CWE - CWE-73: External Control of File Name or Path (4.16)

Weakness ID: 73

Vulnerability Mapping: ALLOWED This CWE ID may be used to map to real-world vulnerabilities
Abstraction: Base Base - a weakness that is still mostly independent of a resource or technology, but with sufficient details to provide specific methods for detection and prevention. Base level weaknesses typically describe issues in terms of 2 or 3 of the following dimensions: behavior, property, technology, language, and resource.

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Description

The product 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.

Common Consequences

This table specifies different individual consequences associated with the weakness. The Scope identifies the application security area that is violated, while the Impact describes the negative technical impact that arises if an adversary succeeds in exploiting this weakness. The Likelihood provides information about how likely the specific consequence is expected to be seen relative to the other consequences in the list. For example, there may be high likelihood that a weakness will be exploited to achieve a certain impact, but a low likelihood that it will be exploited to achieve a different impact.

Scope	Impact	Likelihood
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, or 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.

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 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 allowlists 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 list 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 denylist, 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.

Effectiveness: High

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 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.

Relationships

This table shows the weaknesses and high level categories that are related to this weakness. These relationships are defined as ChildOf, ParentOf, MemberOf and give insight to similar items that may exist at higher and lower levels of abstraction. In addition, relationships such as PeerOf and CanAlsoBe are defined to show similar weaknesses that the user may want to explore.

Relevant to the view "Research Concepts" (CWE-1000)

Nature	Type	ID	Name
ChildOf	Class - a weakness that is described in a very abstract fashion, typically independent of any specific language or technology. More specific than a Pillar Weakness, but more general than a Base Weakness. Class level weaknesses typically describe issues in terms of 1 or 2 of the following dimensions: behavior, property, and resource.	610	Externally Controlled Reference to a Resource in Another Sphere
ChildOf	Class - a weakness that is described in a very abstract fashion, typically independent of any specific language or technology. More specific than a Pillar Weakness, but more general than a Base Weakness. Class level weaknesses typically describe issues in terms of 1 or 2 of the following dimensions: behavior, property, and resource.	642	External Control of Critical State Data
ParentOf	Class - a weakness that is described in a very abstract fashion, typically independent of any specific language or technology. More specific than a Pillar Weakness, but more general than a Base Weakness. Class level weaknesses typically describe issues in terms of 1 or 2 of the following dimensions: behavior, property, and resource.	114	Process Control
CanPrecede	Base - a weakness that is still mostly independent of a resource or technology, but with sufficient details to provide specific methods for detection and prevention. Base level weaknesses typically describe issues in terms of 2 or 3 of the following dimensions: behavior, property, technology, language, and resource.	22	Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')
CanPrecede	Base - a weakness that is still mostly independent of a resource or technology, but with sufficient details to provide specific methods for detection and prevention. Base level weaknesses typically describe issues in terms of 2 or 3 of the following dimensions: behavior, property, technology, language, and resource.	41	Improper Resolution of Path Equivalence
CanPrecede	Base - a weakness that is still mostly independent of a resource or technology, but with sufficient details to provide specific methods for detection and prevention. Base level weaknesses typically describe issues in terms of 2 or 3 of the following dimensions: behavior, property, technology, language, and resource.	59	Improper Link Resolution Before File Access ('Link Following')
CanPrecede	Variant - a weakness that is linked to a certain type of product, typically involving a specific language or technology. More specific than a Base weakness. Variant level weaknesses typically describe issues in terms of 3 to 5 of the following dimensions: behavior, property, technology, language, and resource.	98	Improper Control of Filename for Include/Require Statement in PHP Program ('PHP Remote File Inclusion')
CanPrecede	Base - a weakness that is still mostly independent of a resource or technology, but with sufficient details to provide specific methods for detection and prevention. Base level weaknesses typically describe issues in terms of 2 or 3 of the following dimensions: behavior, property, technology, language, and resource.	434	Unrestricted Upload of File with Dangerous Type

Relevant to the view "Software Development" (CWE-699)

Nature	Type	ID	Name
MemberOf	Category - a CWE entry that contains a set of other entries that share a common characteristic.	399	Resource Management Errors

Relevant to the view "Architectural Concepts" (CWE-1008)

Nature	Type	ID	Name
MemberOf	Category - a CWE entry that contains a set of other entries that share a common characteristic.	1015	Limit Access

Relevant to the view "Seven Pernicious Kingdoms" (CWE-700)

Nature	Type	ID	Name
ChildOf	Class - a weakness that is described in a very abstract fashion, typically independent of any specific language or technology. More specific than a Pillar Weakness, but more general than a Base Weakness. Class level weaknesses typically describe issues in terms of 1 or 2 of the following dimensions: behavior, property, and resource.	20	Improper Input Validation

Modes Of Introduction

The different Modes of Introduction provide information about how and when this weakness may be introduced. The Phase identifies a point in the life cycle at which introduction may occur, while the Note provides a typical scenario related to introduction during the given phase.

Phase	Note
Architecture and Design
Implementation	REALIZATION: This weakness is caused during implementation of an architectural security tactic.

Applicable Platforms

This listing shows possible areas for which the given weakness could appear. These may be for specific named Languages, Operating Systems, Architectures, Paradigms, Technologies, or a class of such platforms. The platform is listed along with how frequently the given weakness appears for that instance.

Languages

Class: Not Language-Specific (Undetermined Prevalence)

Operating Systems

Class: Unix (Often Prevalent)

Class: Windows (Often Prevalent)

Class: macOS (Often Prevalent)

Likelihood Of Exploit

High

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

Reference	Description
CVE-2022-45918	Chain: a learning management tool debugger uses external input to locate previous session logs (CWE-73) and does not properly validate the given path (CWE-20), allowing for filesystem path traversal using "../" sequences (CWE-24)
CVE-2008-5748	Chain: external control of values for user's desired language and theme enables path traversal.
CVE-2008-5764	Chain: external control of user's target language enables remote file inclusion.

Weakness Ordinalities

Ordinality	Description
Primary	(where the weakness exists independent of other weaknesses)

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 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.

Memberships

This MemberOf Relationships table shows additional CWE Categories and Views that reference this weakness as a member. This information is often useful in understanding where a weakness fits within the context of external information sources.

Nature	Type	ID	Name
MemberOf	Category - a CWE entry that contains a set of other entries that share a common characteristic.	723	OWASP Top Ten 2004 Category A2 - Broken Access Control
MemberOf	Category - a CWE entry that contains a set of other entries that share a common characteristic.	752	2009 Top 25 - Risky Resource Management
MemberOf	Category - a CWE entry that contains a set of other entries that share a common characteristic.	877	CERT C++ Secure Coding Section 09 - Input Output (FIO)
MemberOf	Category - a CWE entry that contains a set of other entries that share a common characteristic.	981	SFP Secondary Cluster: Path Traversal
MemberOf	Category - a CWE entry that contains a set of other entries that share a common characteristic.	1348	OWASP Top Ten 2021 Category A04:2021 - Insecure Design
MemberOf	Category - a CWE entry that contains a set of other entries that share a common characteristic.	1403	Comprehensive Categorization: Exposed Resource

Vulnerability Mapping Notes

Usage: ALLOWED

(this CWE ID may be used to map to real-world vulnerabilities)

Reason: Acceptable-Use

Rationale:

This CWE entry is at the Base level of abstraction, which is a preferred level of abstraction for mapping to the root causes of vulnerabilities.

Comments:

Carefully read both the name and description to ensure that this mapping is an appropriate fit. Do not try to 'force' a mapping to a lower-level Base/Variant simply to comply with this preferred level of abstraction.

Notes

Relationship

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.

Maintenance

CWE-114 is a Class, but it is listed a child of CWE-73 in view 1000. This suggests some abstraction problems that should be resolved in future versions.

Taxonomy Mappings

Mapped Taxonomy Name	Node ID	Fit	Mapped Node Name
7 Pernicious Kingdoms			Path Manipulation
Software Fault Patterns	SFP16		Path Traversal

Related Attack Patterns

CAPEC-ID	Attack Pattern Name
CAPEC-13	Subverting Environment Variable Values
CAPEC-267	Leverage Alternate Encoding
CAPEC-64	Using Slashes and URL Encoding Combined to Bypass Validation Logic
CAPEC-72	URL Encoding
CAPEC-76	Manipulating Web Input to File System Calls
CAPEC-78	Using Escaped Slashes in Alternate Encoding
CAPEC-79	Using Slashes in Alternate Encoding
CAPEC-80	Using UTF-8 Encoding to Bypass Validation Logic

References

[REF-6] Katrina Tsipenyuk, Brian Chess and Gary McGraw. "Seven Pernicious Kingdoms: A Taxonomy of Software Security Errors". NIST Workshop on Software Security Assurance Tools Techniques and Metrics. NIST. 2005-11-07. <https://samate.nist.gov/SSATTM_Content/papers/Seven%20Pernicious%20Kingdoms%20-%20Taxonomy%20of%20Sw%20Security%20Errors%20-%20Tsipenyuk%20-%20Chess%20-%20McGraw.pdf>.

[REF-45] OWASP. "OWASP Enterprise Security API (ESAPI) Project". <http://www.owasp.org/index.php/ESAPI>.

Content History

Submissions
Submission Date	Submitter	Organization
2006-07-19 (CWE Draft 3, 2006-07-19)	7 Pernicious Kingdoms
2006-07-19 (CWE Draft 3, 2006-07-19)
Modifications
Modification Date	Modifier	Organization
2008-07-01	Eric Dalci	Cigital
2008-07-01	updated Time_of_Introduction
2008-09-08	CWE Content Team	MITRE
2008-09-08	updated Relationships, Other_Notes, Taxonomy_Mappings, Weakness_Ordinalities
2009-01-12	CWE Content Team	MITRE
2009-01-12	updated Applicable_Platforms, Causal_Nature, Common_Consequences, Demonstrative_Examples, Description, Observed_Examples, Other_Notes, Potential_Mitigations, References, Relationship_Notes, Relationships, Weakness_Ordinalities
2009-03-10	CWE Content Team	MITRE
2009-03-10	updated Potential_Mitigations, Relationships
2009-07-27	CWE Content Team	MITRE
2009-07-27	updated Demonstrative_Examples
2009-10-29	CWE Content Team	MITRE
2009-10-29	updated Common_Consequences, Description
2009-12-28	CWE Content Team	MITRE
2009-12-28	updated Detection_Factors
2010-02-16	CWE Content Team	MITRE
2010-02-16	updated Potential_Mitigations
2011-06-01	CWE Content Team	MITRE
2011-06-01	updated Common_Consequences, Relationships, Taxonomy_Mappings
2011-09-13	CWE Content Team	MITRE
2011-09-13	updated Relationships, Taxonomy_Mappings
2012-05-11	CWE Content Team	MITRE
2012-05-11	updated Demonstrative_Examples, References, Related_Attack_Patterns, Relationships, Taxonomy_Mappings
2012-10-30	CWE Content Team	MITRE
2012-10-30	updated Potential_Mitigations
2014-07-30	CWE Content Team	MITRE
2014-07-30	updated Relationships, Taxonomy_Mappings
2017-11-08	CWE Content Team	MITRE
2017-11-08	updated Applicable_Platforms, Likelihood_of_Exploit, Modes_of_Introduction, Relationships, Taxonomy_Mappings
2020-02-24	CWE Content Team	MITRE
2020-02-24	updated Potential_Mitigations, References, Relationships, Time_of_Introduction, Type
2020-06-25	CWE Content Team	MITRE
2020-06-25	updated Potential_Mitigations, Relationships
2021-03-15	CWE Content Team	MITRE
2021-03-15	updated Maintenance_Notes, Potential_Mitigations
2021-10-28	CWE Content Team	MITRE
2021-10-28	updated Relationships
2023-01-31	CWE Content Team	MITRE
2023-01-31	updated Description, Detection_Factors, Potential_Mitigations
2023-04-27	CWE Content Team	MITRE
2023-04-27	updated Potential_Mitigations, Relationships
2023-06-29	CWE Content Team	MITRE
2023-06-29	updated Mapping_Notes
2023-10-26	CWE Content Team	MITRE
2023-10-26	updated Observed_Examples
Previous Entry Names
Change Date	Previous Entry Name
2008-04-11	Path Manipulation


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Common Weakness Enumeration

CWE-73: External Control of File Name or Path

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