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CWE-73: External Control of File Name or Path

 
External Control of File Name or Path
Weakness ID: 73 (Weakness Class)Status: Draft
+ Description

Description Summary

The software allows user input to control or influence paths or file names that are used in filesystem operations.

Extended Description

This could allow an attacker to access or modify system files or other files that are critical to the application.

Path manipulation errors occur when the following two conditions are met:

1. An attacker can specify a path used in an operation on the filesystem.

2. By specifying the resource, the attacker gains a capability that would not otherwise be permitted.

For example, the program may give the attacker the ability to overwrite the specified file or run with a configuration controlled by the attacker.

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

Languages

All

Operating Systems

UNIX: (Often)

Windows: (Often)

Mac OS: (Often)

+ Common Consequences
ScopeEffect
Integrity
Confidentiality

Technical Impact: Read files or directories; Modify files or directories

The application can operate on unexpected files. Confidentiality is violated when the targeted filename is not directly readable by the attacker.

Integrity
Confidentiality
Availability

Technical Impact: Modify files or directories; Execute unauthorized code or commands

The application can operate on unexpected files. This may violate integrity if the filename is written to, or if the filename is for a program or other form of executable code.

Availability

Technical Impact: DoS: crash / exit / restart; DoS: resource consumption (other)

The application can operate on unexpected files. Availability can be violated if the attacker specifies an unexpected file that the application modifies. Availability can also be affected if the attacker specifies a filename for a large file, or points to a special device or a file that does not have the format that the application expects.

+ Likelihood of Exploit

High to Very High

+ Detection Methods

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.

+ Demonstrative Examples

Example 1

The following code uses input from an HTTP request to create a file name. The programmer has not considered the possibility that an attacker could provide a file name such as "../../tomcat/conf/server.xml", which causes the application to delete one of its own configuration files (CWE-22).

(Bad Code)
Example Language: Java 
String rName = request.getParameter("reportName");
File rFile = new File("/usr/local/apfr/reports/" + rName);
...
rFile.delete();

Example 2

The following code uses input from a configuration file to determine which file to open and echo back to the user. If the program runs with privileges and malicious users can change the configuration file, they can use the program to read any file on the system that ends with the extension .txt.

(Bad Code)
Example Language: Java 
fis = new FileInputStream(cfg.getProperty("sub")+".txt");
amt = fis.read(arr);
out.println(arr);
+ Observed Examples
ReferenceDescription
Chain: external control of values for user's desired language and theme enables path traversal.
Chain: external control of user's target language enables remote file inclusion.
+ Potential Mitigations

Phase: Architecture and Design

When the set of filenames is limited or known, create a mapping from a set of fixed input values (such as numeric IDs) to the actual filenames, 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 provide this capability.

Phases: Architecture and Design; Operation

Run your code in a "jail" or similar sandbox environment that enforces strict boundaries between the process and the operating system. This may effectively restrict all access to files within a particular directory.

Examples include the Unix chroot jail and AppArmor. In general, managed code may provide some protection.

This may not be a feasible solution, and it only limits the impact to the operating system; the rest of your application may still be subject to compromise.

Be careful to avoid CWE-243 and other weaknesses related to jails.

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.

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.

Phase: Implementation

Use a built-in path canonicalization function (such as realpath() in C) that produces the canonical version of the pathname, which effectively removes ".." sequences and symbolic links (CWE-23, CWE-59).

Phases: Installation; Operation

Use OS-level permissions and run as a low-privileged user to limit the scope of any successful attack.

Phases: Operation; Implementation

If you are using PHP, configure your application so that it does not use register_globals. During implementation, develop your 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.

Phase: Testing

Use automated static analysis tools that target this type of weakness. Many modern techniques use data flow analysis to minimize the number of false positives. This is not a perfect solution, since 100% accuracy and coverage are not feasible.

Phase: Testing

Use dynamic tools and techniques that interact with the software using large test suites with many diverse inputs, such as fuzz testing (fuzzing), robustness testing, and fault injection. The software's operation may slow down, but it should not become unstable, crash, or generate incorrect results.

Phase: Testing

Use tools and techniques that require manual (human) analysis, such as penetration testing, threat modeling, and interactive tools that allow the tester to record and modify an active session. These may be more effective than strictly automated techniques. This is especially the case with weaknesses that are related to design and business rules.

+ Weakness Ordinalities
OrdinalityDescription
Primary
(where the weakness exists independent of other weaknesses)
+ Relationships
NatureTypeIDNameView(s) this relationship pertains toView(s)
ChildOfWeakness ClassWeakness Class20Improper Input Validation
Development Concepts (primary)699
Seven Pernicious Kingdoms (primary)700
ChildOfWeakness ClassWeakness Class610Externally Controlled Reference to a Resource in Another Sphere
Research Concepts1000
ChildOfWeakness ClassWeakness Class642External Control of Critical State Data
Research Concepts (primary)1000
ChildOfCategoryCategory723OWASP Top Ten 2004 Category A2 - Broken Access Control
Weaknesses in OWASP Top Ten (2004) (primary)711
ChildOfCategoryCategory7522009 Top 25 - Risky Resource Management
Weaknesses in the 2009 CWE/SANS Top 25 Most Dangerous Programming Errors (primary)750
ChildOfCategoryCategory877CERT C++ Secure Coding Section 09 - Input Output (FIO)
Weaknesses Addressed by the CERT C++ Secure Coding Standard (primary)868
ChildOfCategoryCategory981SFP Secondary Cluster: Path Traversal
Software Fault Pattern (SFP) Clusters (primary)888
CanPrecedeWeakness ClassWeakness Class22Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')
Research Concepts1000
CanPrecedeWeakness BaseWeakness Base41Improper Resolution of Path Equivalence
Research Concepts1000
CanPrecedeWeakness BaseWeakness Base59Improper Link Resolution Before File Access ('Link Following')
Research Concepts1000
CanPrecedeWeakness BaseWeakness Base98Improper Control of Filename for Include/Require Statement in PHP Program ('PHP Remote File Inclusion')
Research Concepts1000
CanPrecedeWeakness BaseWeakness Base434Unrestricted Upload of File with Dangerous Type
Research Concepts1000
CanAlsoBeWeakness BaseWeakness Base99Improper Control of Resource Identifiers ('Resource Injection')
Research Concepts1000
+ Relationship Notes

The external control of filenames can be the primary link in chains with other file-related weaknesses, as seen in the CanPrecede relationships. This is because software systems use files for many different purposes: to execute programs, load code libraries, to store application data, to store configuration settings, record temporary data, act as signals or semaphores to other processes, etc.

However, those weaknesses do not always require external control. For example, link-following weaknesses (CWE-59) often involve pathnames that are not controllable by the attacker at all.

The external control can be resultant from other issues. For example, in PHP applications, the register_globals setting can allow an attacker to modify variables that the programmer thought were immutable, enabling file inclusion (CWE-98) and path traversal (CWE-22). Operating with excessive privileges (CWE-250) might allow an attacker to specify an input filename that is not directly readable by the attacker, but is accessible to the privileged program. A buffer overflow (CWE-119) might give an attacker control over nearby memory locations that are related to pathnames, but were not directly modifiable by the attacker.

+ Taxonomy Mappings
Mapped Taxonomy NameNode IDFitMapped Node Name
7 Pernicious KingdomsPath Manipulation
CERT C++ Secure CodingFIO01-CPPBe careful using functions that use file names for identification
CERT C++ Secure CodingFIO02-CPPCanonicalize path names originating from untrusted sources
Software Fault PatternsSFP16Path Traversal
+ References
[REF-21] OWASP. "OWASP Enterprise Security API (ESAPI) Project". <http://www.owasp.org/index.php/ESAPI>.
+ Content History
Submissions
Submission DateSubmitterOrganizationSource
7 Pernicious KingdomsExternally Mined
Modifications
Modification DateModifierOrganizationSource
2008-07-01Eric DalciCigitalExternal
updated Time_of_Introduction
2008-09-08CWE Content TeamMITREInternal
updated Relationships, Other_Notes, Taxonomy_Mappings, Weakness_Ordinalities
2009-01-12CWE Content TeamMITREInternal
updated Applicable_Platforms, Causal_Nature, Common_Consequences, Demonstrative_Examples, Description, Observed_Examples, Other_Notes, Potential_Mitigations, References, Relationship_Notes, Relationships, Weakness_Ordinalities
2009-03-10CWE Content TeamMITREInternal
updated Potential_Mitigations, Relationships
2009-07-27CWE Content TeamMITREInternal
updated Demonstrative_Examples
2009-10-29CWE Content TeamMITREInternal
updated Common_Consequences, Description
2009-12-28CWE Content TeamMITREInternal
updated Detection_Factors
2010-02-16CWE Content TeamMITREInternal
updated Potential_Mitigations
2011-06-01CWE Content TeamMITREInternal
updated Common_Consequences, Relationships, Taxonomy_Mappings
2011-09-13CWE Content TeamMITREInternal
updated Relationships, Taxonomy_Mappings
2012-05-11CWE Content TeamMITREInternal
updated Demonstrative_Examples, References, Related_Attack_Patterns, Relationships, Taxonomy_Mappings
2012-10-30CWE Content TeamMITREInternal
updated Potential_Mitigations
2014-07-30CWE Content TeamMITREInternal
updated Relationships, Taxonomy_Mappings
Previous Entry Names
Change DatePrevious Entry Name
2008-04-11Path Manipulation
Page Last Updated: July 30, 2014