryptographic operations (such as token verification or encrypted session handling) are performed server-side and error responses differ based on padding validity.

Consider an Axum endpoint that accepts an encrypted authentication token, decrypts it using a RSA-OAEP scheme, and then queries Cockroachdb to validate the user. If the implementation does not use constant-time comparison and instead returns distinct HTTP status codes or response bodies for padding errors versus database errors, the service behaves as a padding oracle. An attacker can automate requests that probe the oracle by submitting modified ciphertexts and observing whether the service reports a padding failure or proceeds to a database lookup (which may produce a different error, such as a missing row).

In a typical Axum + Cockroachdb setup, the flow might look like this: the client sends an encrypted cookie or Authorization header; Axum decrypts it using a private key; if decryption succeeds, the application executes a SQL query like SELECT * FROM users WHERE id = $1 against Cockroachdb; if the padding is invalid, decryption fails early and a 400 is returned. The distinction between a 400 and a 404/500 enables the oracle. Cockroachdb itself does not introduce the vulnerability, but its use in the flow means the attacker can infer correctness indirectly via whether the query is attempted, especially if secondary errors (connection issues, constraint violations) differ from padding errors.

This becomes a security issue when the application does not enforce uniform error handling and timing across all failure paths. The Axum middleware or route handlers must ensure that any cryptographic operation failure results in the same response shape and timing as any other failure before reaching Cockroachdb. Without this, the unauthenticated attack surface includes the error behavior of the combined stack, which can be exploited to recover plaintext or session tokens over time.

Compliance mappings such as OWASP API Top 10 (2023) A02:2023 — Cryptographic Failures, and frameworks like PCI-DSS, highlight the need for strong cryptography and consistent error handling. middleBrick scans can detect indicators of such oracle behavior by analyzing endpoint variance in response codes and timing across repeated requests, flagging inconsistencies that may enable a Bleichenbacher-style attack in this Axum + Cockroachdb deployment.

To mitigate Bleichenbacher-style padding oracle risks in an Axum application using Cockroachdb, ensure that cryptographic operations never reveal distinguishability via status codes, response body differences, or timing variations. Always perform constant-time comparisons and move database interaction out of the cryptographic verification path.

Below is a secure Axum handler pattern that separates concerns and avoids leaking oracle behavior. It first attempts decryption in constant time, and only after successful decryption does it proceed to Cockroachdb. Errors are normalized to a generic failure response.

use axum::{{
    async_trait, body::Body, extract::FromRequest, http::{request::Parts, StatusCode},
    response::IntoResponse, routing::post, Json, Router,
}};
use rsa::{PaddingScheme, PublicKey, RSAPrivateKey};
use serde::{Deserialize, Serialize};
use std::time::Instant;
use crate::db::get_db_pool; // your Cockroachdb pool wrapper

#[derive(Deserialize)]
struct TokenRequest {
    ciphertext: String, // base64-encoded
}

#[derive(Serialize)]
struct GenericError {
    message: String,
}

async fn decrypt_constant_time(private_key: &RSAPrivateKey, data_b64: &str) -> Result, rsa::Error> {
    // Decode without branching on content
    let ciphertext = base64::decode(data_b64)?;
    let padding = PaddingScheme::new_pkcs1v15_decrypt();
    // rsa::RSAPrivateKey::decrypt returns Result, _> on padding or decryption failure
    private_key.decrypt(padding, &ciphertext)
}

async fn handler(
    Json(req): Json,
) -> Result)> {
    // 1) Constant-time decryption; any error yields the same generic response
    let private_key = get_private_key(); // load once at startup
    let decrypted = match decrypt_constant_time(&private_key, &req.ciphertext).await {
        Ok(bytes) => bytes,
        Err(_) => {
            // Always return the same shape and status to avoid oracle signaling
            return Err((StatusCode::BAD_REQUEST, Json(GenericError { message: "invalid request".to_string() })));
        }
    };

    // 2) Only after successful decryption do we interact with Cockroachdb
    let user_id = match std::str::from_utf8(&decrypted) {
        Ok(s) => s.trim(),
        Err(_) => return Err((StatusCode::BAD_REQUEST, Json(GenericError { message: "invalid request".to_string() }))),
    };

    let pool = get_db_pool().map_err(|_| (StatusCode::INTERNAL_SERVER_ERROR, Json(GenericError { message: "service unavailable".to_string() })))?;
    // Use parameterized query to avoid SQL injection; handle errors generically
    let row = sqlx::query!( "SELECT id, email FROM users WHERE id = $1", user_id )
        .fetch_optional(&pool)
        .await
        .map_err(|_| (StatusCode::INTERNAL_SERVER_ERROR, Json(GenericError { message: "service unavailable".to_string() })))?;

    match row {
        Some(rec) => Ok(Json(serde_json::json!({ "id": rec.id, "email": rec.email }))),
        None => Err((StatusCode::NOT_FOUND, Json(GenericError { message: "not found".to_string() }))),
    }
}

// Example router setup
pub fn routes() -> Router {
    Router::new().route("/login/token", post(handler))
}

Key points in this remediation:

• Constant-time decryption: The decryption function returns an error for any padding mismatch, and the handler treats all decryption errors identically, preventing timing or status-code differences that could be probed.
• Separation of concerns: Database access occurs only after successful decryption, but errors from Cockroachdb are also normalized to a generic 500 response to avoid distinguishing between "invalid token" and "database unavailable."
• Parameterized SQL: Using $1 placeholders with sqlx avoids injection and keeps error paths consistent.
• Uniform response shape: The GenericError structure ensures that an attacker cannot differentiate causes based on body content.

Additionally, rotate keys regularly, prefer authenticated encryption with associated data (AEAD) where feasible, and use middleware to enforce global error handling so that no route inadvertently returns distinguishable errors. middleBrick can be used in your workflow (via the CLI: middlebrick scan <url>, the GitHub Action to gate PRs, or the MCP Server in your IDE) to validate that error responses remain consistent across failure modes and that no endpoint exposes timing or status-code side channels that could facilitate a Bleichenbacher attack in this stack.