Difference between revisions of "Secure Coding Cheat Sheet"

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* Do not enable Directory Listing on your server
* Do not enable Directory Listing on your server
== Buffer Overflow ==
== Concurrancy and Race Conditions ==
== Concurrancy and Race Conditions ==

Revision as of 05:01, 8 December 2015



The goal of this document is to create high level guideline for secure coding practices. The goal is to keep the overall size of the document condensed and easy to digest. Individuals seeking addition information on the specific areas should refer to the included links to learn more.

How To Use This Document

The information listed below are generally acceptable secure coding practices; however, it is recommend that organizations consider this a base template and update individual sections with secure coding recommendations specific to the organization's policies and risk tolerance.


User Authentication

Authentication mechanisms

Authentication can be performed by using single factor or using multifactor. Typically, single factor authentication involves one of the following:

1. What you know

2. What you have

3. Who you are

4. Where you are

Whereas multifactor authentication uses more than one of the above mentioned factors

When a user is authentication with a password and a PIN, it is still a single factor authentication, as both password and PIN are what a user has. However, if a PIN is generated off a device / soft token, then it becomes a multifactor authentication, as the PIN is generated off a device / token that you have.

For transactions that process "high risk" data (like credit cards, health information, personally identifiable data) use multifactor authentication

If you are using only client-side authentication, ensure it can not be bypassed, manipulated or misused.

User names

For authentication purposes, user names should be unique. Scenarios that process high risk / critical data choose a unique user name for the user. Eg in case of banking applications, shopping applications, applications that process personally identifiable data etc.

Password Complexity

For more information on password complexity, please see https://www.owasp.org/index.php/Authentication_Cheat_Sheet#Implement_Proper_Password_Strength_Controls.

Password Rotation

Password rotation should be required for privileged accounts within applications at a frequency of every 90 days

Online Password Guessing and Password Attacks

Applications must defend against online password guessing attempts by one of the following methods:

  • Account Lockout - Lock account after 5 failed password attempts
  • Temporary Account Lockout- Temporarily lock account after 5 failed password attempts
  • Anti-automation Captcha - Require a captcha to be successfully completed after 5 failed password attempts

Additional Reading

Password Reset Functions

Email Verification Functions

If the application requires verification of ownership of an email address then observe the following

  • Email verification links should only satisfy the requirement of verify email address ownership and should not provide the user with an authenticated session (e.g. the user must still authenticate as normal to access the application).
  • Email verification codes must expire after the first use or expire after 8 hours if not used.

Password Storage

  • Passwords should be stored in a format, such as Bcrypt, that is resistant to high speed offline brute force attacks
  • Password storage using hashing algorithms plus a per user salt are good, but not sufficient.
  • Do not store passwords in code or in configuration files.

Server Authentication

When using SSL / TLS, ensure that the server identity is established by following a trust chain to a known root certificate

Do not rely upon IP numbers or DNS names in establishing identity.

Ensure all internal and external connections (user and entity) go through an appropriate and adequate form of authentication.

Ensure that this control cannot be bypassed.

Logging and Monitoring

  • Ensure that all failures are logged and reviewed
  • Ensure that all password failures are logged and reviewed
  • Ensure that all account lockouts are logged and reviewed

Session Management


Session ID Length

  • Session tokens should be 128-bit or greater

Session ID Creation

  • The session tokens should be handled by the web server if possible or generated via a cryptographically secure random number generator.

Inactivity Time Out

  • Authenticated sessions should timeout after determined period of inactivity - 15 minutes is recommended.

Secure Flag

  • The "Secure" flag should be set during every set-cookie. This will instruct the browser to never send the cookie over HTTP. The purpose of this flag is to prevent the accidental exposure of a cookie value if a user follows an HTTP link.

HTTP-Only Flag

  • The "HTTP-Only" flag should be set to disable malicious script access to the cookie values, such as the session ID


  • Upon logout the session ID should be invalidated on the server side and deleted on the client via expiring and overwriting the value.

Access Control


Presentation Layer

  • It is recommended to not display links or functionality that is not accessible to a user. The purpose is to minimize unnecessary access controls messages and minimize privileged information from being unnecessarily provided to users.

Business Layer

  • Ensure that an access control verification is performed before an action is executed within the system. A user could craft a custom GET or POST message to attempt to execute unauthorized functionality.

Data Layer

  • Ensure that an access control verification is performed to check that the user is authorized to act upon the target data. Do not assume that a user authorized to perform action X is able to necessarily perform this action on all data sets.

Input Data Validation


Output Encoding

Preventing XSS and Content Security Policy

  • All user data controlled must be encoded when returned in the html page to prevent the execution of malicious data (e.g. XSS). For example <script> would be returned as &lt;script&gt;
  • The type of encoding is specific to the context of the page where the user controlled data is inserted. For example, HTML entity encoding is appropriate for data placed into the HTML body. However, user data placed into a script would need JavaScript specific output encoding

Detailed information on XSS prevention here: OWASP XSS Prevention Cheat Sheet

Preventing SQL Injection

  • It's not realistic to always know if a piece of data is user controlled, therefore parameterized queries should be used whenever a method/function accepts data and uses this data as part of the SQL statement.
  • String concatenation to build any part of a SQL statement with user controlled data creates a SQL injection vulnerability.
  • Parameterized queries are a guaranteed approach to prevent SQL injection.

Further Reading: SQL Injection Prevention Cheat Sheet

Preventing OS Injection

  • Avoid sending user controlled data to the OS as much as possible
  • Ensure that a robust escaping routine is in place to prevent the user from adding additional characters that can be executed by the OS ( e.g. user appends | to the malicious data and then executes another OS command). Remember to use a positive approach when constructing escaping routinges. Example

Further Reading: Reviewing Code for OS Injection

Preventing XML Injection

  • In addition to the existing input validation, define a positive approach which escapes/encodes characters that can be interpreted as xml. At a minimum this includes the following: < > " ' &
  • If accepting raw XML then more robust validation is necessary. This can be complex. Please contact the infrastructure security team for additional discussion

Cross Site Request Forgery

Preventing CSRF

  • Any state changing operation requires a secure random token (e.g CSRF token) to prevent against CSRF attacks
  • Characteristics of a CSRF Token
    • Unique per user & per user session
    • Tied to a single user session
    • Large random value
    • Generated by a cryptographically secure random number generator
  • The CSRF token is added as a hidden field for forms or within the URL if the state changing operation occurs via a GET
  • The server rejects the requested action if the CSRF token fails validation

Preventing Malicious Site Framing (ClickJacking)

Set the x-frame-options header for all responses containing HTML content. The possible values are "DENY" or "SAMEORIGIN".

   DENY will block any site (regardless of domain) from framing the content.
   SAMEORIGIN will block all sites from framing the content, except sites within the same domain.

The "DENY" setting is recommended unless a specific need has been identified for framing.

Secure Transmission / Network Layer security

When To Use SSL/TLS

  • All points from the login page to the logout page must be served over HTTPS.
  • Ensure that the page where a user completes the login form is accessed over HTTPS. This is in addition to POST'ing the form over HTTPS.
  • All authenticated pages must be served over HTTPS. This includes css, scripts, images. Failure to do so creates a vector for man in the middle attack and also causes the browser to display a mixed SSL warning message.

Implement HTTP Strict Transport Security (HSTS)

  • Applications that are served exclusively over HTTPS should utilize HSTS to instruct compatible browsers to not allow HTTP connections to the domain

File Uploads

Upload Verification

  • Use input validation to ensure the uploaded filename uses an expected extension type
  • Ensure the uploaded file is not larger than a defined maximum file size

Upload Storage

  • Use a new filename to store the file on the OS. Do not use any user controlled text for this filename or for the temporary filename.
  • Store all user uploaded files on a separate domain (e.g. mozillafiles.net vs mozilla.org). Archives should be analyzed for malicious content (anti-malware, static analysis, etc)

Public Serving of Uploaded Content

  • Ensure the image is served with the correct content-type (e.g. image/jpeg, application/x-xpinstall)

Beware of "special" files

  • The upload feature should be using a whitelist approach to only allow specific file types and extensions. However, it is important to be aware of the following file types that, if allowed, could result in security vulnerabilities.
  • "crossdomain.xml" allows cross-domain data loading in Flash, Java and Silverlight. If permitted on sites with authentication this can permit cross-domain data theft and CSRF attacks. Note this can get pretty complicated depending on the specific plugin version in question, so its best to just prohibit files named "crossdomain.xml" or "clientaccesspolicy.xml".
  • ".htaccess" and ".htpasswd" provides server configuration options on a per-directory basis, and should not be permitted. See http://en.wikipedia.org/wiki/Htaccess

Upload Verification

  • Use image rewriting libraries to verify the image is valid and to strip away extraneous content.
  • Set the extension of the stored image to be a valid image extension based on the detected content type of the image from image processing (e.g. do not just trust the header from the upload).
  • Ensure the detected content type of the image is within a list of defined image types (jpg, png, etc)

Error Handling

Typical types of errors: • The result of business logic conditions not being met. • The result of the environment wherein the business logic resides fails. • The result of upstream or downstream systems upon which the application depends fail. • Technical hardware / physical failure.

To address these errors:

  • Ensure that all method/function calls that return a value have proper error handling and return value checking
  • Ensure that exceptions and error conditions are properly handled
  • Ensure that no system errors can be returned to the user
  • Ensure that the application fails in a secure manner
  • Ensure resources are released if an error occurs
  • Ensure that stack trace is not thrown to the user
  • If the language in question has a finally method, use it. The finally method is always called. The finally method can be used to release resources referenced by the method that threw the exception.

This is very important. An example would be if a method gained a database connection from a pool of connections, and an exception occurred without finally, the connection object shall not be returned to the pool for some time (until the timeout). This can lead to pool exhaustion. The method finally() is called even if no exception is thrown.

Logging and Auditing



  • All protocols and algorithms for authentication and secure communication should be well vetted by the cryptographic community.
  • Ensure certificates are properly validated against the hostnames/users ie whom they are meant for.
  • Avoid using wildcard certificates unless there is a business need for it
  • Maintain a cryptographic standard to ensure that the developer community knows about the approved ciphersuits for network security protocols, algorithms, permitted use, cryptoperiods and Key Management

Cookie Management

  • Ensure that sensitive information is not comprised, by ensuring that sensitive information is not persistent / encrypting / stored on a need basis for the duration of the need
  • Ensure that unauthorized activities cannot take place via cookie manipulation
  • Ensure secure flag is set to prevent accidental transmission over “the wire” in a non-secure manner
  • Determine if all state transitions in the application code properly check for the cookies and enforce their use
  • Ensure entire cookie should be encrypted if sensitive data is persisted in the cookie
  • Define all cookies being used by the application, their name and why they are needed

Secure Deployment

  • Secure access to with authentication and authorisation to configuration files, directories, and resources on the host so that direct access to such artifacts is disallowed
  • Use a “deny all” rule to deny access to resources on the hosts and then grant access on need basis
  • In Apache HTTP server, ensure directories like WEB-INF and META-INF are protected. If permissions for a directory and subdirectories are specified in .htaccess file, ensure that it is protected using the “deny all” rule
  • While using Struts framework, ensure that JSP files are not accessible directly by denying access to *.jsp files in web.xml
  • Maintain a clean environment. remove files that contain source code but are not used by the application.
  • Ensure production environment does not contain any source code / development tools and that the production
  • Ensure environment contains only compiled code / executables.
  • Remove test code / debug code (that might contain backdoors).
  • Remove commented code and meta tags as they might contain sensitive data.
  • If applicable, obfuscate your code to ensure that reverse engineering is avoided

Unvalidated Redirects and Forwards Cheat Sheet


Common Vulnerabilities

SQL Injection


Cross Site Scripting




Cross Site Request Forgery


Security Misconfigurations

  • Diable all the services, ports, protocols and daemons that are not required.
  • Change all the default and vendor supplied passwords
  • Protect servers by grouping all similar functions into a VLAN
  • White wash error messages such that no internal workings are revealed
  • Prevent stack traces from leaving the container.
  • Authorising access to the least amount of data/ least number of pages that is possible

Insecure Direct Object references


Directory Listing

  • Do not enable Directory Listing on your server

Concurrancy and Race Conditions

  • Use a locking mechanism to lock shared resources
  • Obtain a lock on shared resources before it is read


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