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Difference between revisions of "Log Injection"

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(Re-designed by explaining the readers what is Log injection and when it is possible in detail also provided adequate prevention measures.)
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==Description==
 
==Description==
  
−
Writing unvalidated user input to log files can allow an attacker to forge log entries or inject malicious content into the logs.
+
First to understand Log injection, lets first know what a log file is.
  
−
Log forging vulnerabilities occur when:
+
'''Log file''' - It is basically a file, probably residing on some server, database (depending on what kind of logs it notes down) or any other essential networking components, that keeps an accurate record of all the transactions and events taking place interminably from the start until the service has decided to terminate.
  
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# Data enters an application from an untrusted source.
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All major and vital decisions in relation to the web application are taken place by analyzing these logs.
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# The data is written to an application or system log file.  
 
  
−
Applications typically use log files to store a history of events or transactions for later review, statistics gathering, or debugging. Depending on the nature of the application, the task of reviewing log files may be performed manually on an as-needed basis or automated with a tool that automatically culls logs for important events or trending information.
+
So this points out 2 important things:
 +
* How important logs are to every single organization existing over the web!
 +
* Log files exist in the backend of practically anywhere you navigate on the web.
 +
'''So how does log poisoning work?'''
  
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Interpretation of the log files may be hindered or misdirected if an attacker can supply data to the application that is subsequently logged verbatim. In the most benign case, an attacker may be able to insert false entries into the log file by providing the application with input that includes appropriate characters. If the log file is processed automatically, the attacker can render the file unusable by corrupting the format of the file or injecting unexpected characters. A more subtle attack might involve skewing the log file statistics. Forged or otherwise, corrupted log files can be used to cover an attacker's tracks or even to implicate another party in the commission of a malicious act [1]. In the worst case, an attacker may inject code or other commands into the log file and take advantage of a vulnerability in the log processing utility [2].
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First a PHP code is injected in the log file, and it can be achieved by doing something like:
 +
<code><nowiki>https://www.somedomain.tld/index.php?file=</nowiki><?php echo phpinfo(); ?></code>
  
−
==Examples==
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As it is an HTTP GET request, it will end up in the log file. This stage it is called '''log file poisoning'''. Although the above request will most likely display an error, by re-reading the web server's log file, the code will be executed.
  
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The following web application code attempts to read an integer value from a request object. If the value fails to parse as an integer, then the input is logged with an error message indicating what happened.
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'''Code Execution'''
  
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<pre>
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Now the web server log file is poisoned with the previously injected PHP code, but '''loading the web server's log file''' the following will happen:
−
...
 
−
String val = request.getParameter("val");
 
−
try {
 
−
int value = Integer.parseInt(val);
 
−
}
 
−
catch (NumberFormatException) {
 
−
log.info("Failed to parse val = " + val);
 
−
}
 
−
...
 
−
</pre>
 
  
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If a user submits the string "twenty-one" for val, the following entry is logged:
+
So the most important factor we can note here is, log files are not executable! But if the server side programming language is the same as the code injected(which is the obvious case) the server while loading the log file:
 +
# (PHP) reads the log file.
 +
# (PHP) detects the (PHP) code in the log file.
 +
# (PHP) will parse the (PHP) code and parse its output to the user's browser, displaying the (PHP) info.
 +
Also the most important thing to note down is the way it works:
 +
* ''(Log_injection(Code_Execution(parsing)))''
 +
# Read access to log files is enough to execute code after they're poisoned (this requires LFI)
  
−
<pre>
+
'''Now let's understand when can all this work?'''
−
INFO: Failed to parse val=twenty-one
 
−
</pre>
 
  
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However, if an attacker submits the string "twenty-one%0a%0aINFO:+User+logged+out%3dbadguy", the following entry is logged:
+
Let's consider we have found a LFI(Local File Inclusion) vulnerability
 +
<nowiki>http://vulnerable_host/preview.php?file=example.html</nowiki>
 +
If an attacker is lucky enough, and instead of selecting the appropriate page from the array by its name, the script directly includes the input parameter, it is possible to include arbitrary files on the server (Also while at it lets assume for a moment that the PHP open_basedir* is not configured).
  
−
<pre>
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Now this would load the preview.php file which is located in the same directory as where the web pages served.
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INFO: Failed to parse val=twenty-one
 
  
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INFO: User logged out=badguy
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'''The POC(Proof of Concept) would be: ''To load a log_file file(in our case).'''''
−
</pre>
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<nowiki>http://vulnerable_host/preview.php?file=../../../../etc/access_log</nowiki>  
 +
In this scenario, let's assume that the log file is loaded and displayed in the user's browser. In this case it's already bad enough because you can basically load any file that the www-user has read access to. And something like this would be displayed:
 +
root:x:0:0:root:/root:/bin/bash bin:x:1:1:bin:/bin:/sbin/nologin daemon:x:2:2:daemon:/sbin:/sbin/nologin alex:x:500:500:alex:/home/alex:/bin/bash margo:x:501:501::/home/margo:/bin/bash ...
 +
So this answers when a PHP code can be injected to the log file.
  
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Clearly, attackers can use this same mechanism to insert arbitrary log entries.
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So the above equation can be revised as:
 +
 
 +
''LFI(Log_injection(Code_Execution(parsing)))''
 +
 
 +
== How can Log Injection be prevented? ==
 +
* We need to include input validation both at server and client’s end. The suspected characters can be sanitized and replaced.
 +
* The application used to render logs has to be thoroughly scrutinized for vulnerabilities.
 +
 
 +
* Instead of directly passing messages via parameters, log codes could be passed. However it is still exploitable.
 +
* Don’t use API Calls to log actions, as there API’s will be visible in browser network calls.
 +
* In case of a need, pass user ids or publicly non identifiable values as parameters in logging endpoints.
 +
* Use proper error codes and identifiable error messages.
 +
 
 +
== References ==
 +
* https://security.stackexchange.com/questions/197871/are-log-files-executable-files<nowiki/>(Self asked question)
 +
* https://medium.com/@shatabda/security-log-injection-what-how-a510cfc0f73b
 +
* https://www.geeksforgeeks.org/log-injection/
 +
* https://affinity-it-security.com/what-is-log-injection/
  
 
==References==
 
==References==

Revision as of 14:30, 18 November 2018

This is an Attack. To view all attacks, please see the Attack Category page.


Last revision (mm/dd/yy): 11/18/2018

Description

First to understand Log injection, lets first know what a log file is.

Log file - It is basically a file, probably residing on some server, database (depending on what kind of logs it notes down) or any other essential networking components, that keeps an accurate record of all the transactions and events taking place interminably from the start until the service has decided to terminate.

All major and vital decisions in relation to the web application are taken place by analyzing these logs.

So this points out 2 important things:

  • How important logs are to every single organization existing over the web!
  • Log files exist in the backend of practically anywhere you navigate on the web.

So how does log poisoning work?

First a PHP code is injected in the log file, and it can be achieved by doing something like: 

https://www.somedomain.tld/index.php?file=<?php echo phpinfo(); ?>

As it is an HTTP GET request, it will end up in the log file. This stage it is called log file poisoning. Although the above request will most likely display an error, by re-reading the web server's log file, the code will be executed.

Code Execution

Now the web server log file is poisoned with the previously injected PHP code, but loading the web server's log file the following will happen:

So the most important factor we can note here is, log files are not executable! But if the server side programming language is the same as the code injected(which is the obvious case) the server while loading the log file:

  1. (PHP) reads the log file.
  2. (PHP) detects the (PHP) code in the log file.
  3. (PHP) will parse the (PHP) code and parse its output to the user's browser, displaying the (PHP) info.

Also the most important thing to note down is the way it works:

  • (Log_injection(Code_Execution(parsing)))
  1. Read access to log files is enough to execute code after they're poisoned (this requires LFI)

Now let's understand when can all this work?

Let's consider we have found a LFI(Local File Inclusion) vulnerability 

http://vulnerable_host/preview.php?file=example.html

If an attacker is lucky enough, and instead of selecting the appropriate page from the array by its name, the script directly includes the input parameter, it is possible to include arbitrary files on the server (Also while at it lets assume for a moment that the PHP open_basedir* is not configured).

Now this would load the preview.php file which is located in the same directory as where the web pages served.

The POC(Proof of Concept) would be: To load a log_file file(in our case). 

http://vulnerable_host/preview.php?file=../../../../etc/access_log

In this scenario, let's assume that the log file is loaded and displayed in the user's browser. In this case it's already bad enough because you can basically load any file that the www-user has read access to. And something like this would be displayed: 

root:x:0:0:root:/root:/bin/bash bin:x:1:1:bin:/bin:/sbin/nologin daemon:x:2:2:daemon:/sbin:/sbin/nologin alex:x:500:500:alex:/home/alex:/bin/bash margo:x:501:501::/home/margo:/bin/bash ...

So this answers when a PHP code can be injected to the log file.

So the above equation can be revised as:

LFI(Log_injection(Code_Execution(parsing)))

How can Log Injection be prevented?

  • We need to include input validation both at server and client’s end. The suspected characters can be sanitized and replaced.
  • The application used to render logs has to be thoroughly scrutinized for vulnerabilities.
  • Instead of directly passing messages via parameters, log codes could be passed. However it is still exploitable.
  • Don’t use API Calls to log actions, as there API’s will be visible in browser network calls.
  • In case of a need, pass user ids or publicly non identifiable values as parameters in logging endpoints.
  • Use proper error codes and identifiable error messages.

References

References

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