Security analysis with logging

09/06/2021

Knowing the offensive techniques is the best way to protect yourself from computer attacks. In the field of #cybersecurity, an excellent logging allows collecting a lot of information on the techniques used during the attempts. The analysis of intrusion attempts, injection and information gathering is essential to defend a computer system. In this publication, we discuss log analysis concepts with concrete examples as a way to raise awareness of cybersecurity.

Where to find logs files?

Depending on the server configuration, the logs can be located in several places. Indeed, the process configuration usually allows moving the log file to a different directory than the default one. In the following examples, the default configuration of an Ubuntu server is used.

Composition of a log entry

Conventionally, the entry in a log includes a date, time, user, process, status and description about the event.

Example of process log analysis

The list of default process logs and log directories can be displayed using the following command :

Let’s take an example to analyze the last 10 syslog entries.

We can quickly observe that there is a problem with the banning. In fact, the firewall does not ban the MAC address and no other process bans these intrusion attempts. Note that Fail2Ban is used, but banning by MAC address is not a Fail2Ban feature. We must therefore find the source of the problem. Since these logs look like a brute force attempt, we can analyze the authentication file :

The authentication log shows two processes that generate log entries with intrusion attempts, sshd and wordpress. Finally, from this information we can increase the security by configuring the two processes.

Example of query analysis logs

The two most used types of requests are GET and POST. The GET type takes parameters (called params) in the url. For example, the request GET www.google.ca ?cookie=123 includes the parameter cookie whose value is 123. Without limitations, the response of a GET request can be an html page, a string or a JSON object. In general, the GET request is used to retrieve information from the state of the params.

By convention, the POST request is generally used to add, modify and delete information on the server. To do this, the POST request generally uses the body of the request to send information to the server.

One of the problems that can occur with these two types of requests is injection. If the parameters or the body are not parsed correctly, then injection is likely. A simple solution to validate the params or the body is to add a data schema validator. Let’s take an example with an AJAX request code on a Wordpress server :

add_action(’wp_ajax_endpoint’,’save_secret_info’);

function save_secret_info(){
||!isset($_POST[’secret’]) ||!wp_verify_nonce($_POST[’secret’],’algosol−nonce’)){
$output="Permission denied: invalid nonce"; log_ajax($output);
wp_die("permission denied");
}else{
if(is_string($_POST[’secret’])){ //A more secure way to save secret log_ajax("saved"); wp_die("saved"); }else{ log_ajax("secret n’est pas une chaine de caracteres"); wp_die("invalidformat"); } } } functionlog_ajax($message){
$date=date(’Y−m−dH:i:s’);$user=get_current_user();
$process="AJAX"; error_log($date."".$user."".$process."".\$message);
}


According to the previous code, each validation condition of the query is recorded in the logs. This example demonstrates the basis of a schema-based validation, but this example could be much more precise.

Example of stack trace logs

When a malicious request is sent to the server, many conditions are to be expected. It may happen that some conditions are not validated by the processes and the request analysis stage. In this case, the execution of the request may lead to undesirable program behaviour. If the source code of the program is not known to the attacker, then the attempts will have the form of probability. For example, the success of a Shellcode sent in a request is more difficult due to execution constraints and therefore it is more likely that several attempts will be necessary.

When a badly parsed request is executed by a computer system, the execution of the request frequently generates execution errors. The execution errors are usually displayed in the stack trace and the programming languages also allow generating the stack trace or to generate an exception.

Once again, logs can help to detect this type of event. Besides, the stack trace often includes enough information to correct the problem by a programmer.

Example of response logs

As mentioned earlier, the two most commonly used types of requests are GET requests and POST requests. When accessing a web page, we use the GET request which includes state parameters. In addition, it is possible to determine the programs used by the web service. Moreover, free online tools exist. More concretely, the response sent by the server includes a lot of information.

For GET requests, it is enough to inspect the page. For example, it is possible to know if a website is designed with Wordpress by searching for the prefix 'wp-'. The following figure shows 76 results for the prefix 'wp-'. So, the probability that this site is written with Wordpress is very high.

Note that it is possible to analyze other types of requests. For example, we can look at the response headers to see if Cloudflare is being used as a proxy. The figure refcloudflare shows an example of a response for a POST request type.

Conclusion

This publication brings together fundamental analysis concepts for the protection of computer systems. Indeed, event logging is a goldmine of information about intrusion attempts and misuse of computer systems. This publication is only an introduction to log event analysis, as there are more effective ways to automatically analyze log events. Knowing the offensive techniques is the best way to protect yourself from computer attacks.

PDF: https://www.algorithmesolutions.com/publication/security_logging_en
Sha256: edbc102b9324a83f481c91065b8419d19836b7c6738684bdb7df2f9826da23a1
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