Untrusted input used in the replacement string could therefore inject PHP code to be executed. Web applications, by definition, will include various files necessary to service any given request. By manipulating the request path or its parameters, it may be possible to provoke the server into including unintended local files by taking advantage of flawed logic in its routing, dependency management, autoloading or other processes.
Such manipulations outside of what the web application was designed to handle can have unforeseen effects. For example, an application might unwittingly expose routes intended only for command line usage. The application may also expose other classes whose constructors perform tasks not a recommended way to design classes but it happens. Many applications maintain a range of logs which are often displayable to authorised users from a HTML interface.
As a result, they are a prime target for attackers wishing to disguise other attacks, mislead log reviewers, or even mount a subsequent attack on the users of the monitoring application used to read and analyse the logs. The vulnerability of logs depends on the controls put in place over the writing of logs and ensuring that log data is treated as an untrusted source of data when it comes to performing any monitoring or analysis of the log entries.
For example, a programmer might log failed login attempts using a string of the following format:. If this string, from untrusted input were inserted into the log the attacker would have successfully disguised their failed login attempt as an innocent failure by the Admin user to login.
Adding a successful retry attempt makes the data even less suspicious. Of course, the point here is that an attacker can append all manner of log entries. They can also inject XSS vectors, and even inject characters to mess with the display of the log entries in a console. Injection may also target log format interpreters. If an analyser tool uses regular expressions to parse a log entry to break it up into data fields, an injected string could be carefully constructed to ensure the regex matches an injected surplus field instead of the correct field.
For example, the following entry might pose a few problems:. More nefarious attacks using Log Injection may attempt to build on a Directory Traversal attack to display a log in a browser. In the right circumstances, injecting PHP code into a log message and calling up the log file in the browser can lead to a successful means of Code Injection which can be carefully formatted and executed at will by the attacker.
Enough said there. The simplest defence against Log Injections is to sanitise any outbound log messages using an allowed characters whitelist. We could, for example, limit all logs to alphanumeric characters and spaces. Messages detected outside of this character list may be construed as being corrupt leading to a log message concerning a potential LFI to notify you of the potential attempt. A secondary defence may be to encode the untrusted input portion into something like base64 which maintains a limited allowed characters profile while still allowing a wide range of information to be stored in text.
Path Traversal also Directory Traversal Attacks are attempts to influence backend operations that read from or write to files in the web application by injecting parameters capable of manipulating the file paths employed by the backend operation. This seems completely counterintuitive but it means that the following two function calls using absolute file paths i. In a Path Traversal vulnerability, the common factor is that the path to a file is manipulated to instead point at a different file.
This is commonly achieved by injecting a series of.. The Dot-Dot-Slash sequence allows an attacker to tell the system to navigate or backtrack up to the parent directory.
Of course, path traversals are not just for backtracking. An attacker can also inject new path elements to access child directories which may be inaccessible from a browser, e. XML is so ubiquitous that it can also be found in use on the web application server, in browsers as the format of choice for XMLHttpRequest requests and responses, and in browser extensions.
Where the browser is an active participant in an XML exchange, consideration should be given to XML as a request format where authenticated users, via a Cross-Site Scripting attack, may be submitting XML which is actually written by an attacker. XML allows you to expand on the standard entity set by defining custom entities within the XML document itself.
Custom entities can be defined by including them directly in an optional DOCTYPE and the expanded value they represent may reference an external resource to be included. It is this capacity of ordinary XML to carry custom references which can be expanded with the contents of an external resources that gives rise to an XXE vulnerability.
Under normal circumstances, untrusted inputs should never be capable of interacting with our system in unanticipated ways and XXE is almost certainly unexpected for most programmers making it an area of particular concern. Custom entities obviously have a benefit in representing repetitive text and XML with shorter named entities. However, in keeping with our theme of not trusting outside inputs, we need to be very careful as to what all the XML our application is consuming is really up to.
For example, this one is definitely not of the harmless variety:. The file retrieved will be interpreted as XML unless it avoids the special characters that trigger that interpretation thus making the scope of local file content disclosure limited.
In this case the attacker could inject his malicious input somehow and if he can craft a URL or a Form that will get back the input in XML, then he could send it to his targets. When they open it the malicious code gets executed. XML documents can be downloaded from the web app.
Leave a Reply Cancel reply Your email address will not be published. When we examine the XXE vulnerability, we get this advisory.
In fact, the professional version of Burp will go far enough to use Burp Collaborator, an external service that Burp can use to help with discovering vulnerabilities or exploiting targets. Basically, the Collaborator requests that Burp sends to the vulnerable application are designed to call out to Burp Collaborator via both DNS resolution and web requests, which, if successful, will inform your Burp instance that the target machine executed the malicious payload.
Most vulnerabilities can be discovered by examining or timing the response, but sometimes, using an external service like Burp Collaborator can take things to the next level.
We will need the unmolested version of the request. XXEinjector operates a bit differently, in comparison to Burp excluding Collaborator. Notice that in the manual injection method Proof of Concept section along with the Burp approach, we rely on the fact that the server is ultimately echoing out the injected entity somehow.
This is a luxury that we may not always have in a vulnerable application. Burp Collaborator is similar to XXEinjector in that they both utilize out-of-band techniques. This technique requires that the application can connect to the attacker's site, which means egress filtering comes into play when attacking externally.
XXEinjector can enumerate egress ports for us, which is a nice feature to help in getting the tool to work. It is important to note that XXEinjector can be a bit finicky since the attack is more sophisticated than sending one request. In that same request from XXEinjector, we call upon two other entities that can only be executed if file.
This is a huge part of why XXEinjector is finicky. This helps get the job done more robustly and quietly. The request looks something like this. Line of interest is bolded. There are three entities at play, "remote", "int", and "trick". The moment XXEinjector fires off this request, it starts a server on port 80 can be changed , waiting to serve file. We then define the two missing entities from the previous request, "int" and "trick". Note that "trick" is an entity defined within the other entity, "int".
Lastly, notice that this payload is not encoded. Some characters are encoded, due to the embedding of one entity within another, but the request itself is not base64 encoded.
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