[Owasp-Pune] Security Principles

Dharmesh Mehta dharmeshmm at owasp.org
Thu Apr 10 06:54:15 EDT 2008


>From J.D. Meier


If you know the underlying principles for security, you can be more
effective in your security design.  While working on Improving Web
Application Security: Threats and
Countermeasures<http://msdn2.microsoft.com/en-us/library/ms994921.aspx>,
my team focused on creating a durable set of security principles.  The
challenge was to make the principles more useful.  It's one thing to know
the principles, but another to turn it into action.

*Turning Insights Into Action*

To make the principles more useful, we organized them using our Security
Frame <http://blogs.msdn.com/jmeier/pages/security-frame.aspx>.  Our
Security Frame is a set of actionable, relevant categories that shape your
key engineering and deployment decisions.  With the Security Frame we could
quickly find principles related to authentication, or authorization or input
validation ... etc.

Once we had these principles and this organizing frame, we could then
evaluate technologies against it to find effective, principle-based
techniques.  For example, when we analyzed doing input and data validation
in ASP.NET, we focused on finding the best ways to constrain, reject, and
sanitize input.  For constraining input, we focused on checking for length,
range, format and type.  Using these strategies both shortened our learning
curve and improved our results.

*Core Security Principles*

We started with a firm foundation of core security principles.  These
influenced the rest of our security design principles.  Here's the core
security principles we started with:

   - *Adopt the principle of least privilege - *Processes that run script
   or execute code should run under a least privileged account to limit the
   potential damage that can be done if the process is compromised
   - *Use defense in depth*.   Place check points within each of the
   layers and subsystems within your application. The check points are the
   gatekeepers that ensure that only authenticated and authorized users are
   able to access the next downstream layer.
   - *Don't trust user input.*  Applications should thoroughly validate
   all user input before performing operations with that input. The validation
   may include filtering out special characters.
   - *Use secure defaults*.   If your application demands features that
   force you to reduce or change default security settings, test the effects
   and understand the implications before making the change
   - *Don't rely on security by obscurity*.   Trying to hide secrets by
   using misleading variable names or storing them in odd file locations does
   not provide security. In a game of hide-and-seek, it's better to use
   platform features or proven techniques for securing your data.
   - *Check at the gate*.   Checking the client at the gate refers to
   authorizing the user at the first point of authentication (for example,
   within the Web application on the Web server), and determining which
   resources and operations (potentially provided by downstream services) the
   user should be allowed to access.
   - *Assume external systems are insecure*.  If you don't own it, don't
   assume security is taken care of for you.
   - *Reduce Surface Area*   Avoid exposing information that is not
   required. By doing so, you are potentially opening doors that can lead to
   additional vulnerabilities. Also, handle errors gracefully; don't expose any
   more information than is required when returning an error message to the end
   user.
   - *Fail to a secure mode*.   your application fails, make sure it does
   not leave sensitive data unprotected. Also, do not provide too much detail
   in error messages; meaning don't include details that could help an attacker
   exploit a vulnerability in your application. Write detailed error
   information to the Windows event log.
   - *Security is a concern across all of your application layers and
   tiers*.   Remember you are only as secure as your weakest link.
   - *If you don't use it, disable it*.   You can remove potential points
   of attack by disabling modules and components that your application does not
   require. For example, if your application doesn't use output caching, then
   you should disable the ASP.NET output cache module. If a future
   security vulnerability is found in the module, your application is not
   threatened.

*Frame for Organizing Security Design Principles*

Rather than a laundry list of security principles, you can use the Security
Frame as a way to organize and share security principles:

   - Auditing and Logging
   - Authentication
   - Authorization
   - Configuration Management
   - Cryptography
   - Exception Management
   - Input / Data Validation
   - Sensitive Data
   - Session Management

*Auditing and Logging*

Here's our security design principles for auditing and logging:

   - *Audit and log access across application tiers*.   Audit and log
   access across the tiers of your application for non-repudiation. Use a
   combination of application-level logging and platform auditing features,
   such as Windows, IIS, and SQL Server auditing.
   - *Consider identity flow*.   You have two basic choices. You can flow
   the caller's identity at the operating system level or you can flow the
   caller's identity at the application level and use trusted identities to
   access back-end resources.
   - *Log key events*.   The types of events that should be logged
   include successful and failed logon attempts, modification of data,
   retrieval of data, network communications, and administrative functions such
   as the enabling or disabling of logging. Logs should include the time of the
   event, the location of the event including the machine name, the identity of
   the current user, the identity of the process initiating the event, and a
   detailed description of the event
   - *Protect log files.*   Protect log files using  access control lists
   and restrict access to the log files. This makes it more difficult for
   attackers to tamper with log files to cover their tracks. Minimize the
   number of individuals who can manipulate the log files. Authorize access
   only to highly trusted accounts such as administrators.
   - *Back up and analyze log files regularly*.   There's no point in
   logging activity if the log files are never analyzed. Log files should be
   removed from production servers on a regular basis. The frequency of removal
   is dependent upon your application's level of activity. Your design should
   consider the way that log files will be retrieved and moved to offline
   servers for analysis. Any additional protocols and ports opened on the Web
   server for this purpose must be securely locked down.

*Authentication*

Here's our security design principles for authentication:

   - *Separate public and restricted areas*.   A public area of your site
   can be accessed by any user anonymously. Restricted areas can be accessed
   only by specific individuals and the users must authenticate with the site.
   By partitioning your site into public and restricted access areas, you can
   apply separate authentication and authorization rules across the site.
   - *Use account lockout policies for end-user accounts*.   Disable
   end-user accounts or write events to a log after a set number of failed
   logon attempts. With Forms authentication, these policies are the
   responsibility of the application and must be incorporated into the
   application design. Be careful that account lockout policies cannot be
   abused in denial of service attacks.
   - *Support password expiration periods*. Passwords should not be
   static and should be changed as part of routine password maintenance through
   password expiration periods. Consider providing this type of facility during
   application design.
   - *Be able to disable accounts*.  If the system is compromised, being
   able to deliberately invalidate credentials or disable accounts can prevent
   additional attacks.
   - *Do not store passwords in user stores*.  If you must verify
   passwords, it is not necessary to actually store the passwords. Instead,
   store a one way hash value and then re-compute the hash using the
   user-supplied passwords. To mitigate the threat of dictionary attacks
   against the user store, use strong passwords and incorporate a random salt
   value with the password.
   - *Require strong passwords*.   Do not make it easy for attackers to
   crack passwords. There are many guidelines available, but a general practice
   is to require a minimum of eight characters and a mixture of uppercase and
   lowercase characters, numbers, and special characters. Whether you are using
   the platform to enforce these for you, or you are developing your own
   validation, this step is necessary to counter brute-force attacks where an
   attacker tries to crack a password through systematic trial and error. Use
   regular expressions to help with strong password validation.
   - *Do not send passwords over the wire in plaintext*.   Plaintext
   passwords sent over a network are vulnerable to eavesdropping. To address
   this threat, secure the communication channel, for example, by using SSL to
   encrypt the traffic.
   - *Protect authentication cookies*.   A stolen authentication cookie
   is a stolen logon. Protect authentication tickets using encryption and
   secure communication channels. Also limit the time interval in which an
   authentication ticket remains valid, to counter the spoofing threat that can
   result from replay attacks, where an attacker captures the cookie and uses
   it to gain illicit access to your site. Reducing the cookie timeout does not
   prevent replay attacks but it does limit the amount of time the attacker has
   to access the site using the stolen cookie.

*Authorization*

Here's our security design principles for authorization:

   - *Use multiple gatekeepers.*   By combining multiple gatekeepers
   across layers and tiers, you can develop an effective authorization
   strategy.
   - *Restrict user access to system-level resources*.   System level
   resources include files, folders, registry keys, Active Directory objects,
   database objects, event logs, and so on. Use access control lists to
   restrict which users can access what resources and the types of operations
   that they can perform. Pay particular attention to anonymous Internet user
   accounts; lock these down on resources that explicitly deny access to
   anonymous users.
   - *Consider authorization granularity*.   There are three common
   authorization models, each with varying degrees of granularity and
   scalability: (1.) the impersonation model providing per end user
   authorization granularity, (2.) the trusted subsystem model uses the
   application's process identity for resource access, and (3.) the hybrid
   model uses multiple trusted service identities for downstream resource
   access. The most granular approach relies on impersonation. The
   impersonation model provides per end user authorization granularity.

*Configuration Management*

Here's our security design principles for configuration management:

   - *Protect your administration interfaces*.   It is important that
   configuration management functionality is accessible only by authorized
   operators and administrators. A key part is to enforce strong authentication
   over your administration interfaces, for example, by using certificates. If
   possible, limit or avoid the use of remote administration and require
   administrators to log on locally. If you need to support remote
   administration, use encrypted channels, for example, with SSL or VPN
   technology, because of the sensitive nature of the data passed over
   administrative interfaces.
   - *Protect your configuration store*. Text-based configuration files,
   the registry, and databases are common options for storing application
   configuration data. If possible, avoid using configuration files in the
   application's Web space to prevent possible server configuration
   vulnerabilities resulting in the download of configuration files. Whatever
   approach you use, secure access to the configuration store, for example, by
   using access control lists or database permissions. Also avoid storing
   plaintext secrets such as database connection strings or account
   credentials. Secure these items using encryption and then restrict access to
   the registry key, file, or table that contains the encrypted data.
   - *Maintain separate administration privileges*.   If the
   functionality supported by the features of your application's configuration
   management varies based on the role of the administrator, consider
   authorizing each role separately by using role-based authorization. For
   example, the person responsible for updating a site's static content should
   not necessarily be allowed to change a customer's credit limit.
   - *Use least privileged process and service accounts*.  An important
   aspect of your application's configuration is the process accounts used to
   run the Web server process and the service accounts used to access
   downstream resources and systems. Make sure these accounts are set up as
   least privileged. If an attacker manages to take control of a process, the
   process identity should have very restricted access to the file system and
   other system resources to limit the damage that can be done.

*Cryptography *

Here's our security design principles for cryptography:

   - *Do not develop your own cryptography*.   Cryptographic algorithms
   and routines are notoriously difficult to develop successfully. As a result,
   you should use the tried and tested cryptographic services provided by the
   platform.
   - *Keep unencrypted data close to the algorithm*.   When passing
   plaintext to an algorithm, do not obtain the data until you are ready to use
   it, and store it in as few variables as possible.
   - *Use the correct algorithm and correct key size*.   It is important
   to make sure you choose the right algorithm for the right job and to make
   sure you use a key size that provides a sufficient degree of security.
   Larger key sizes generally increase security. The following list summarizes
   the major algorithms together with the key sizes that each uses: Data
   Encryption Standard (DES) 64-bit key (8 bytes) , TripleDES 128-bit key or
   192-bit key (16 or 24 bytes) , Rijndael 128–256 bit keys (16–32 bytes) , RSA
   384–16,384 bit keys (48–2,048 bytes) .  For large data encryption, use the
   TripleDES symmetric encryption algorithm. For slower and stronger encryption
   of large data, use Rijndael. To encrypt data that is to be stored for short
   periods of time, you can consider using a faster but weaker algorithm such
   as DES. For digital signatures, use Rivest, Shamir, and Adleman (RSA) or
   Digital Signature Algorithm (DSA). For hashing, use the Secure Hash
   Algorithm (SHA)1.0. For keyed hashes, use the Hash-based Message
   Authentication Code (HMAC) SHA1.0.
   - *Protect your encryption keys*.   An encryption key is a secret
   number used as input to the encryption and decryption processes. For
   encrypted data to remain secure, the key must be protected. If an attacker
   compromises the decryption key, your encrypted data is no longer secure.
   Avoid key management when you can, and when you do need to store encryption
   keys, cycle your keys periodically.

*Exception Management *

Here's our security design principles for exception management:

   - *Do not leak information to the client*.   In the event of a
   failure, do not expose information that could lead to information
   disclosure. For example, do not expose stack trace details that include
   function names and line numbers in the case of debug builds (which should
   not be used on production servers). Instead, return generic error messages
   to the client.
   - *Log detailed error messages*.   Send detailed error messages to the
   error log. Send minimal information to the consumer of your service or
   application, such as a generic error message and custom error log ID that
   can subsequently be mapped to detailed message in the event logs. Make sure
   that you do not log passwords or other sensitive data.
   - *Catch exceptions*.  Use structured exception handling and catch
   exception conditions. Doing so avoids leaving your application in an
   inconsistent state that may lead to information disclosure. It also helps
   protect your application from denial of service attacks. Decide how to
   propagate exceptions internally in your application and give special
   consideration to what occurs at the application boundary.

*Input / Data Validation *

Here's our security design principles for input and data validation:

   - *Assume all input is malicious*.  Input validation starts with a
   fundamental supposition that all input is malicious until proven otherwise.
   Whether input comes from a service, a file share, a user, or a database,
   validate your input if the source is outside your trust boundary.
   - *Centralize your approach*.  Make your input validation strategy a
   core element of your application design. Consider a centralized approach to
   validation, for example, by using common validation and filtering code in
   shared libraries. This ensures that validation rules are applied
   consistently. It also reduces development effort and helps with future
   maintenance.  In many cases, individual fields require specific validation,
   for example, with specifically developed regular expressions. However, you
   can frequently factor out common routines to validate regularly used fields
   such as e-mail addresses, titles, names, postal addresses including ZIP or
   postal codes, and so on.
   - *Do not rely on client-side validation*.   Server-side code should
   perform its own validation. What if an attacker bypasses your client, or
   shuts off your client-side script routines, for example, by disabling
   JavaScript? Use client-side validation to help reduce the number of round
   trips to the server but do not rely on it for security. This is an example
   of defense in depth.
   - *Be careful with canonicalization issues*.   Data in canonical form
   is in its most standard or simplest form. Canonicalization is the process of
   converting data to its canonical form. File paths and URLs are particularly
   prone to canonicalization issues and many well-known exploits are a direct
   result of canonicalization bugs.  You should generally try to avoid
   designing applications that accept input file names from the user to avoid
   canonicalization issues.
   - *Constrain, reject, and sanitize your input*.   The preferred
   approach to validating input is to constrain what you allow from the
   beginning. It is much easier to validate data for known valid types,
   patterns, and ranges than it is to validate data by looking for known bad
   characters. When you design your application, you know what your application
   expects. The range of valid data is generally a more finite set than
   potentially malicious input. However, for defense in depth you may also want
   to reject known bad input and then sanitize the input.
   - *Encrypt sensitive cookie state*.  Cookies may contain sensitive
   data such as session identifiers or data that is used as part of the
   server-side authorization process. To protect this type of data from
   unauthorized manipulation, use cryptography to encrypt the contents of the
   cookie.
   - *Make sure that users do not bypass your checks*.   Make sure that
   users do not bypass your checks by manipulating parameters. URL parameters
   can be manipulated by end users through the browser address text box. For
   example, the URL
http://www.<*YourSite*>/<*YourApp*>/sessionId=10<http://www.%3cyoursite%3e/%3cYourApp%3e/sessionId=10>has
a value of 10 that can be changed to some random number to receive
   different output. Make sure that you check this in server-side code, not in
   client-side JavaScript, which can be disabled in the browser.
   - *Validate all values sent from the client.*   Restrict the fields
   that the user can enter and modify and validate all values coming from the
   client. If you have predefined values in your form fields, users can change
   them and post them back to receive different results. Permit only known good
   values wherever possible. For example, if the input field is for a state,
   only inputs matching a state postal code should be permitted.
   - *Do not trust HTTP header information*.   HTTP headers are sent at
   the start of HTTP requests and HTTP responses. Your Web application should
   make sure it does not base any security decision on information in the HTTP
   headers because it is easy for an attacker to manipulate the header. For
   example, the *referrer *field in the header contains the URL of the
   Web page from where the request originated. Do not make any security
   decisions based on the value of the referrer field, for example, to check
   whether the request originated from a page generated by the Web application,
   because the field is easily falsified.

*Sensitive Data *

Here's our security design principles for sensitive data:

   - *Do not store secrets if you can avoid it.*   Storing secrets in
   software in a completely secure fashion is not possible. An administrator,
   who has physical access to the server, can access the data. For example, it
   is not necessary to store a secret when all you need to do is verify whether
   a user knows the secret. In this case, you can store a hash value that
   represents the secret and compute the hash using the user-supplied value to
   verify whether the user knows the secret.
   - *Do not store secrets in code*.  Do not hard code secrets in code.
   Even if the source code is not exposed on the Web server, it is possible to
   extract string constants from compiled executable files. A configuration
   vulnerability may allow an attacker to retrieve the executable.
   - *Do not store database connections, passwords, or keys in plaintext*.
   Avoid storing secrets such as database connection strings, passwords, and
   keys in plaintext. Use encryption and store encrypted strings.
   - *Avoid storing secrets in the Local Security Authority (LSA)*.
   Avoid the LSA because your application requires administration privileges to
   access it. This violates the core security principle of running with least
   privilege. Also, the LSA can store secrets in only a restricted number of
   slots. A better approach is to use DPAPI.
   - *Use Data Protection API (DPAPI) for encrypting secrets*.   To store
   secrets such as database connection strings or service account credentials,
   use DPAPI. The main advantage to using DPAPI is that the platform system
   manages the encryption/decryption key and it is not an issue for the
   application. The key is either tied to a Windows user account or to a
   specific computer, depending on flags passed to the DPAPI functions.   DPAPI
   is best suited for encrypting information that can be manually recreated
   when the master keys are lost, for example, because a damaged server
   requires an operating system re-install. Data that cannot be recovered
   because you do not know the plaintext value, for example, customer credit
   card details, require an alternate approach that uses traditional symmetric
   key-based cryptography such as the use of triple-DES.
   - *Retrieve sensitive data on demand*.   The preferred approach is to
   retrieve sensitive data on demand when it is needed instead of persisting or
   caching it in memory. For example, retrieve the encrypted secret when it is
   needed, decrypt it, use it, and then clear the memory (variable) used to
   hold the plaintext secret. If performance becomes an issue, consider caching
   along with potential security implications.
   - *Encrypt the data or secure the communication channel*.   If you are
   sending sensitive data over the network to the client, encrypt the data or
   secure the channel. A common practice is to use SSL between the client and
   Web server. Between servers, an increasingly common approach is to use
   IPSec. For securing sensitive data that flows through several
   intermediaries, for example, Web service Simple Object Access Protocol
   (SOAP) messages, use message level encryption.
   - *Do not store sensitive data in persistent cookies*.  Avoid storing
   sensitive data in persistent cookies. If you store plaintext data, the end
   user is able to see and modify the data. If you encrypt the data, key
   management can be a problem. For example, if the key used to encrypt the
   data in the cookie has expired and been recycled, the new key cannot decrypt
   the persistent cookie passed by the browser from the client.
   - *Do not pass sensitive data using the HTTP-GET protocol*.   You
   should avoid storing sensitive data using the HTTP-GET protocol because the
   protocol uses query strings to pass data. Sensitive data cannot be secured
   using query strings and query strings are often logged by the server

*Session Management *

Here's our security design principles for session management:

   - *Use SSL to protect session authentication cookies*.   Do not pass
   authentication cookies over HTTP connections. Set the secure cookie property
   within authentication cookies, which instructs browsers to send cookies back
   to the server only over HTTPS connections.
   - *Encrypt the contents of the authentication cookies*.   Encrypt the
   cookie contents even if you are using SSL. This prevents an attacker viewing
   or modifying the cookie if he manages to steal it through an XSS attack. In
   this event, the attacker could still use the cookie to access your
   application, but only while the cookie remains valid.
   - *Limit session lifetime*.   Reduce the lifetime of sessions to
   mitigate the risk of session hijacking and replay attacks. The shorter the
   session, the less time an attacker has to capture a session cookie and use
   it to access your application.
   - *Protect session state from unauthorized access*.   Consider how
   session state is to be stored. For optimum performance, you can store
   session state in the Web application's process address space. However, this
   approach has limited scalability and implications in Web farm scenarios,
   where requests from the same user cannot be guaranteed to be handled by the
   same server. You should secure the network link from the Web application to
   state store using IPSec or SSL to mitigate the risk of eavesdropping. Also
   consider how the Web application is to be authenticated by the state store.
   Use Windows authentication where possible to avoid passing plaintext
   authentication credentials across the network and to benefit from secure
   Windows account policies.

*Using the Security Design Principles*

This is simply a baseline set of principles so that you don't have to start
from scratch.  You can build on this set and tailor for your specific
context.  I find that while having a set of principles helps, that you can't
stop there.  To share the knowledge and help others use the information,
it's important to encapsulate the principles in patterns as well as show
concrete examples and create precise, actionable guidelines for developers.
Personally, I've found Wikis to be the most effective way to share and
manage the information.

*Additional Resources*

   - patterns & practices Improving Web Application
Security<http://msdn2.microsoft.com/en-us/library/ms994921.aspx>(MSDN)
   - patterns & practices Security Engineering
Explained<http://msdn2.microsoft.com/en-us/library/ms998382.aspx>(MSDN)
   - Security Design
Principles<http://www.guidanceshare.com/wiki/Security_Design_Principles>(Guidance
Share)

*My Related Posts*

   - Security Frame<http://blogs.msdn.com/jmeier/pages/security-frame.aspx>
   - How To Use Guidance Explorer to Do a Security Code
Inspection<http://blogs.msdn.com/jmeier/archive/2007/12/13/how-to-use-guidance-explorer-to-do-a-security-code-inspection.aspx>
   - Guidance Share
Sweep<http://blogs.msdn.com/jmeier/archive/2008/01/02/guidance-share-sweep.aspx>


View article...<http://blogs.msdn.com/jmeier/archive/2008/04/07/security-principles.aspx>
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