Difference between revisions of "Crawling Code"

Revision as of 17:34, 25 January 2009

Crawling code is the practice of scanning a code base of the review target in question. It is, in effect, looking for key pointers wherein a possible security vulnerability might reside. Certain APIs are related to interfacing to the external world or file IO or user management, which are key areas for an attacker to focus on. In crawling code we look for APIs relating to these areas. We also need to look for business logic areas which may cause security issues, but generally these are bespoke methods which have bespoke names and can not be detected directly, even though we may touch on certain methods due to their relationship with a certain key API.

We also need to look for common issues relating to a specific language; issues that may not be *security* related but which may affect the stability/availability of the application in the case of extraordinary circumstances. Other issues when performing a code review are areas such a simple copyright notice in order to protect one’s intellectual property.

Crawling code can be done manually or in an automated fashion using automated tools. Tools as simple as grep or wingrep can be used. Other tools are available which would search for key words relating to a specific programming language.

The following sections shall cover the function of crawing code for Java/J2EE, .NET and Classic ASP. This section is best used in conjunction with the transactional analysis section also detailed in this guide.

Searching for Key Indicators

The basis of the code review is to locate and analyse areas of code which may have application security implications. Assuming the code reviewer has a thorough understanding of the code, what it is intended to do, and the context in which it is to be used, firstly one needs to sweep the code base for areas of interest.

This can be done by performing a text search on the code base looking for keywords relating to APIs and functions. Below is a guide for .NET framework 1.1 & 2.0

Searching for Code in .NET

Firstly one needs to be familiar with the tools one can use in order to perform text searching, following this one neesd to know what to look for.

In this section we will assume you have a copy of Visual Studio (VS) .NET at hand. VS has two types of search "Find in Files" and a cmd line tool called Findstr.

The test search tools in XP is not great in my experience and if one has to use this make sure SP2 in installed as it works better. To start off, one should scan thorough the code looking for common patterns or keywords such as "User", "Password", "Pswd", "Key", "Http", etc... This can be done using the "Find in Files" tool in VS or using findstring as follows:

findstr /s /m /i /d:c:\projects\codebase\sec "http" *.*

HTTP Request Strings

Requests from external sources are obviously a key area of a security code review. We need to ensure that all HTTP requests received are data validated for composition, max and min length, and if the data falls with the realms of the parameter white-list. Bottom-line is this is a key area to look at and ensure security is enabled.

request.accepttypes
request.browser
request.files
request.headers
request.httpmethod
request.item
request.querystring
request.form
request.cookies
request.certificate
request.rawurl
request.servervariables
request.url
request.urlreferrer
request.useragent
request.userlanguages
request.IsSecureConnection
request.TotalBytes
request.BinaryRead
InputStream
HiddenField.Value
TextBox.Text
recordSet

HTML Output

Here we are looking for responses to the client. Responses which go unvalidated or which echo external input without data validation are key areas to examine. Many client side attacks result from poor response validation. XSS relies on this somewhat.

response.write
<% =
HttpUtility
HtmlEncode
UrlEncode
innerText
innerHTML

SQL & Database

Locating where a database may be involved in the code is an important aspect of the code review. Looking at the database code will help determine if the application is vulnerable to SQL injection. One aspect of this is to verify that the code uses either SqlParameter, OleDbParameter, or OdbcParameter(System.Data.SqlClient). These are typed and treat parameters as the literal value and not executable code in the database.

exec sp_executesql
execute sp_executesql
select from
Insert

update
delete from where
delete
exec sp_
execute sp_
exec xp_
execute sp_
exec @
execute @
executestatement
executeSQL
setfilter
executeQuery
GetQueryResultInXML
adodb
sqloledb
sql server

driver
Server.CreateObject
.Provider
.Open
ADODB.recordset
New OleDbConnection
ExecuteReader
DataSource

SqlCommand
Microsoft.Jet
SqlDataReader
ExecuteReader
GetString
SqlDataAdapter
CommandType
StoredProcedure
System.Data.sql

Cookies

Cookie manipulation can be key to various application security exploits, such as session hijacking/fixation and parameter manipulation. One should examine any code relating to cookie functionality, as this would have a bearing on session security.

System.Net.Cookie
HTTPOnly
document.cookie

HTML Tags

Many of the HTML tags below can be used for client side attacks such as cross site scripting. It is important to examine the context in which these tags are used and to examine any relevant data validation associated with the display and use of such tags within a web application.

HtmlEncode
URLEncode
<applet>
<frameset>
<embed>
<frame>
<html>
<iframe>
<img>
<style>
<layer>
<ilayer>
<meta>
<object>
<body>
<frame security
<iframe security

Input Controls

The input controls below are server classes used to produce and display web application form fields. Looking for such references helps locate entry points into the application.

system.web.ui.htmlcontrols.htmlinputhidden system.web.ui.webcontrols.hiddenfield system.web.ui.webcontrols.hyperlink system.web.ui.webcontrols.textbox system.web.ui.webcontrols.label system.web.ui.webcontrols.linkbutton system.web.ui.webcontrols.listbox system.web.ui.webcontrols.checkboxlist system.web.ui.webcontrols.dropdownlist

WEB.Config

The .NET Framework relies on .config files to define configuration settings. The .config files are text-based XML files. Many .config files can, and typically do, exist on a single system. Web applications refer to a web.config file located in the application’s root directory. For ASP.NET applications, web.config contains information about most aspects of the application’s operation.

requestEncoding
responseEncoding
trace
authorization
compilation
CustomErrors
httpCookies
httpHandlers
httpRuntime
sessionState
maxRequestLength
debug
forms protection
appSettings
ConfigurationSettings
appSettings
connectionStrings
authentication mode
allow
deny
credentials
identity impersonate
timeout
remote

global.asax

Each application has its own Global.asax if one is required. Global.asax sets the event code and values for an application using scripts. One must ensure that application variables do not contain sensitive information, as they are accessible to the whole application and to all users within it.

Application_OnAuthenticateRequest
Application_OnAuthorizeRequest
Session_OnStart
Session_OnEnd

Logging

Logging can be a source of information leakage. It is important to examine all calls to the logging subsystem and to determine if any sensitive information is being logged. Common mistakes are logging userID in conjunction with passwords within the authentication functionality or logging database requests which may contains sensitive data.

log4net
Console.WriteLine
System.Diagnostics.Debug
System.Diagnostics.Trace

Machine.config

Its important that many variables in machine.config can be overridden in the web.config file for a particular application.

validateRequest
enableViewState
enableViewStateMac

Threads and Concurrency

Locating code that contains multithreaded functions. Concurrency issues can result in race conditions which may result in security vulnerabilities. The Thread keyword is where new threads objects are created. Code that uses static global variables which hold sensitive security information may cause session issues. Code that uses static constructors may also cause issues between threads. Not synchronizing the Dispose method may cause issues if a number of threads call Dispose at the same time, this may cause resource release issues.

Thread
Dispose

Class Design

Public and Sealed relate to the design at class level. Classes which are not intended to be derived from should be sealed. Make sure all class fields are Public for a reason. Don't expose anything you don't need to.

Public
Sealed

Reflection, Serialization

Code may be generated dynamically at runtime. Code that is generated dynamically as a function of external input may give rise to issues. If your code contains sensitive data, does it need to be serialized?

Serializable
AllowPartiallyTrustedCallersAttribute
GetObjectData
StrongNameIdentityPermission
StrongNameIdentity
System.Reflection

Exceptions & Errors

Ensure that the catch blocks do not leak information to the user in the case of an exception. Ensure when dealing with resources that the finally block is used. Having trace enabled is not great from an information leakage perspective. Ensure customised errors are properly implemented.

catch{
Finally
trace enabled
customErrors mode

Crypto

If cryptography is used then is a strong enough cipher used, i.e. AES or 3DES? What size key is used? The larger the better. Where is hashing performed? Are passwords that are being persisted hashed? They should be. How are random numbers generated? Is the PRNG "random enough"?

RNGCryptoServiceProvider
SHA
MD5
base64
xor
DES
RC2
System.Random
Random
System.Security.Cryptography

Storage

If storing sensitive data in memory, I recommend one uses the following.

SecureString
ProtectedMemory

Authorization, Assert & Revert

Bypassing the code access security permission? Not a good idea. Also below is a list of potentially dangerous permissions such as calling unmanaged code, outside the CLR.

.RequestMinimum
.RequestOptional
Assert
Debug.Assert
CodeAccessPermission
ReflectionPermission.MemberAccess
SecurityPermission.ControlAppDomain
SecurityPermission.UnmanagedCode
SecurityPermission.SkipVerification
SecurityPermission.ControlEvidence
SecurityPermission.SerializationFormatter
SecurityPermission.ControlPrincipal
SecurityPermission.ControlDomainPolicy
SecurityPermission.ControlPolicy

Legacy Methods

printf
strcpy