Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')ID: 22 | Date: (C)2012-05-14 (M)2022-10-10 | Type: weakness | Status: 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 DescriptionMany 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 PlatformsLanguage Class: Language-independent Time Of Introduction - Architecture and Design
- Implementation
Related Attack Patterns Common Consequences Scope | Technical Impact | Notes |
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IntegrityConfidentialityAvailability | 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 Name | Description | Effectiveness | Notes |
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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 Phase | Strategy | Description | Effectiveness | Notes |
---|
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 DesignOperation | 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 DesignOperation | 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 DesignOperation | 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. | | | OperationImplementation | 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. | | |
RelationshipsPathname 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 CWE | Type | View | Chain |
---|
CWE-22 ChildOf CWE-893 | Category | CWE-888 | |
Demonstrative Examples (Details) - 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)
- 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/".
- 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)
- 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)
- 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 - CVE-2010-0467 : Newsletter module allows reading arbitrary files using "../" sequences.
- CVE-2009-4194 : FTP server allows deletion of arbitrary files using ".." in the DELE command.
- CVE-2009-4053 : FTP server allows creation of arbitrary directories using ".." in the MKD command.
- CVE-2009-0244 : OBEX FTP service for a Bluetooth device allows listing of directories, and creation or reading of files using ".." sequences..
- CVE-2009-4013 : Software package maintenance program allows overwriting arbitrary files using "../" sequences.
- CVE-2009-4449 : Bulletin board allows attackers to determine the existence of files using the avatar.
- CVE-2009-4581 : PHP program allows arbitrary code execution using ".." in filenames that are fed to the include() function.
- CVE-2010-0012 : Overwrite of files using a .. in a Torrent file.
- CVE-2010-0013 : Chat program allows overwriting files using a custom smiley request.
- 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 Taxynomy | Id | Name | Fit |
---|
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: - M. Howard D. LeBlanc .Writing Secure Code 2nd Edition. Microsoft. Section:'Chapter 11, "Directory Traversal and Using Parent Paths (..)"
Page 370'. Published on 2002.
- OWASP .OWASP Enterprise Security API (ESAPI) Project.
- OWASP .Testing for Path Traversal (OWASP-AZ-001).
- Johannes Ullrich .Top 25 Series - Rank 7 - Path Traversal. SANS Software Security Institute. 2010-03-09.
- Sean Barnum Michael Gegick .Least Privilege. Published on 2005-09-14.
- 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.
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