Security

This skill provides guidance on writing secure code and identifying security vulnerabilities during code review.

Core Principles

•Defense in Depth: Layer multiple security controls; don't rely on a single defense
•Least Privilege: Grant minimum permissions necessary for functionality
•Don't Trust Input: Validate and sanitize all external input
•Keep It Simple: Complex code is harder to secure
•Fail Secure: When errors occur, fail closed rather than open

OWASP Top 10 (2025)

The OWASP Top 10 represents the most critical web application security risks.

A01: Broken Access Control

Failures in enforcing what authenticated users are allowed to do.

Vulnerabilities:

•Missing access control checks on sensitive operations
•Insecure Direct Object References (IDOR) — accessing resources by ID without authorization checks
•Privilege escalation — users accessing admin functionality
•CORS misconfiguration allowing unauthorized API access
•Bypassing access control by modifying URLs, parameters, or JWT tokens

Prevention:

•Deny by default; explicitly grant access
•Implement access control checks server-side, not client-side
•Use consistent access control mechanisms throughout the application
•Log access control failures and alert on repeated failures
•Invalidate sessions on logout and set appropriate timeouts

A02: Security Misconfiguration

Insecure default configurations, incomplete setups, or misconfigured security settings.

Vulnerabilities:

•Default credentials left unchanged
•Unnecessary features enabled (ports, services, pages, accounts)
•Error messages exposing sensitive information (stack traces, database errors)
•Missing security headers
•Outdated or vulnerable software configurations

Prevention:

•Automate secure configuration deployment
•Remove unused features and dependencies
•Review cloud storage permissions (S3 buckets, etc.)
•Send security directives via headers (CSP, X-Content-Type-Options, etc.)
•Use different credentials across environments

A03: Software Supply Chain Failures

Vulnerabilities introduced through dependencies, build systems, or distribution infrastructure.

Vulnerabilities:

•Using components with known vulnerabilities
•Outdated or unmaintained dependencies
•Not verifying package integrity
•Typosquatting attacks (malicious packages with similar names)
•Compromised build pipelines

Prevention:

•Maintain inventory of all dependencies and their versions
•Continuously monitor for vulnerabilities (Dependabot, Snyk, etc.)
•Only obtain packages from official sources
•Verify package signatures and checksums
•Review dependency changes in pull requests
•Use lock files to pin dependency versions

A04: Cryptographic Failures

Failures related to cryptography that expose sensitive data.

Vulnerabilities:

•Transmitting data in cleartext (HTTP, FTP, SMTP)
•Using weak or deprecated algorithms (MD5, SHA1, DES)
•Weak or default cryptographic keys
•Not enforcing encryption (missing TLS, weak TLS configuration)
•Improper key management (hardcoded keys, keys in source control)

Prevention:

•Encrypt all sensitive data in transit and at rest
•Use strong, current algorithms (AES-256, RSA-2048+, SHA-256+)
•Generate keys using cryptographically secure random generators
•Store keys securely, separate from encrypted data
•Use TLS 1.2+ with strong cipher suites
•Never implement custom cryptography

A05: Injection

Untrusted data sent to an interpreter as part of a command or query.

Vulnerabilities:

•SQL Injection — malicious SQL in user input
•NoSQL Injection — malicious queries in document databases
•OS Command Injection — shell commands in user input
•LDAP Injection — malicious LDAP queries
•Cross-Site Scripting (XSS) — malicious scripts in web pages

Prevention:

•Use parameterized queries or prepared statements (never string concatenation)
•Use ORM frameworks correctly
•Validate and sanitize all input (allowlist preferred over denylist)
•Escape output appropriate to context (HTML, JavaScript, SQL, etc.)
•Use Content Security Policy (CSP) headers
•Apply least privilege to database accounts

A06: Insecure Design

Security flaws from missing or ineffective security controls in the design phase.

Vulnerabilities:

•Missing threat modeling
•No rate limiting on expensive operations
•Missing input validation requirements
•Inadequate segregation of duties
•Business logic flaws

Prevention:

•Establish secure development lifecycle
•Use threat modeling for critical features
•Define security requirements and acceptance criteria
•Use secure design patterns and reference architectures
•Integrate security testing into CI/CD

A07: Authentication Failures

Failures in authentication mechanisms.

Vulnerabilities:

•Credential stuffing (using breached username/password lists)
•Brute force attacks (no rate limiting)
•Permitting weak passwords
•Missing or ineffective multi-factor authentication
•Session fixation or improper session invalidation
•Exposing session IDs in URLs

Prevention:

•Delegate authentication to trusted 3rd parties
•Implement multi-factor authentication
•Enforce strong password policies
•Rate limit and lock out after failed attempts
•Use secure session management (regenerate IDs on login)
•Never ship with default credentials
•Use secure (slow) password hashing (bcrypt, Argon2, scrypt)

A08: Data Integrity Failures

Failures to protect data and code from unauthorized modification.

Vulnerabilities:

•Insecure deserialization — untrusted data deserialized without validation
•Missing integrity checks on software updates
•Unsigned or unverified CI/CD pipelines
•Trusting serialized objects from untrusted sources

Prevention:

•Use digital signatures to verify integrity
•Validate all serialized data from untrusted sources
•Implement integrity checks in CI/CD pipelines
•Review code and configuration changes
•Avoid serializing sensitive data

A09: Security Logging and Alerting Failures

Insufficient logging, monitoring, and alerting.

Vulnerabilities:

•Not logging security-relevant events
•Logs not containing enough detail for forensics
•No monitoring or alerting on suspicious activity
•Logs stored only locally, vulnerable to tampering
•Missing audit trails for sensitive operations

Prevention:

•Log all authentication attempts (success and failure)
•Log all access control failures
•Log all input validation failures
•Include context: who, what, when, where
•Store logs centrally, append-only
•Implement real-time alerting for critical events

A10: Mishandling of Exceptional Conditions

Improper error handling that leads to security vulnerabilities.

Vulnerabilities:

•Failing open instead of closed
•Exposing sensitive information in error messages
•Resource exhaustion from unhandled exceptions
•Logic errors from unexpected states
•Race conditions and time-of-check/time-of-use (TOCTOU) bugs

Prevention:

•Catch and handle all exceptions appropriately
•Fail secure — deny access on error
•Use generic error messages for users; log details server-side
•Test error handling paths explicitly
•Use timeouts and circuit breakers

Language-Specific Guidelines

Ruby / Rails

•Use strong_parameters to whitelist permitted attributes
•Use parameterized queries (ActiveRecord does this by default, but beware of raw SQL)
•Enable CSRF protection (on by default in Rails)
•Use html_safe sparingly and never on user input
•Use SecureRandom for tokens, not rand
•Set config.force_ssl = true in production
•Use Brakeman for static security analysis

JavaScript / Node.js

•Avoid eval() and Function() constructor with user input
•Use parameterized queries with database drivers
•Sanitize HTML with libraries like DOMPurify
•Use helmet middleware for security headers
•Validate JSON schemas for API input
•Use npm audit or yarn audit regularly
•Be cautious with dangerouslySetInnerHTML in React

Bash

•Quote all variables: "$var" not $var
•Never use eval with user input
•Avoid command substitution with untrusted data
•Use -- to separate options from arguments
•Validate and sanitize all input before use
•Use arrays for command arguments to avoid injection

Security Review Checklist

When reviewing code for security: