Format String in Flask with Cockroachdb

A format string vulnerability occurs when user-controlled input is passed directly into a formatting function such as str.format or percent-style formatting without proper sanitization. In a Flask application using CockroachDB, this typically manifests when constructing SQL strings or log messages with unchecked user input. CockroachDB, like other SQL databases, expects parameterized queries; however, if a developer builds SQL commands using string interpolation, an attacker can supply format specifiers (e.g., %s, %x) that cause unintended reads from memory or writes to variables, leading to information disclosure or instability.

Consider a Flask route that retrieves a user record by username and logs the query. If the logging or query construction uses format strings unsafely, the interaction between Flask request handling and CockroachDB connectivity becomes a vector:

from flask import Flask, request
import psycopg2

app = Flask(__name__)

@app.route('/user')
def get_user():
    username = request.args.get('username', '')
    # Unsafe: building SQL via string formatting
    query = 'SELECT id, email FROM users WHERE username = \'%s\'' % username
    conn = psycopg2.connect(
        host='cockroachdb-host',
        port=26257,
        dbname='mydb',
        user='myuser',
        password='mypassword'
    )
    cur = conn.cursor()
    cur.execute(query)
    result = cur.fetchone()
    cur.close()
    conn.close()
    return {'user': result}

In this example, username is directly interpolated into the SQL string. An attacker could supply a value like \' OR 1=1 -- to manipulate the query, or use format specifiers in a log message if the application logs the query string unsafely. Although CockroachDB correctly parameterizes queries when used properly, the vulnerability is introduced at the Flask layer before the database interaction occurs. The database itself does not introduce format string issues, but the way the application builds commands for CockroachDB exposes the risk.

Additionally, if the application uses Python’s logging module and logs user input with format strings, an attacker-supplied format specifier can cause the logging library to read stack memory, potentially leaking sensitive data such as database connection parameters or session tokens. This is a classic format string bug enabled by the combination of dynamic request data in Flask and improper string handling around CockroachDB operations.

Remediation focuses on ensuring that user input never participates in string formatting for SQL or logging. Use parameterized queries with placeholders supported by the CockroachDB-compatible PostgreSQL driver (psycopg2). This guarantees that input is treated strictly as data, not executable code or format directives.

Correct usage with CockroachDB in Flask:

from flask import Flask, request
import psycopg2

app = Flask(__name__)

@app.route('/user')
def get_user():
    username = request.args.get('username', '')
    # Safe: parameterized query with placeholders
    conn = psycopg2.connect(
        host='cockroachdb-host',
        port=26257,
        dbname='mydb',
        user='myuser',
        password='mypassword'
    )
    cur = conn.cursor()
    cur.execute('SELECT id, email FROM users WHERE username = %s', (username,))
    result = cur.fetchone()
    cur.close()
    conn.close()
    return {'user': result}

In this fixed version, %s is a placeholder in a parameterized query, and username is passed as a separate parameter. The psycopg2 driver ensures proper escaping and type handling, eliminating format string risks. Do not use string formatting to construct SQL, even with trusted-seeming inputs.

For logging, configure the logger to avoid interpreting format specifiers from user data. Use logging.Logger.info(msg) with explicit arguments rather than string interpolation:

import logging
logger = logging.getLogger(__name__)

# Instead of this:
# logger.info('Query for user: %s' % username)

# Do this:
logger.info('Query for user: %s', username)

By passing the variable as an extra argument, the logging module handles formatting safely and avoids triggering format string bugs. These practices align with secure handling of CockroachDB connections in Flask and mitigate both SQL injection and format string attack surfaces.