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734 Chapter 19 n Finding Vulnerabilities in Source Code NOTE Not all dangerous functions are restricted by safe mode, and some restrictions are affected by other configuration options. Furthermore, there are various ways to bypass some safe mode restrictions. Safe mode should not be considered a panacea to security issues within PHP applications. Safe mode has been removed from PHP version 6. Magic Quotes If the magic_quotes_gpc directive is enabled, any single quote, double quote, backslash, and NULL characters contained within request parameters are automatically escaped using a backslash. If the magic_quotes_sybase directive is enabled, single quotes are instead escaped using a single quote. This option is designed to protect vulnerable code containing unsafe database calls from being exploitable via malicious user input. When reviewing the application codebase to identify any SQL injection flaws, you should be aware of whether magic quotes are enabled, because this affects the application’s handling of input. Using magic quotes does not prevent all SQL injection attacks. As described in Chapter 9, an attack that injects into a numeric field does not need to use single quotation marks. Furthermore, data whose quotes have been escaped may still be used in a second-order attack when it is subsequently read back from the database. The magic quotes option may result in undesirable modification of user input, when data is being processed in a context that does not require any escaping. This can result in the addition of slashes that need to be removed using the stripslashes function. Some applications perform their own escaping of relevant input by passing individual parameters through the addslashes function only when required. If magic quotes are enabled in the PHP configuration, this approach results in double-escaped characters. Doubled-up slashes are interpreted as literal backslashes, leaving the potentially malicious character unescaped. Because of the limitations and anomalies of the magic quotes option, it is recommended that prepared statements be used for safe database access and that the magic quotes option be disabled. NOTE The magic quotes option has been removed from PHP version 6. Miscellaneous Table 19-10 lists some miscellaneous configuration options that can affect the security of PHP applications.
Chapter 19 n Finding Vulnerabilities in Source Code 735 Table 19-10: Miscellaneous PHP Configuration Options OPTION allow_url_fopen allow_url_include display_errors file_uploads upload_tmp_dir DESCRIPTION If disabled, this directive prevents some file functions from accessing remote files (as described previously). If disabled, this directive prevents the PHP file include functions from being used to include a remote file. If disabled, this directive prevents PHP errors from being reported to the user’s browser. The log_errors and error_log options can be used to record error information on the server for diagnostic purposes. If enabled, this directive causes PHP to allow file uploads over HTTP. This directive can be used to specify the temporary directory used to store uploaded files. This can be used to ensure that sensitive files are not stored in a worldreadable location. Perl This section describes ways to acquire user-supplied input, ways to interact with the user’s session, potentially dangerous APIs, and security-relevant configuration options on the Perl platform. The Perl language is notorious for allowing developers to perform the same task in a multitude of ways. Furthermore, numerous Perl modules can be used to meet different requirements. Any unusual or proprietary modules in use should be closely reviewed to identify whether they use any powerful or dangerous functions and thus may introduce the same vulnerabilities as if the application made direct use of those functions. CGI.pm is a widely used Perl module for creating web applications. It provides the APIs you are most likely to encounter when performing a code review of a web application written in Perl. Identifying User-Supplied Data The functions listed in Table 19-11 are all members of the CGI query object.
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The Web Application Hacker’s Hand
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About the Authors Dafydd Stuttard i
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MDSec: The Authors’ Company Dafyd
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Acknowledgments We are indebted to
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Contents Introduction xxiii Chapter
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Contents xi Chapter 5 Bypassing Cli
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Contents xiii Testing Restrictions
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Contents xv Payloads for XSS Attack
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Contents xvii “Off-by-One” Vuln
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Contents xix Technical Challenges F
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Contents xxi 12 Miscellaneous Check
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xxiv Introduction attacker. If you
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xxvi Introduction Chapters 6, 7, an
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xxviii Introduction internal workin
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xxx Introduction If you want to foc
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xxxii Introduction web users that d
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CHAPTER 1 Web Application (In)secur
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Chapter 1 n Web Application (In)sec
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CHAPTER 3 Web Application Technolog
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Chapter 3 n Web Application Technol
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CHAPTER 5 Bypassing Client-Side Con
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Chapter 5 n Bypassing Client-Side C
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CHAPTER 7 Attacking Session Managem
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Chapter 7 n Attacking Session Manag
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CHAPTER 8 Attacking Access Controls
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Chapter 8 n Attacking Access Contro
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288 Chapter 9 n Attacking Data Stor
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358 Chapter 10 n Attacking Back-End
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406 Chapter 11 n Attacking Applicat
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432 Chapter 12 n Attacking Users: C
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CHAPTER 13 Attacking Users: Other T
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Chapter 13 n Attacking Users: Other
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CHAPTER 16 Attacking Native Compile
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670 Chapter 18 n Attacking the Appl
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784 Chapter 20 n A Web Application
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854 Index n A-A hacker’s methodol
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856 Index n B-B meaningful, 212 URL
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858 Index n C-C XSS, 450-451 “cha
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860 Index n D-D CSS. See Cascading
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862 Index n F-F detecting hits, 574
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864 Index n H-H script injection, 8
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866 Index n J-J error messages, 388
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868 Index n M-O secret questions, 1
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870 Index n Q-R user input, 379-380
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872 Index n S-S second-order XSS. S
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874 Index n T-T signatures of commo
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876 Index n V-W provisos, 305-306 S
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878 Index n X-Z WHERE clause DELETE
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