Improper Control of Filename for Include/Require Statement in PHP Program ('PHP Remote File Inclusion')
Draft Variant
Structure: Simple
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.
In certain versions and configurations of PHP, this can allow an attacker to specify a URL to a remote location from which the product 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.
Common Consequences 1
Scope: IntegrityConfidentialityAvailability
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.
Detection Methods 2
Manual AnalysisHigh
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.Automated Static Analysis
The external control or influence of filenames can often be detected using automated static analysis that models data flow within the product.
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 12
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 [REF-1482].
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 [REF-185] 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 Client-Side Enforcement of Server-Side Security. 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.
Phase: Architecture and DesignOperation
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 Creation of chroot Jail Without Changing Working Directory and other weaknesses related to jails.
Effectiveness: Limited
Phase: Architecture and DesignOperation
Strategy: Environment Hardening
Run your code using the lowest privileges that are required to accomplish the necessary tasks [REF-76]. 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 list 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. This 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, denylists 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 lists that limit the character set to be used. If feasible, only allow a single "." character in the filename to avoid weaknesses such as Relative Path Traversal, and exclude directory separators such as "/" to avoid Absolute Path Traversal. Use a list of allowable file extensions, which will help to avoid Unrestricted Upload of File with Dangerous Type.
Do not rely exclusively on a filtering mechanism that removes potentially dangerous characters. This is equivalent to a denylist, which may be incomplete (Incomplete List of Disallowed Inputs). 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 (Collapse of Data into Unsafe Value). 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.
Effectiveness: High
Phase: Architecture and DesignOperation
Strategy: Attack Surface Reduction
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.
Phase: Architecture and DesignImplementation
Strategy: Attack Surface Reduction
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 [REF-1481].
Effectiveness: Moderate
Phase: OperationImplementation
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.
Phase: OperationImplementation
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 Improper Neutralization of Directives in Dynamically Evaluated Code ('Eval Injection'), Variable Extraction Error, 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
Demonstrative Examples 1
The following code, victim.php, 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.
Code Example:
Bad
PHP
phpThe 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:
Code Example:
Attack
bashUpon 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:
Code Example:
Bad
PHP
phpAn attacker could now go a step further in our example and provide a request string as follows:
Code Example:
Attack
bashThe 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:
Code Example:
Attack
bashNote 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 20
CVE-2004-0285Modification of assumed-immutable configuration variable in include file allows file inclusion via direct request.
CVE-2004-0030Modification of assumed-immutable configuration variable in include file allows file inclusion via direct request.
CVE-2004-0068Modification of assumed-immutable configuration variable in include file allows file inclusion via direct request.
CVE-2005-2157Modification of assumed-immutable configuration variable in include file allows file inclusion via direct request.
CVE-2005-2162Modification of assumed-immutable configuration variable in include file allows file inclusion via direct request.
CVE-2005-2198Modification of assumed-immutable configuration variable in include file allows file inclusion via direct request.
CVE-2004-0128Modification of assumed-immutable variable in configuration script leads to file inclusion.
CVE-2005-1864PHP file inclusion.
CVE-2005-1869PHP file inclusion.
CVE-2005-1870PHP file inclusion.
CVE-2005-2154PHP local file inclusion.
CVE-2002-1704PHP remote file include.
CVE-2002-1707PHP remote file include.
CVE-2005-1964PHP remote file include.
CVE-2005-1681PHP remote file include.
CVE-2005-2086PHP remote file include.
CVE-2004-0127Directory traversal vulnerability in PHP include statement.
CVE-2005-1971Directory traversal vulnerability in PHP include statement.
CVE-2005-3335PHP 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.
CVE-2009-1936chain: library file sends a redirect if it is directly requested but continues to execute, allowing remote file inclusion and path traversal.
References 7
Testing for Path Traversal (OWASP-AZ-001)
OWASP
ID: REF-185
Least Privilege
Sean Barnum and Michael Gegick
14-09-2005
ID: REF-76
A Study in Scarlet
Shaun Clowes
ID: REF-951
Top 25 Series - Rank 13 - PHP File Inclusion
Johannes Ullrich
SANS Software Security Institute
11-03-2010
ID: REF-953
D3FEND: Application Layer Firewall
D3FEND
ID: REF-1481
D3FEND: D3-TL Trusted Library
D3FEND
ID: REF-1482
Likelihood of Exploit
High
Applicable Platforms
Languages:
PHP : Often
Modes of Introduction
Implementation
Related Attack Patterns
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 (Relative Path Traversal) attack techniques to access files that may contain previously-injected PHP code, such as web access logs.Affected Resources
- File or Directory
Related Weaknesses
Taxonomy Mapping
- PLOVER
- OWASP Top Ten 2007
- WASC
Notes
Relationship
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.