CWE-22: Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')
Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')
Weakness ID: 22 (Weakness Class)
Status: Draft
Description
Description Summary
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 neutralizespecial 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.
Alternate Terms
Directory traversal
Path traversal:
"Path traversal" is preferred over "directory traversal," but both
terms are attack-focused.
Terminology Notes
Like other weaknesses, terminology is often based on the types of
manipulations used, instead of the underlying weaknesses. Some people use
"directory traversal" only to refer to the injection of ".." and equivalent
sequences whose specific meaning is to traverse directories.
Other variants like "absolute pathname" and "drive letter" have the
*effect* of directory traversal, but some people may not call it such, since
it doesn't involve ".." or equivalent.
Time of Introduction
Architecture and Design
Implementation
Applicable Platforms
Languages
Language-independent
Common Consequences
Scope
Effect
Integrity
Confidentiality
Availability
Technical Impact: 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
Technical Impact: 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
Technical Impact: 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
Technical Impact: 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.
Likelihood of Exploit
High to Very High
Detection Methods
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.
Effectiveness: 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.
Effectiveness: High
Demonstrative Examples
Example 1
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.
While the programmer intends to access files such as
"/users/cwe/profiles/alice" or "/users/cwe/profiles/bob", there is no
verification of the incoming user parameter. An attacker could provide a
string such as:
(Attack)
../../../etc/passwd
The program would generate a profile pathname like this:
(Result)
/users/cwe/profiles/../../../etc/passwd
When the file is opened, the operating system resolves the "../"
during path canonicalization and actually accesses this file:
(Result)
/etc/passwd
As a result, the attacker could read the entire text of the password
file.
Notice how this code also contains an error message information leak (CWE-209) if the user parameter does not produce a file that exists: the full pathname is provided. Because of the lack of output encoding of the file that is retrieved, there might also be a cross-site scripting problem (CWE-79) if profile contains any HTML, but other code would need to be examined.
Example 2
In the example below, the path to a dictionary file is read from a
system property and used to initialize a File object.
However, the path is not validated or modified to prevent it from
containing relative or absolute path sequences before creating the File
object. This allows anyone who can control the system property to
determine what file is used. Ideally, the path should be resolved
relative to some kind of application or user home directory.
Example 3
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.
(Bad Code)
Example
Language: Perl
my $Username = GetUntrustedInput();
$Username =~ s/\.\.\///;
my $filename = "/home/user/" . $Username;
ReadAndSendFile($filename);
Since the regular expression does not have the /g global match
modifier, it only removes the first instance of "../" it comes across.
So an input value such as:
(Attack)
../../../etc/passwd
will have the first "../" stripped, resulting in:
(Result)
../../etc/passwd
This value is then concatenated with the /home/user/ directory:
(Result)
/home/user/../../etc/passwd
which causes the /etc/passwd file to be retrieved once the operating system has resolved the ../ sequences in the pathname. This leads to relative path traversal (CWE-23).
Example 4
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/".
(Bad Code)
Example
Language: Java
String path = getInputPath();
if (path.startsWith("/safe_dir/"))
{
File f = new File(path);
f.delete()
}
An attacker could provide an input such as this:
(Attack)
/safe_dir/../important.dat
The software assumes that the path is valid because it starts with the
"/safe_path/" sequence, but the "../" sequence will cause the program to
delete the important.dat file in the parent directory
Example 5
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.
When submitted the Java servlet's doPost method will receive the
request, extract the name of the file from the Http request header, read
the file contents from the request and output the file to the local
upload directory.
(Bad Code)
Example
Language: Java
public class FileUploadServlet extends HttpServlet {
BufferedWriter bw = new BufferedWriter(new
FileWriter(uploadLocation+filename, true));
for (String line; (line=br.readLine())!=null; )
{
if (line.indexOf(boundary) == -1) {
bw.write(line);
bw.newLine();
bw.flush();
}
} //end of for loop
bw.close();
} catch (IOException ex) {...}
// output successful upload response HTML
page
}
// output unsuccessful upload response HTML
page
else
{...}
}
...
}
This code does not check the filename that is provided in the header, so an attacker can use "../" sequences to write to files outside of the intended directory. Depending on the executing environment, the attacker may be able to specify arbitrary files to write to, leading to a wide variety of consequences, from code execution, XSS (CWE-79), or system crash.
Also, this code does not perform a check on the type of the file being uploaded. This could allow an attacker to upload any executable file or other file with malicious code (CWE-434).
chain: library file sends a redirect if it is
directly requested but continues to execute, allowing remote file inclusion
and path traversal.
Potential Mitigations
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: 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
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:
realpath() in C
getCanonicalPath() in Java
GetFullPath() in ASP.NET
realpath() or abs_path() in Perl
realpath() in PHP
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.
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.
Effectiveness: 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.
Phases: Architecture and Design; Operation
Strategy: 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.
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 [R.22.3] provide this capability.
Phases: Architecture and Design; Operation
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 CWE-243 and other weaknesses related to jails.
Effectiveness: 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.
Phases: Architecture and Design; Operation
Strategy: 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.
Phase: 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.
Phases: Operation; Implementation
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 CWE-95, CWE-621, and similar issues.
Other Notes
Incomplete diagnosis or reporting of vulnerabilities can make it difficult
to know which variant is affected. For example, a researcher might say that
"..\" is vulnerable, but not test "../" which may also be vulnerable.
Any combination of the items below can provide its own variant, e.g.
"//../" is not listed (CVE-2004-0325).
Weakness Ordinalities
Ordinality
Description
Primary
(where
the weakness exists independent of other weaknesses)
Resultant
(where
the weakness is typically related to the presence of some other
weaknesses)
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).
Research Gaps
Many variants of path traversal attacks are probably under-studied with respect to root cause. CWE-790 and CWE-182 begin to cover part of this gap.
Affected Resources
File/Directory
Relevant Properties
Equivalence
Functional Areas
File processing
Causal Nature
Explicit
Taxonomy Mappings
Mapped Taxonomy Name
Node ID
Fit
Mapped Node Name
PLOVER
Path Traversal
OWASP Top Ten 2007
A4
CWE_More_Specific
Insecure Direct Object Reference
OWASP Top Ten 2004
A2
CWE_More_Specific
Broken Access Control
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
[R.22.1] [REF-11] M. Howard and
D. LeBlanc. "Writing Secure Code". Chapter 11, "Directory Traversal and Using Parent Paths (..)"
Page 370. 2nd Edition. Microsoft. 2002.
[R.22.6] [REF-7] Mark Dowd, John McDonald
and Justin Schuh. "The Art of Software Security Assessment". Chapter 9, "Filenames and Paths", Page
503.. 1st Edition. Addison Wesley. 2006.
Description
