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CWE
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Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')

ID: 22Date: (C)2012-05-14   (M)2017-12-01
Type: weaknessStatus: DRAFT
Abstraction Type: Class





Description

The software uses external input to construct a pathname that is intended to identify a file or directory that is located underneath a restricted parent directory, but the software does not properly neutralize special elements within the pathname that can cause the pathname to resolve to a location that is outside of the restricted directory.

Extended Description

Many file operations are intended to take place within a restricted directory. By using special elements such as ".." and "/" separators, attackers can escape outside of the restricted location to access files or directories that are elsewhere on the system. One of the most common special elements is the "../" sequence, which in most modern operating systems is interpreted as the parent directory of the current location. This is referred to as relative path traversal. Path traversal also covers the use of absolute pathnames such as "/usr/local/bin", which may also be useful in accessing unexpected files. This is referred to as absolute path traversal.

In many programming languages, the injection of a null byte (the 0 or NUL) may allow an attacker to truncate a generated filename to widen the scope of attack. For example, the software may add ".txt" to any pathname, thus limiting the attacker to text files, but a null injection may effectively remove this restriction.

Likelihood of Exploit: High to Very High

Applicable Platforms
Language Class: Language-independent

Time Of Introduction

  • Architecture and Design
  • Implementation

Related Attack Patterns

Common Consequences

ScopeTechnical ImpactNotes
Integrity
Confidentiality
Availability
 
Execute unauthorized code or commands
 
The attacker may be able to create or overwrite critical files that are used to execute code, such as programs or libraries.
 
Integrity
 
Modify files or directories
 
The attacker may be able to overwrite or create critical files, such as programs, libraries, or important data. If the targeted file is used for a security mechanism, then the attacker may be able to bypass that mechanism. For example, appending a new account at the end of a password file may allow an attacker to bypass authentication.
 
Confidentiality
 
Read files or directories
 
The attacker may be able read the contents of unexpected files and expose sensitive data. If the targeted file is used for a security mechanism, then the attacker may be able to bypass that mechanism. For example, by reading a password file, the attacker could conduct brute force password guessing attacks in order to break into an account on the system.
 
Availability
 
DoS: crash / exit / restart
 
The attacker may be able to overwrite, delete, or corrupt unexpected critical files such as programs, libraries, or important data. This may prevent the software from working at all and in the case of a protection mechanisms such as authentication, it has the potential to lockout every user of the software.
 

Detection Methods

NameDescriptionEffectivenessNotes
Automated Static Analysis
 
Automated techniques can find areas where path traversal weaknesses exist. However, tuning or customization may be required to remove or de-prioritize path-traversal problems that are only exploitable by the software's administrator - or other privileged users - and thus potentially valid behavior or, at worst, a bug instead of a vulnerability.
 
High
 
 
Manual Static Analysis
 
Manual white box techniques may be able to provide sufficient code coverage and reduction of false positives if all file access operations can be assessed within limited time constraints.
 
High
 
 

Potential Mitigations

PhaseStrategyDescriptionEffectivenessNotes
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
 
 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.
 
  
Implementation
 
Input Validation
 
Inputs should be decoded and canonicalized to the application's current internal representation before being validated (CWE-180). Make sure that the application does not decode the same input twice (CWE-174). Such errors could be used to bypass whitelist validation schemes by introducing dangerous inputs after they have been checked.
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). This includes:

 
  
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.
 
  
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.
 
Architecture and Design
Operation
 
Environment Hardening
 
Run your code using the lowest privileges that are required to accomplish the necessary tasks [R.22.5]. 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.
 
  
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.22.3] provide this capability.
 
  
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
 
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.
 
  
Implementation
 
 Ensure that error messages only contain minimal details that are useful to the intended audience, and nobody else. The messages need to strike the balance between being too cryptic and not being cryptic enough. They should not necessarily reveal the methods that were used to determine the error. Such detailed information can be used to refine the original attack to increase the chances of success.
If errors must be tracked in some detail, capture them in log messages - but consider what could occur if the log messages can be viewed by attackers. Avoid recording highly sensitive information such as passwords in any form. Avoid inconsistent messaging that might accidentally tip off an attacker about internal state, such as whether a username is valid or not.
In the context of path traversal, error messages which disclose path information can help attackers craft the appropriate attack strings to move through the file system hierarchy.
 
  
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.
 
  

Relationships
Pathname equivalence can be regarded as a type of canonicalization error.
Some pathname equivalence issues are not directly related to directory traversal, rather are used to bypass security-relevant checks for whether a file/directory can be accessed by the attacker (e.g. a trailing "/" on a filename could bypass access rules that don't expect a trailing /, causing a server to provide the file when it normally would not).

Related CWETypeViewChain
CWE-22 ChildOf CWE-893 Category CWE-888  

Demonstrative Examples   (Details)

  1. In the example below, the path to a dictionary file is read from a system property and used to initialize a File object. (Demonstrative Example Id DX-18)
  2. The following code attempts to validate a given input path by checking it against a white list and once validated delete the given file. In this specific case, the path is considered valid if it starts with the string "/safe_dir/".
  3. The following code could be for a social networking application in which each user's profile information is stored in a separate file. All files are stored in a single directory. (Demonstrative Example Id DX-27)
  4. The following code demonstrates the unrestricted upload of a file with a Java servlet and a path traversal vulnerability. The HTML code is the same as in the previous example with the action attribute of the form sending the upload file request to the Java servlet instead of the PHP code. (Demonstrative Example Id DX-22)
  5. The following code takes untrusted input and uses a regular expression to filter "../" from the input. It then appends this result to the /home/user/ directory and attempts to read the file in the final resulting path. (Demonstrative Example Id DX-2)

Observed Examples

  1. CVE-2010-0467 : Newsletter module allows reading arbitrary files using "../" sequences.
  2. CVE-2009-4194 : FTP server allows deletion of arbitrary files using ".." in the DELE command.
  3. CVE-2009-4053 : FTP server allows creation of arbitrary directories using ".." in the MKD command.
  4. CVE-2009-0244 : OBEX FTP service for a Bluetooth device allows listing of directories, and creation or reading of files using ".." sequences..
  5. CVE-2009-4013 : Software package maintenance program allows overwriting arbitrary files using "../" sequences.
  6. CVE-2009-4449 : Bulletin board allows attackers to determine the existence of files using the avatar.
  7. CVE-2009-4581 : PHP program allows arbitrary code execution using ".." in filenames that are fed to the include() function.
  8. CVE-2010-0012 : Overwrite of files using a .. in a Torrent file.
  9. CVE-2010-0013 : Chat program allows overwriting files using a custom smiley request.
  10. CVE-2008-5748 : Chain: external control of values for user's desired language and theme enables path traversal.

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

White Box Definitions
None

Black Box Definitions
None

Taxynomy Mappings

TaxynomyIdNameFit
PLOVER  Path Traversal
 
 
OWASP Top Ten 2007 A4
 
Insecure Direct Object Reference
 
CWE_More_Specific
 
OWASP Top Ten 2004 A2
 
Broken Access Control
 
CWE_More_Specific
 
CERT C Secure Coding FIO02-C
 
Canonicalize path names originating from untrusted sources
 
 
WASC 33
 
Path Traversal
 
 
CERT C++ Secure Coding FIO02-CPP
 
Canonicalize path names originating from untrusted sources
 
 

References:

  1. M. Howard D. LeBlanc .Writing Secure Code 2nd Edition. Microsoft. Section:'Chapter 11, "Directory Traversal and Using Parent Paths (..)" Page 370'. Published on 2002.
  2. OWASP .OWASP Enterprise Security API (ESAPI) Project.
  3. OWASP .Testing for Path Traversal (OWASP-AZ-001).
  4. Johannes Ullrich .Top 25 Series - Rank 7 - Path Traversal. SANS Software Security Institute. 2010-03-09.
  5. Sean Barnum Michael Gegick .Least Privilege. Published on 2005-09-14.
  6. Mark Dowd John McDonald Justin Schuh .The Art of Software Security Assessment 1st Edition. Addison Wesley. Section:'Chapter 9, "Filenames and Paths", Page 503.'. Published on 2006.
CVE    1372
CVE-2004-2717
CVE-2004-2745
CVE-2004-2747
CVE-2004-2750
...

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