Command Injection in Flask with Hmac Signatures

Command injection occurs when an attacker can inject and execute arbitrary system commands on the host. In Flask applications that use Hmac Signatures to validate webhook or API requests, the vulnerability arises when the application embeds user-influenced data into shell commands without proper sanitization, even though the request is authenticated with a valid Hmac signature. A typical pattern is using a query parameter, header, or JSON field to drive an external tool or script, and then passing that value directly to functions such as os.system, subprocess.run, or subprocess.Popen with shell=True. Because the Hmac signature confirms the request originates from a trusted source, developers may skip input validation, inadvertently allowing a signed but malicious payload to execute arbitrary commands.

Consider a Flask route that rebuilds an index for a search engine. The route expects an Hmac-SHA256 signature in a header, verifies it, and then invokes a system utility using a user-supplied index_name. If the developer does not validate or sanitize index_name, an attacker who knows or guesses a valid signature (e.g., from logs or a leaked configuration) can supply an index name such as test; cat /etc/passwd or $(id). When the command is constructed with string interpolation and executed with shell=True, the injected shell operators run additional commands, leading to command injection despite the Hmac check. This pattern is common in integrations that call tools like ffmpeg, convert, or custom scripts where parameters are driven by external input.

Another scenario involves file paths or identifiers used in constructed commands. An attacker might supply a filename like ../../../etc/hosts or embed newline characters to chain commands. Even with Hmac protecting the endpoint from unauthenticated callers, the application’s trust boundary is limited to signature verification; it does not automatically neutralize shell metacharacters or enforce strict allowlists. Because the scan category includes Unsafe Consumption and Input Validation, middleBrick can detect such risky patterns where signed inputs directly influence subprocess creation, highlighting missing sanitization and overly permissive command construction.

Real-world command injection in this context aligns with OWASP API Top 10 A03:2023 Injection and parallels typical system-level vulnerabilities seen in web frameworks. The presence of Hmac Signatures does not mitigate command injection; it only authenticates the request. If the underlying command construction is unsafe, the authenticated request becomes a vector for more precise attacks, such as reading sensitive files, spawning reverse shells, or manipulating system state. This is why input validation and strict command construction are essential regardless of authentication mechanisms.

Remediation focuses on avoiding shell interpretation of user data and validating inputs even after Hmac verification. The safest approach is to bypass the shell entirely and use the list form of subprocess APIs, which does not invoke /bin/sh and therefore prevents metacharacter injection. Below are concrete, realistic code examples that demonstrate a vulnerable pattern and its secure fix in a Flask application.

Vulnerable Example

The following snippet shows a Flask route that verifies an Hmac signature but then builds a shell command using string formatting, creating a command injection risk:

import hashlib
import hmac
import os
from flask import Flask, request

app = Flask(__name__)
SECRET_KEY = b'super-secret-key'

def verify_hmac(data, signature):
    expected = hmac.new(SECRET_KEY, data, hashlib.sha256).hexdigest()
    return hmac.compare_digest(expected, signature)

@app.route('/reindex')
def reindex():
    index_name = request.args.get('index', '')
    signature = request.headers.get('X-Hub-Signature-256', '').replace('sha256=', '')
    payload = index_name.encode('utf-8')
    if not verify_hmac(payload, signature):
        return 'Invalid signature', 401
    # Vulnerable: shell=True with user input
    os.system(f'python build_index.py --name {index_name}')
    return 'OK', 200

Secure Remediation: Use subprocess with a list and strict allowlist validation

The fixed version validates index_name against an allowlist of known safe values and uses subprocess.run without shell=True. This prevents command injection while still performing the intended operation:

import hashlib
import hmac
import subprocess
from flask import Flask, request

app = Flask(__name__)
SECRET_KEY = b'super-secret-key'
ALLOWED_INDEXES = {'products', 'users', 'orders', 'logs'}

def verify_hmac(data, signature):
    expected = hmac.new(SECRET_KEY, data, hashlib.sha256).hexdigest()
    return hmac.compare_digest(expected, signature)

@app.route('/reindex')
def reindex():
    index_name = request.args.get('index', '')
    signature = request.headers.get('X-Hub-Signature-256', '').replace('sha256=', '')
    payload = index_name.encode('utf-8')
    if not verify_hmac(payload, signature):
        return 'Invalid signature', 401
    # Validate against an allowlist
    if index_name not in ALLOWED_INDEXES:
        return 'Invalid index name', 400
    # Secure: list arguments, no shell
    result = subprocess.run(
        ['python', 'build_index.py', '--name', index_name],
        capture_output=True,
        text=True,
        timeout=30
    )
    if result.returncode != 0:
        return f'Build failed: {result.stderr}', 502
    return 'OK', 200

Additional recommendations include avoiding passing raw command strings to tools like curl or wget via shell; instead, use library-native clients or subprocess with explicit arguments. For cases where shell features are truly required, rigorously sanitize input by allowing only alphanumeric characters and a limited set of safe symbols, and prefer explicit mapping to safe command components rather than dynamic concatenation. The middleBrick CLI can be used to scan the Flask service for unsafe subprocess usage and input validation gaps, while the GitHub Action can enforce a minimum security score before deployments that involve such endpoints.