A directory traversal consists in exploiting insufficient security validation / sanitization of user-supplied input file names, so that characters representing "traverse to parent directory" are passed through to the file APIs.
The goal of this attack is to order an application to access a computer file
Computer file
A computer file is a block of arbitrary information, or resource for storing information, which is available to a computer program and is usually based on some kind of durable storage. A file is durable in the sense that it remains available for programs to use after the current program has finished...
that is not intended to be accessible. This attack exploits a lack of security (the software is acting exactly as it is supposed to) as opposed to exploiting a bug in the code.
Directory traversal is also known as the ../ (dot dot slash) attack, directory
Directory (file systems)
In computing, a folder, directory, catalog, or drawer, is a virtual container originally derived from an earlier Object-oriented programming concept by the same name within a digital file system, in which groups of computer files and other folders can be kept and organized.A typical file system may...
climbing, and backtracking. Some forms of this attack are also canonicalization
Canonicalization
In computer science, canonicalization , is a process for converting data that has more than one possible representation into a "standard", "normal", or canonical form...
attacks.
Example
A typical example of vulnerable application in PHP
PHP
PHP is a general-purpose server-side scripting language originally designed for web development to produce dynamic web pages. For this purpose, PHP code is embedded into the HTML source document and interpreted by a web server with a PHP processor module, which generates the web page document...
code is:
$template = 'red.php';
if (isset($_COOKIE['TEMPLATE']))
$template = $_COOKIE['TEMPLATE'];
include ("/home/users/phpguru/templates/" . $template);
?>
An attack against this system could be to send the following HTTP request:
GET /vulnerable.php HTTP/1.0
Cookie: TEMPLATE=../../../../../../../../../etc/passwd
Generating a server response such as:
HTTP/1.0 200 OK
Content-Type: text/html
Server: Apache
The repeated ../ characters after /home/users/phpguru/templates/ has caused include to traverse to the root directory, and then include the Unix password file /etc/passwd
Passwd (file)
In Unix-like operating systems the /etc/passwd file is a text-based database of information about users that may login to the system or other operating system user identities that own running processes....
In Unix-like operating systems the /etc/passwd file is a text-based database of information about users that may login to the system or other operating system user identities that own running processes....
is a common file used to demonstrate directory traversal, as it is often used by crackers
Hacker (computer security)
In computer security and everyday language, a hacker is someone who breaks into computers and computer networks. Hackers may be motivated by a multitude of reasons, including profit, protest, or because of the challenge...
Password cracking is the process of recovering passwords from data that has been stored in or transmitted by a computer system. A common approach is to repeatedly try guesses for the password...
the passwords.
However, in more recent Unix systems, the passwd file does not contain the hashed passwords. They are, instead, located in the shadow file which cannot be read by unprivileged users on the machine. It is however, still useful for account enumeration on the machine, as it still displays the user accounts on the system.
Variations of directory traversal
Listed below are some known directory traversal attack strings:
A Unix-like operating system is one that behaves in a manner similar to a Unix system, while not necessarily conforming to or being certified to any version of the Single UNIX Specification....
Microsoft Windows is a series of operating systems produced by Microsoft.Microsoft introduced an operating environment named Windows on November 20, 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces . Microsoft Windows came to dominate the world's personal...
DOS, short for "Disk Operating System", is an acronym for several closely related operating systems that dominated the IBM PC compatible market between 1981 and 1995, or until about 2000 if one includes the partially DOS-based Microsoft Windows versions 95, 98, and Millennium Edition.Related...
directory traversal uses the ..\ characters.
Today, many Windows programs or APIs also accept Unix-like
Unix-like
A Unix-like operating system is one that behaves in a manner similar to a Unix system, while not necessarily conforming to or being certified to any version of the Single UNIX Specification....
directory traversal characters.
Each partition has a separate root directory (labeled C:\ for a particular partition C) and there is no common root directory above that. This means that for most directory vulnerabilities on Windows, the attack is limited to a single partition.
This sort of attack was frequently used to exploit a vulnerability fixed in Microsoft Security Bulletin MS08-067.
In computer science, canonicalization , is a process for converting data that has more than one possible representation into a "standard", "normal", or canonical form...
In World Wide Web, a query string is the part of a Uniform Resource Locator that contains data to be passed to web applications such as CGI programs....
for dangerous characters such as:
..
..\
../
to prevent directory traversal. However, the query string is usually URI decoded before use. Therefore these applications are vulnerable to percent encoded
Percent-encoding
Percent-encoding, also known as URL encoding, is a mechanism for encoding information in a Uniform Resource Identifier under certain circumstances. Although it is known as URL encoding it is, in fact, used more generally within the main Uniform Resource Identifier set, which includes both Uniform...
In computer science, canonicalization , is a process for converting data that has more than one possible representation into a "standard", "normal", or canonical form...
UTF-8 is a multibyte character encoding for Unicode. Like UTF-16 and UTF-32, UTF-8 can represent every character in the Unicode character set. Unlike them, it is backward-compatible with ASCII and avoids the complications of endianness and byte order marks...
Bruce Schneier is an American cryptographer, computer security specialist, and writer. He is the author of several books on general security topics, computer security and cryptography, and is the founder and chief technology officer of BT Managed Security Solutions, formerly Counterpane Internet...
Unicode is a computing industry standard for the consistent encoding, representation and handling of text expressed in most of the world's writing systems...
support to their Web server, a new way of encoding ../ was introduced into their code, causing their attempts at directory traversal prevention to be circumvented.
Multiple percent encodings, such as
%c1%1c
%c0%af
translated into / or \ characters.
Percent encodings were decoded into the corresponding 8-bit characters by Microsoft webserver. This has historically been correct behavior as Windows and DOS
DOS
DOS, short for "Disk Operating System", is an acronym for several closely related operating systems that dominated the IBM PC compatible market between 1981 and 1995, or until about 2000 if one includes the partially DOS-based Microsoft Windows versions 95, 98, and Millennium Edition.Related...
Canonical is an adjective derived from canon. Canon comes from the greek word κανών kanon, "rule" or "measuring stick" , and is used in various meanings....
The American Standard Code for Information Interchange is a character-encoding scheme based on the ordering of the English alphabet. ASCII codes represent text in computers, communications equipment, and other devices that use text...
UTF-8 is a multibyte character encoding for Unicode. Like UTF-16 and UTF-32, UTF-8 can represent every character in the Unicode character set. Unlike them, it is backward-compatible with ASCII and avoids the complications of endianness and byte order marks...
was not canonical, and several strings were now string encodings translatable into the same string. Microsoft performed the anti-traversal checks without UTF-8 canonicalization
Canonicalization
In computer science, canonicalization , is a process for converting data that has more than one possible representation into a "standard", "normal", or canonical form...
In mathematics and computer science, hexadecimal is a positional numeral system with a radix, or base, of 16. It uses sixteen distinct symbols, most often the symbols 0–9 to represent values zero to nine, and A, B, C, D, E, F to represent values ten to fifteen...
In mathematics and computer science, hexadecimal is a positional numeral system with a radix, or base, of 16. It uses sixteen distinct symbols, most often the symbols 0–9 to represent values zero to nine, and A, B, C, D, E, F to represent values ten to fifteen...
In formal languages, which are used in mathematical logic and theoretical computer science, a string is a finite sequence of symbols that are chosen from a set or alphabet....
comparisons. Malformed percent encodings, such as %c0%9v was also utilized.
Possible methods to prevent directory traversal
A possible algorithm for preventing directory traversal would be to:
Process URI requests that do not result in a file request, e.g., executing a hook into user code, before continuing below.
When a URI request for a file/directory is to be made, build a full path to the file/directory if it exists, and normalize all characters (e.g., %20 converted to spaces).
It is assumed that a 'Document Root' fully qualified, normalized, path is known, and this string has a length N. Assume that no files outside this directory can be served.
Ensure that the first N characters of the fully qualified path to the requested file is exactly the same as the 'Document Root'.
If so, allow the file to be returned.
If not, return an error, since the request is clearly out of bounds from what the web-server should be allowed to serve.
A chroot on Unix operating systems is an operation that changes the apparent root directory for the current running process and its children. A program that is run in such a modified environment cannot name files outside the designated directory tree. The term "chroot" may refer to the chroot...
may be subject to directory traversal using if the chroot jail is incorrectly created. Possible directory traversal attack vectors are open file descriptor
File descriptor
In computer programming, a file descriptor is an abstract indicator for accessing a file. The term is generally used in POSIX operating systems...
In computing, the working directory of a process is a directory of a hierarchical file system, if any, dynamically associated with each process. When the process refers to a file using a simple file name or relative path , the reference is interpreted relative to the current working directory of...
