Format String in Axum with Cockroachdb

A format string vulnerability occurs when user-controlled input is directly used in functions that interpret format specifiers (e.g., println!, format!) without proper sanitization. In an Axum application interfacing with CockroachDB, this typically arises when building SQL strings or logging messages using unchecked user input. For example, concatenating a request parameter directly into a SQL query string and then passing the resulting string to a logging or formatting routine can expose placeholders like %s or %x to user control. An attacker can supply crafted input such as '; DROP TABLE users; -- %s %s %s, which may lead to information disclosure through log output or, in broader contexts, enable SQL injection when the string is later executed. CockroachDB, while strict in its SQL syntax, does not inherently prevent misuse of format strings at the application layer; vulnerabilities arise when Axum routes forward user data into format strings that are either logged or used to construct queries without parameterization. The risk is compounded when developers use macros like format! to assemble queries dynamically, inadvertently exposing internal state or error messages that reveal database structure or sensitive data. This combination emphasizes the importance of treating all user input as untrusted and avoiding string interpolation for SQL or logging.

To prevent format string issues in Axum when working with CockroachDB, always use parameterized queries and avoid string formatting for SQL statements. Axum integrates with database libraries such as sqlx or diesel, which support prepared statements that separate SQL logic from data. Below is a concrete example using sqlx with CockroachDB in an Axum handler:

use axum::{routing::get, Router};
use sqlx::postgres::PgPoolOptions;
use std::net::SocketAddr;

#[derive(serde::Deserialize)]
struct UserQuery {
    user_id: i32,
}

async fn get_user_handler(
    user_query: axum::extract::Query,
    pool: &sqlx::PgPool,
) -> Result {
    let user = sqlx::query!("SELECT username, email FROM users WHERE id = $1", user_query.user_id)
        .fetch_one(pool)
        await
        .map_err(|e| (axum::http::StatusCode::INTERNAL_SERVER_ERROR, e.to_string()))?;

    Ok(format!(\"User: {} ({}),\\n\", user.username, user.email))
}

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let pool = PgPoolOptions::new()
        .connect("postgresql://user:password@localhost:26257/mydb?sslmode=require")
        .await
        .expect("Failed to connect to CockroachDB");

    let app = Router::new().route("/user", get(get_user_handler));

    let listener = tokio::net::TcpListener::bind("0.0.0.0:3000").await.unwrap();
    axum::serve(listener, app).await.unwrap();
}

In this example, the SQL query uses $1 as a placeholder, and user_query.user_id is passed as a parameter. This ensures that user input is never interpreted as part of the SQL command or formatting string, effectively mitigating format string risks. Avoid using format! or macros that concatenate SQL strings with user input, such as:

// Unsafe pattern to avoid
let query = format!(\"SELECT * FROM users WHERE id = {}", user_input);
sqlx::query(&query).fetch_one(&pool).await;

Additionally, ensure that any logging within Axum handlers does not directly embed user input into format strings. Use structured logging with key-value pairs instead of string interpolation. For example, with the tracing crate:

use tracing::info;

info!(user_id = user_query.user_id, "Fetching user data");

By following these practices, applications using Axum and CockroachDB can avoid format string vulnerabilities while maintaining compatibility with CockroachDB’s SQL dialect.