Command Injection in Axum with Api Keys

Command injection occurs when an API passes untrusted input directly to a system shell or to process-spawning functions without validation or sanitization. In Axum, building an endpoint that accepts an API key and then uses its value in a shell command can create a critical security risk. An API key is often treated as an opaque credential, but if it is concatenated into a command string without proper escaping, an attacker who knows or guesses the key may be able to inject shell metacharacters.

Consider a scenario where a developer logs or executes a helper script using the API key value, for example to trigger an external tool or to pass it to a backend service via a shell command. If the implementation does not treat the key as untrusted, characters such as ;, &, |, &&, or backticks can allow an attacker to chain arbitrary commands. This becomes a command injection vector even though the API key itself is intended to be secret; the secrecy does not prevent injection if the value is used unsafely.

In Axum, handlers often extract headers or query parameters into strongly typed extractors. If an API key is extracted and then passed to a function that builds a shell command using string interpolation, the application can execute unintended instructions on the host. For instance, suppose an endpoint receives api_key as a header and forwards it to a monitoring script via std::process::Command. An attacker could supply a key like validkey; cat /etc/passwd. Without input validation or shell-escaping, the command may execute both the intended program and the injected cat command, leading to information disclosure.

Another subtle risk arises when API keys are stored or logged in environments where shell metacharacters can affect log parsing or automation. Even if the immediate command invocation appears safe, downstream scripts or imported libraries might later use the key in a shell context, effectively expanding the attack surface. This highlights the importance of validating and sanitizing API key usage at the point where external processes are spawned, regardless of whether the key is considered sensitive.

middleBrick detects such patterns by analyzing the unauthenticated attack surface and identifying unsafe use of inputs in command construction. It flags instances where API key values are reflected into shell contexts without proper escaping or isolation. These findings align with common attack patterns such as those cataloged in the OWASP API Top 10 and can lead to severe consequences including unauthorized file access or arbitrary code execution.

Remediation focuses on avoiding shell invocation entirely and treating API keys as opaque strings that must never be interpolated into commands. When external tooling is required, prefer structured APIs or SDKs instead of shelling out. If shell usage is unavoidable, use safe process-spawning methods that do not invoke a shell, and validate input strictly.

Example of vulnerable code in Axum where an API key is concatenated into a shell command:

use axum::{routing::get, Router};
use std::process::Command;

async fn handler(api_key: String) -> String {
    // Unsafe: directly using API key in shell command
    let output = Command::new("sh")
        .arg("-c")
        .arg(format!("/usr/local/bin/helper --key {}", api_key))
        .output()
        .expect("failed to execute process");
    String::from_utf8_lossy(&output.stdout).to_string()
}

let app = Router::new().route("/run", get(handler));

In this example, if api_key contains shell metacharacters, an attacker can execute arbitrary commands. The fix is to avoid the shell and pass arguments directly, or to sanitize the key if it must be used in a controlled context.

Safe approach without shell invocation:

use axum::{routing::get, Router};
use std::process::Command;

async fn handler(api_key: String) -> String {
    // Safe: pass arguments directly without shell interpretation
    let output = Command::new("/usr/local/bin/helper")
        .arg("--key")
        .arg(api_key)
        .output()
        .expect("failed to execute process");
    String::from_utf8_lossy(&output.stdout).to_string()
}

let app = Router::new().route("/run", get(handler));

By omitting the shell, arguments are passed literally, and metacharacters are not interpreted as commands. This eliminates command injection risk while still providing the key to the binary.

If logging or auxiliary tooling requires shell usage, apply strict validation to the API key format. For example, enforce alphanumeric plus limited punctuation using a regex and reject any input that does not match. Additionally, avoid logging the raw key or ensure logs are structured to prevent injection into log-processing scripts.

middleBrick supports these safe practices by scanning for command construction patterns that include API key values in shell contexts. It highlights the need to avoid string-based command assembly and encourages the use of direct argument passing. The scanner also maps findings to relevant compliance frameworks, helping teams align with secure coding standards.