https://github.com/FallibleInc/security-guide-for-developers/blob/master/security-checklist.md

AUTHENTICATION SYSTEMS (Signup/Signin/2 Factor/Password reset)

•  Use HTTPS everywhere.
•  Store password hashes using Bcrypt (no salt necessary - Bcrypt does it for you).
•  Destroy the session identifier after logout.
•  Destroy all active sessions on reset password (or offer to).
•  Must have the state parameter in OAuth2.
•  No open redirects after successful login or in any other intermediate redirects.
•  When parsing Signup/Login input, sanitize for javascript://, data://, CRLF characters.
•  Set secure, httpOnly cookies.
•  In Mobile OTP based mobile verification, do not send the OTP back in the response when generate OTP or Resend OTPAPI is called.
•  Limit attempts to Login, Verify OTP, Resend OTP and generate OTP APIs for a particular user. Have an exponential backoff set or/and something like a captcha based challenge.
•  Check for randomness of reset password token in the emailed link or SMS.
•  Set an expiration on the reset password token for a reasonable period.
•  Expire the reset token after it has been successfully used.

USER DATA & AUTHORIZATION

•  Any resource access like, my cart, my history should check the logged in user's ownership of the resource using session id.
•  Serially iterable resource id should be avoided. Use /me/orders instead of /user/37153/orders. This acts as a sanity check in case you forgot to check for authorization token.
•  Edit email/phone number feature should be accompanied by a verification email to the owner of the account.
•  Any upload feature should sanitize the filename provided by the user. Also, for generally reasons apart from security, upload to something like S3 (and post-process using lambda) and not your own server capable of executing code.
•  Profile photo upload feature should sanitize all the EXIF tags also if not required.
•  For user ids and other ids, use RFC compliant UUID instead of integers. You can find an implementation for this for your language on Github.
•  JWT are awesome. Use them if required for your single page app/APIs.

ANDROID / IOS APP

•  salt from payment gateways should not be hardcoded.
•  secret / auth token from 3rd party SDK's should not be hardcoded.
•  API calls intended to be done server to server should not be done from the app.
•  In Android, all the granted permissions should be carefully evaluated.
•  On iOS, store sensitive information (authentication tokens, API keys, etc.) in the system keychain. Do not store this kind of information in the user defaults.
•  Certificate pinning is highly recommended.

SECURITY HEADERS & CONFIGURATIONS

•  Add CSP header to mitigate XSS and data injection attacks. This is important.
•  Add CSRF header to prevent cross site request forgery. Also add SameSite attributes on cookies.
•  Add HSTS header to prevent SSL stripping attack.
•  Add your domain to the HSTS Preload List
•  Add X-Frame-Options to protect against Clickjacking.
•  Add X-XSS-Protection header to mitigate XSS attacks.
•  Update DNS records to add SPF record to mitigate spam and phishing attacks.
•  Add subresource integrity checks if loading your JavaScript libraries from a third party CDN. For extra security, add the require-sri-for CSP-directive so you don't load resources that don't have an SRI sat.
•  Use random CSRF tokens and expose business logic APIs as HTTP POST requests. Do not expose CSRF tokens over HTTP for example in an initial request upgrade phase.
•  Do not use critical data or tokens in GET request parameters. Exposure of server logs or a machine/stack processing them would expose user data in turn.

SANITIZATION OF INPUT

•  Sanitize all user inputs or any input parameters exposed to user to prevent XSS.
•  Always use parameterized queries to prevent SQL Injection.
•  Sanitize user input if using it directly for functionalities like CSV import.
•  Sanitize user input for special cases like robots.txt as profile names in case you are using a url pattern like coolcorp.io/username.
•  Do not hand code or build JSON by string concatenation ever, no matter how small the object is. Use your language defined libraries or framework.
•  Sanitize inputs that take some sort of URLs to prevent SSRF.
•  Sanitize Outputs before displaying to users.

OPERATIONS

•  If you are small and inexperienced, evaluate using AWS elasticbeanstalk or a PaaS to run your code.
•  Use a decent provisioning script to create VMs in the cloud.
•  Check for machines with unwanted publicly open ports.
•  Check for no/default passwords for databases especially MongoDB & Redis.
•  Use SSH to access your machines; do not setup a password, use SSH key-based authentication instead.
•  Install updates timely to act upon zero day vulnerabilities like Heartbleed, Shellshock.
•  Modify server config to use TLS 1.2 for HTTPS and disable all other schemes. (The tradeoff is good.)
•  Do not leave the DEBUG mode on. In some frameworks, DEBUG mode can give access full-fledged REPL or shells or expose critical data in error messages stacktraces.
•  Be prepared for bad actors & DDOS - use a hosting service that has DDOS mitigation.
•  Set up monitoring for your systems, and log stuff (use New Relic or something like that).
•  If developing for enterprise customers, adhere to compliance requirements. If AWS S3, consider using the feature to encrypt data. If using AWS EC2, consider using the feature to use encrypted volumes (even boot volumes can be encrypted now).

PEOPLE

•  Set up an email (e.g. security@coolcorp.io) and a page for security researchers to report vulnerabilities.
•  Depending on what you are making, limit access to your user databases.
•  Be polite to bug reporters.
•  Have your code review done by a fellow developer from a secure coding perspective. (More eyes)
•  In case of a hack or data breach, check previous logs for data access, ask people to change passwords. You might require an audit by external agencies depending on where you are incorporated.
•  Set up Netflix's Scumblr to hear about talks about your organization on social platforms and Google search.