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CWE
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Improper Control of Filename for Include/Require Statement in PHP Program ('PHP File Inclusion')

ID: 98Date: (C)2012-05-14   (M)2012-11-08
Type: weaknessStatus: DRAFT
Abstraction Type: Base





Description

The PHP application receives input from an upstream component, but it does not restrict or incorrectly restricts the input before its usage in "require," "include," or similar functions.

Extended Description

In certain versions and configurations of PHP, this can allow an attacker to specify a URL to a remote location from which the software will obtain the code to execute. In other cases in association with path traversal, the attacker can specify a local file that may contain executable statements that can be parsed by PHP.

Likelihood of Exploit: High to Very High

Applicable Platforms
Language: Often
Language: PHP

Time Of Introduction

  • Implementation
  • Architecture and Design

Related Attack Patterns

Common Consequences

ScopeTechnical ImpactNotes
Integrity
Confidentiality
Availability
 
Execute unauthorized code or commands
 
The attacker may be able to specify arbitrary code to be executed from a remote location. Alternatively, it may be possible to use normal program behavior to insert php code into files on the local machine which can then be included and force the code to execute since php ignores everything in the file except for the content between php specifiers.
 

Detection Methods

NameDescriptionEffectivenessNotes
Manual Analysis
 
Manual white-box analysis can be very effective for finding this issue, since there is typically a relatively small number of include or require statements in each program.
 
High
 
 
Automated Static Analysis
 
The external control or influence of filenames can often be detected using automated static analysis that models data flow within the software.
Automated static analysis might not be able to recognize when proper input validation is being performed, leading to false positives - i.e., warnings that do not have any security consequences or require any code changes. If the program uses a customized input validation library, then some tools may allow the analyst to create custom signatures to detect usage of those routines.
 
  

Potential Mitigations

PhaseStrategyDescriptionEffectivenessNotes
Architecture and Design
 
Libraries or Frameworks
 
Use a vetted library or framework that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid.
 
  
Architecture and Design
 
Enforcement by Conversion
 
When the set of acceptable objects, such as filenames or URLs, is limited or known, create a mapping from a set of fixed input values (such as numeric IDs) to the actual filenames or URLs, and reject all other inputs.
For example, ID 1 could map to "inbox.txt" and ID 2 could map to "profile.txt". Features such as the ESAPI AccessReferenceMap [R.98.1] provide this capability.
 
  
Architecture and Design
 
 For any security checks that are performed on the client side, ensure that these checks are duplicated on the server side, in order to avoid CWE-602. Attackers can bypass the client-side checks by modifying values after the checks have been performed, or by changing the client to remove the client-side checks entirely. Then, these modified values would be submitted to the server.
 
  
Architecture and Design
Operation
 
Sandbox or Jail
 
Run the code in a "jail" or similar sandbox environment that enforces strict boundaries between the process and the operating system. This may effectively restrict which files can be accessed in a particular directory or which commands can be executed by the software.
OS-level examples include the Unix chroot jail, AppArmor, and SELinux. In general, managed code may provide some protection. For example, java.io.FilePermission in the Java SecurityManager allows the software to specify restrictions on file operations.
This may not be a feasible solution, and it only limits the impact to the operating system; the rest of the application may still be subject to compromise.
Be careful to avoid CWE-243 and other weaknesses related to jails.
 
Limited
 
The effectiveness of this mitigation depends on the prevention capabilities of the specific sandbox or jail being used and might only help to reduce the scope of an attack, such as restricting the attacker to certain system calls or limiting the portion of the file system that can be accessed.
 
Architecture and Design
Operation
 
Environment Hardening
 
Run your code using the lowest privileges that are required to accomplish the necessary tasks [R.98.2]. If possible, create isolated accounts with limited privileges that are only used for a single task. That way, a successful attack will not immediately give the attacker access to the rest of the software or its environment. For example, database applications rarely need to run as the database administrator, especially in day-to-day operations.
 
  
Implementation
 
Input Validation
 
Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a whitelist of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does.
When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, "boat" may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as "red" or "blue."
Do not rely exclusively on looking for malicious or malformed inputs (i.e., do not rely on a blacklist). A blacklist is likely to miss at least one undesirable input, especially if the code's environment changes. This can give attackers enough room to bypass the intended validation. However, blacklists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright.
When validating filenames, use stringent whitelists that limit the character set to be used. If feasible, only allow a single "." character in the filename to avoid weaknesses such as CWE-23, and exclude directory separators such as "/" to avoid CWE-36. Use a whitelist of allowable file extensions, which will help to avoid CWE-434.
Do not rely exclusively on a filtering mechanism that removes potentially dangerous characters. This is equivalent to a blacklist, which may be incomplete (CWE-184). For example, filtering "/" is insufficient protection if the filesystem also supports the use of "\" as a directory separator. Another possible error could occur when the filtering is applied in a way that still produces dangerous data (CWE-182). For example, if "../" sequences are removed from the ".../...//" string in a sequential fashion, two instances of "../" would be removed from the original string, but the remaining characters would still form the "../" string.
 
  
Architecture and Design
Operation
 
Identify and Reduce Attack Surface
 
Store library, include, and utility files outside of the web document root, if possible. Otherwise, store them in a separate directory and use the web server's access control capabilities to prevent attackers from directly requesting them. One common practice is to define a fixed constant in each calling program, then check for the existence of the constant in the library/include file; if the constant does not exist, then the file was directly requested, and it can exit immediately.
This significantly reduces the chance of an attacker being able to bypass any protection mechanisms that are in the base program but not in the include files. It will also reduce the attack surface.
 
  
Architecture and Design
Implementation
 
Identify and Reduce Attack Surface
 
Understand all the potential areas where untrusted inputs can enter your software: parameters or arguments, cookies, anything read from the network, environment variables, reverse DNS lookups, query results, request headers, URL components, e-mail, files, filenames, databases, and any external systems that provide data to the application. Remember that such inputs may be obtained indirectly through API calls.
Many file inclusion problems occur because the programmer assumed that certain inputs could not be modified, especially for cookies and URL components.
 
  
Operation
 
Firewall
 
Use an application firewall that can detect attacks against this weakness. It can be beneficial in cases in which the code cannot be fixed (because it is controlled by a third party), as an emergency prevention measure while more comprehensive software assurance measures are applied, or to provide defense in depth.
 
Moderate
 
An application firewall might not cover all possible input vectors. In addition, attack techniques might be available to bypass the protection mechanism, such as using malformed inputs that can still be processed by the component that receives those inputs. Depending on functionality, an application firewall might inadvertently reject or modify legitimate requests. Finally, some manual effort may be required for customization.
 
Operation
Implementation
 
Environment Hardening
 
Develop and run your code in the most recent versions of PHP available, preferably PHP 6 or later. Many of the highly risky features in earlier PHP interpreters have been removed, restricted, or disabled by default.
 
  
Operation
Implementation
 
Environment Hardening
 
When using PHP, configure the application so that it does not use register_globals. During implementation, develop the application so that it does not rely on this feature, but be wary of implementing a register_globals emulation that is subject to weaknesses such as CWE-95, CWE-621, and similar issues.
Often, programmers do not protect direct access to files intended only to be included by core programs. These include files may assume that critical variables have already been initialized by the calling program. As a result, the use of register_globals combined with the ability to directly access the include file may allow attackers to conduct file inclusion attacks. This remains an extremely common pattern as of 2009.
 
  
Operation
 
Environment Hardening
 
Set allow_url_fopen to false, which limits the ability to include files from remote locations.
 
High
 
Be aware that some versions of PHP will still accept ftp:// and other URI schemes. In addition, this setting does not protect the code from path traversal attacks (CWE-22), which are frequently successful against the same vulnerable code that allows remote file inclusion.
 

Relationships
This is frequently a functional consequence of other weaknesses. It is usually multi-factor with other factors (e.g. MAID), although not all inclusion bugs involve assumed-immutable data. Direct request weaknesses frequently play a role.
Can overlap directory traversal in local inclusion problems.

Related CWETypeViewChain
CWE-98 ChildOf CWE-727 Category CWE-711  

Demonstrative Examples   (Details)

  1. The following code attempts to include a function contained in a separate PHP page on the server. It builds the path to the file by using the supplied 'module_name' parameter and appending the string '/function.php' to it.

Observed Examples

  1. CVE-2004-0285 : Modification of assumed-immutable configuration variable in include file allows file inclusion via direct request.
  2. CVE-2004-0030 : Modification of assumed-immutable configuration variable in include file allows file inclusion via direct request.
  3. CVE-2004-0068 : Modification of assumed-immutable configuration variable in include file allows file inclusion via direct request.
  4. CVE-2005-2157 : Modification of assumed-immutable configuration variable in include file allows file inclusion via direct request.
  5. CVE-2005-2162 : Modification of assumed-immutable configuration variable in include file allows file inclusion via direct request.
  6. CVE-2005-2198 : Modification of assumed-immutable configuration variable in include file allows file inclusion via direct request.
  7. CVE-2004-0128 : Modification of assumed-immutable variable in configuration script leads to file inclusion.
  8. CVE-2005-1864 : PHP file inclusion.
  9. CVE-2005-1869 : PHP file inclusion.
  10. CVE-2005-1870 : PHP file inclusion.
  11. CVE-2005-2154 : PHP local file inclusion.
  12. CVE-2002-1704 : PHP remote file include.
  13. CVE-2002-1707 : PHP remote file include.
  14. CVE-2005-1964 : PHP remote file include.
  15. CVE-2005-1681 : PHP remote file include.
  16. CVE-2005-2086 : PHP remote file include.
  17. CVE-2004-0127 : Directory traversal vulnerability in PHP include statement.
  18. CVE-2005-1971 : Directory traversal vulnerability in PHP include statement.
  19. CVE-2005-3335 : PHP file inclusion issue, both remote and local; local include uses ".." and "%00" characters as a manipulation, but many remote file inclusion issues probably have this vector.

For more examples, refer to CVE relations in the bottom box.

White Box Definitions
None

Black Box Definitions
None

Taxynomy Mappings

TaxynomyIdNameFit
PLOVER  PHP File Include
 
 
OWASP Top Ten 2007 A3
 
Malicious File Execution
 
CWE_More_Specific
 
WASC 5
 
Remote File Inclusion
 
 

References:

  1. OWASP .Testing for Path Traversal (OWASP-AZ-001).
  2. Sean Barnum Michael Gegick .Least Privilege. Published on 2005-09-14.
  3. Shaun Clowes .A Study in Scarlet.
  4. Stefan Esser .Suhosin.
  5. Johannes Ullrich .Top 25 Series - Rank 13 - PHP File Inclusion. SANS Software Security Institute. 2010-03-11.

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