Command Injection in Rocket with Hmac Signatures

Command Injection occurs when an attacker can cause an application to execute arbitrary operating system commands. In Rocket, this risk can emerge when you build signed routes using Hmac Signatures and then use parts of the validated payload to construct shell commands. Even with a valid signature verifying integrity and authenticity, the application may pass unchecked external input to a shell executor, bypassing signature trust because the signature only guarantees the data has not been tampered with, not that it is safe to use as a command.

Consider a Rocket endpoint that accepts a filename and a user identifier, validates them via an Hmac-signed query or body, and then invokes a system utility such as tar or gzip. If the developer concatenates the user-supplied filename directly into the command string, an attacker who does not need to break the Hmac can supply values like report.txt; cat /etc/passwd or report.txt && id. Because the command is built from data presumed trustworthy after signature verification, the shell will execute the injected segment with the same privileges as the Rocket process. This is not a flaw in Hmac itself, but a failure to treat validated input as untrusted when it flows into an OS command.

Real-world patterns include using std::process::Command with .arg() incorrectly by joining a raw string, or invoking a script that builds commands via interpolation. Attackers may also probe for behaviors described in the OWASP API Top 10 (e.g., Improper Neutralization of Special Elements used in an OS Command) and common CVEs related to deserialization or injection in API endpoints. The presence of Hmac Signatures can give a false sense of security: the signature confirms the request came from an expected source and has not been modified, but it does not enforce least privilege, sandboxing, or input validation for shell execution.

An additional subtlety arises when the signed payload includes dynamic parts used to select commands or arguments. For example, a signed enum indicating an operation type (backup, cleanup, export) might be mapped directly to a shell binary path. If mapping is done naively, an attacker who can influence runtime configuration or discover non-signature-covered endpoints might pivot to dangerous binaries. Therefore, treating Hmac-verified data as safe for shell construction is a design error, not a cryptographic weakness.

Remediation centers on strict separation between authenticated metadata (protected by Hmac Signatures) and shell command construction. Never concatenate validated data into shell commands; use structured arguments and avoid the shell entirely when possible. Below are concrete Rocket examples in Rust that show safe patterns.

1) Safe command execution with explicit arguments and no shell involvement:

use rocket::get;
use rocket::serde::json::Json;
use rocket_hmac::{Hmac, HmacRequestGuard};
use std::process::Command;

#[derive(serde::Deserialize)]
struct ReportParams {
    filename: String,
}

#[get("/report?&signature")]
async fn generate_report(
    params: HmacRequestGuard<ReportParams>,
) -> Result<String, String> {
    // params is verified by Hmac Signatures; treat filename as untrusted
    let filename = &params.filename;

    // Validate filename format strictly (e.g., alphanumeric + limited extensions)
    if !filename.chars().all(|c| c.is_ascii_alphanumeric() || c == '.' || c == '-') {
        return Err("Invalid filename".into());
    }

    // Use explicit arguments; do not invoke a shell
    let output = Command::new("tar")
        .arg("-czf")
        .arg(format!("{}.tar.gz", filename))
        .arg(filename)
        .output()
        .map_err(|e| format("Execution error: {}", e))?;

    if output.status.success() {
        Ok(format!("Created {}.tar.gz", filename))
    } else {
        Err(format!("Tar failed: {}", String::from_utf8_lossy(&output.stderr)))
    }
}

2) Using a command router with a verified enum to avoid dynamic binary selection:

use rocket::get;
use rocket_hmac::{Hmac, HmacRequestGuard};
use std::process::Command;

#[derive(serde::Deserialize)]
enum Operation {
    Backup,
    Cleanup,
}

#[derive(serde::Deserialize)]
struct TaskParams {
    op: Operation,
    path: String,
}

#[get("/task?&signature")]
async fn run_task(
    params: HmacRequestGuard<TaskParams>,
) -> Result<String, String> {
    let path = &params.path;

    // Strict validation for path (e.g., must be under an allowed directory)
    if !path.starts_with("/safe/base/") {
        return Err("Path not allowed".into());
    }

    let cmd = match params.op {
        Operation::Backup => Command::new("/usr/bin/backup"),
        Operation::Cleanup => Command::new("/usr/bin/cleanup"),
    };

    let output = cmd
        .arg(path)
        .arg("--force")
        .output()
        .map_err(|e| format("Execution error: {}", e))?;

    if output.status.success() {
        Ok("Task completed".into())
    } else {
        Err(format!("Task failed: {}", String::from_utf8_lossy(&output.stderr)))
    }
}

3) If you must invoke a shell (e.g., for complex pipelines), use a shell-safe escaping library and avoid concatenation of untrusted strings. However, prefer the above approaches that avoid the shell entirely.

By combining Hmac Signatures for request authentication with rigorous input validation and process construction, you mitigate Command Injection while preserving the integrity checks the signatures provide.