Log Injection

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Description

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.

Writing invalidated user input to log files can allow an attacker to forge log entries or inject malicious content into the logs. This is called log injection.

Log injection vulnerabilities occur when:

  1. Data enters an application from an untrusted source.
  2. The data is written to an application or system log file.

Successful log injection attacks can cause:

  1. Injection of new/bogus log events (log forging via log injection)
  2. Injection of XSS attacks, hoping that the malicious log event isviewed in a vulnerable web application
  3. Injection of commands that parsers (like PHP parsers) could execute

Log Forging

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.

Log Forging Example

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.

...
String val = request.getParameter("val");
try {
    int value = Integer.parseInt(val);
}
catch (NumberFormatException) {
    log.info("Failed to parse val = " + val);
}
...

If a user submits the string “twenty-one” for val, the following entry is logged:

INFO: Failed to parse val=twenty-one

However, if an attacker submits the string “twenty-one%0a%0aINFO:+User+logged+out%3dbadguy”, the following entry is logged:

INFO: Failed to parse val=twenty-one

INFO: User logged out=badguy

Clearly, attackers can use this same mechanism to insert arbitrary log entries.

Code Execution via Log Injection

PHP code can easily be added to a log file, for example:

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

This stage it is called log file poisoning. If the log file is staged on a public directory and can be accessed via a HTTP GET request, the embedded PHP command may execute in certain circumstances. This is a form of Command Injection via Log Injection.

References

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