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Input validation is less about specific vulnerabilities

Security takes a layered approach to reduce the risk to our organization. Input validation is the perfect example of one of these layers. In most cases, input validation is 1 factor in a multi-pronged approach to protecting against common vulnerabilities.

Take any course on secure development and they will, or should, mention input validation as a mitigating control for so many vulnerabilities. You might notice that it always comes with a but. Use input validation, but also use output encoding. Use input validation, but also use parameterized queries.

The reason for this is that input validation that covers every potential vulnerability is hard, if not impossible. Even input validation for a specific vulnerability can be complex and difficult. We have to walk this line of good validation, but also a useable application.

I always recommend not trying to look for specific vulnerabilities or attack strings within an input validation function. That turns into a blacklist of attacks that is impossible to keep up with. In addition, you may be looking for attack strings that don’t matter to that input.

One of the challenges is not knowing where the input will actually be used. Is the data going to the database? Is the data going to a database and then going to the browser? If the data is going to the browser, what context will it be in? If you don’t know how that data will be used, how do you know what attack strings to look for?

Instead of focusing on vulnerabilities, you should spend the time to define what good data looks like for that field. What type of data is expected? Is it an integer, a date, a string? What are the valid range of values? How long should the data be? I will discuss these in just a moment.

These limitations will not stop all attacks. There is no argument there. They are just going to help make sure the data is acceptable or the field. It is possible that a cross-site scripting attack string is valid input. That can be ok. You have limited the potential attacks. Then there is the but, remember? In this case, you would have output encoding to protect the app from cross-site scripting.

Let’s look at some of these items in more detail.

Type
One of the easiest checks to implement is checking the data type of the input field. If you are expecting a date data type and the input received is not a valid date then it is invalid and should be rejected. Numbers are also fairly simple to validate to ensure they match the type expected. Many of the most popular frameworks provide methods to determine if a string matches a specific data type. Use these features to validate this type of data. Unfortunately, when the data type is a free form string then the check isn’t as useful. This is why we have other checks that we will perform.

Range
You have determined that the input is of the correct type, say a number or a date. Now it is time to validate that it falls within the correct range of acceptable values. Imagine that you have an age field on a form. Depending on your business cases, lets say that a valid age is 0-150. It is a simple step to ensure that the value entered falls into this valid range. Another example is a date field. What if the date represents a signed date and can’t be in the future. In this case, it is important to ensure that the value entered is not later than the current day.

Length
This is really more for the free form text fields to validate that the input doesn’t exceed the expected length for the field. In many cases, this lines up with the database or back end storage. For example, in SQL Server you may specify a first name field that is 100 characters or a state field that only allows two characters for the abbreviation. Limiting down the length of the input can significantly inhibit some of the attacks that are out there. Two characters can be tough to exploit, while possible depending on the application design.

Format
Some fields may have a specific format they must match. For example, a social security number may have to match DDD-DD-DDDD. An account number may have a specific format of DDDDDSSS. There are a lot of options for specific formats. If you verify that the data matches the expected format, this can drastically cut down on potential vulnerabilities because many attacks will not be possible when matching the format.

White Lists
White lists can be a very effective control during input validation. Depending on the type of data that is being accepted it may be possible to indicate that only alphabetical characters are acceptable. In other cases you can open the white list up to other special characters but then deny everything else. It is important to remember that even white lists can allow malicious input depending on the values required to be accepted. 

Black Lists
Black lists are another control that can be used when there are specifically known bad characters that shouldn’t be allowed. Like white lists, this doesn’t mean that attack payloads can’t get through, but it should make them more difficult.

Regular Expressions
Regular expressions help with white and black lists as well as pattern matching. With the former it is just a matter of defining what the acceptable characters are. Pattern matching is a little different and really aligns more with the range item discussed earlier. Pattern matching is great for free form fields that have a specific pattern. A social security number or an email address. These fields have an exact format that can be matched, allowing you to determine if it is right or not.

Test Your Validation
Make sure that you test the validation routines to ensure they are working as expected. If the field shouldn’t allow negative numbers, make sure that it rejects negative numbers. If the field should only allow email addresses then ensure that it rejects any pattern that doesn’t match a valid email address.

Validate Server Side
Maybe this should have been first? Client side validation is a great feature for the end user. They get immediate feedback to what is wrong with their data without having that round trip to the server and back. THAT IS EASILY BYPASSED!! It is imperative that the validation is also checked on the server. It is too easy to bypass client side validation so this must be done at the server where the user cannot intercept it and bypass it.

Don’t Try To Solve All Problems
Don’t try to solve all output issues with input validation. It is common for someone to try and block cross site scripting with input validation, it is what WAFs do right? But is that the goal? What if that input isn’t even sent to a browser? Are you also going to try and block SQL Injection, LDAP Injection, Command Injection, XML Injection, Parameter Manipulation, etc. with input validation? Now we are getting back to an overly complex solution when there are other solutions for these issues. These types of items shouldn’t be ignored and that is why we have the regular expressions and white lists to help decrease the chance that these payloads make it into the system. We also have output encoding and parameterized queries that help with these additional issues. Don’t spend all of your time focusing on input validation and forget about where this data is going and protecting it there. 

Input validation is only half of the solution, the other half is implemented when the data is transmitted from the application to another system (ie. database, web browser, command shell). Just like we don’t want to rely solely on output encoding, we don’t want to do that with input validation. Assess the application and its needs to determine what solution is going to provide the best results. If you are not doing any input validation, start as soon as possible. It is your first line of defense. Start small with the Type/Range/Length checks. When those are working as expected then you can start working into the more advanced input validation techniques. Make sure the output encoding routines are also inplace.

Don’t overwhelm yourself with validating every possible attack. This is one cog in the system. There should also be other controls like monitoring and auditing that catch things as well. Implementing something for input validation is better than nothing, but don’t let it be a false sense of security. Just because you check the length of a string and limit it to 25 characters, don’t think a payload can’t be sent and exploited. It is, however, raising the bar (albeit just a little). 

Take the time to assess what you are doing for input validation and work with the business to determine what valid inputs are for the different fields. You can’t do fine validation if you don’t understand the data being received. Don’t forget that data comes in many different forms or encodings. Convert all data to a specific encoding before you perform any validation on it.

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Ep. 115: Is CSRF Really Dead?

In 2020, Chrome will default the SameSite attribute to Lax on all cookies. SameSite helps mitigate CSRF, but does that mean CSRF is Dead?

For more info go to https://www.developsec.com or follow us on twitter (@developsec).

Join the conversations.. join our slack channel. Email james@developsec.com for an invitation.

 DevelopSec provides application security training to add value to your application security program. Contact us today to see how we can help.

[Read more…] about Ep. 115: Is CSRF Really Dead?

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Ep. 114: Investing in People for Better Application Security

In this episode, James talks about investing in the development teams to increase application security priorities.

For more info go to https://www.developsec.com or follow us on twitter (@developsec).

Join the conversations.. join our slack channel. Email james@developsec.com for an invitation.

 DevelopSec provides application security training to add value to your application security program. Contact us today to see how we can help.

[Read more…] about Ep. 114: Investing in People for Better Application Security

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XSS in Script Tag

Cross-site scripting is a pretty common vulnerability, even with many of the new advances in UI frameworks. One of the first things we mention when discussing the vulnerability is to understand the context. Is it HTML, Attribute, JavaScript, etc.? This understanding helps us better understand the types of characters that can be used to expose the vulnerability.

In this post, I want to take a quick look at placing data within a <script> tag. In particular, I want to look at how embedded <script> tags are processed. Let’s use a simple web page as our example.

<html>
	<head>
	</head>
	<body>
	<script>
		var x = "<a href=test.html>test</a>";
	</script>
	</body>
</html>

The above example works as we expect. When you load the page, nothing is displayed. The link tag embedded in the variable is rated as a string, not parsed as a link tag. What happens, though, when we embed a <script> tag? 

<html>
	<head>
	</head>
	<body>
	<script>
		var x = "<script>alert(9)</script>";
	</script>
	</body>
</html>

In the above snippet, actually nothing happens on the screen. Meaning that the alert box does not actually trigger. This often misleads people into thinking the code is not vulnerable to cross-site scripting. if the link tag is not processed, why would the script tag be. In many situations, the understanding is that we need to break out of the (“) delimiter to start writing our own JavaScript commands. For example, if I submitted a payload of (test”;alert(9);t = “). This type of payload would break out of the x variable and add new JavaScript commands. Of course, this doesn’t work if the (“) character is properly encoded to not allow breaking out.

Going back to our previous example, we may have overlooked something very simple. It wasn’t that the script wasn’t executing because it wasn’t being parsed. Instead, it wasn’t executing because our JavaScript was bad. Our issue was that we were attempting to open a <script> within a <script>. What if we modify our value to the following:

<html>
	<head>
	</head>
	<body>
	<script>
		var x = "</script><script>alert(9)</script>";
	</script>
	</body>
</html>

In the above code, we are first closing out the original <script> tag and then we are starting a new one. This removes the embedded nuance and when the page is loaded, the alert box will appear.

This technique works in many places where a user can control the text returned within the <script> element. Of course, the important remediation step is to make sure that data is properly encoded when returned to the browser. By default, Content Security Policy may not be an immediate solution since this situation would indicate that inline scripts are allowed. However, if you are limiting the use of inline scripts to ones with a registered nonce would help prevent this technique. This reference shows setting the nonce (https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/script-src).

When testing our applications, it is important to focus on the lack of output encoding and less on the ability to fully exploit a situation. Our secure coding standards should identify the types of encoding that should be applied to outputs. If the encodings are not properly implemented then we are citing a violation of our standards.

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Ep. 102: Intro to Web Security Policies

In this episode James introduces us to the idea of web security policies stored in a security.txt file. We have talked about vulnerability disclosure before and this ties directly into that conversation.

Link to Draft: https://tools.ietf.org/html/draft-foudil-securitytxt-03

Link to form to create the file: https://securitytxt.org/

Link to our blog post: https://www.developsec.com/2018/06/26/overview-of-web-security-policies/

For more info go to https://www.developsec.com or follow us on twitter (@developsec).

Join the conversations.. join our slack channel.  Email james@developsec.com for an invitation.

DevelopSec provides application security consulting and training to add value to your application security program. Contact us today to see how we can help.

[Read more…] about Ep. 102: Intro to Web Security Policies

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Thinking about starting a bug bounty? Do this first.

Application security has become an important topic within our organizations. We have come to understand that the data that we deem sensitive and critical to our business is made available through these applications. With breaches happening all the time, it is critical to take reasonable steps to help protect that data by ensuring that our applications are implementing strong controls.

Over the years, testing has been the main avenue for “implementing” security into applications. We have seen a shift to the left more recently, leading to doing more throughout the entire development cycle, but we still have a ways to go. I am still a firm believer in embedding security into each of the phases as our main means of securing applications. Testing, however, is still a major component of any security program.

Typically, organizations rely on penetration testing to find the flaws in their applications. This is the de facto standard for understanding your risk. Unfortunately, penetration testing for applications has been watered down from what we think about with network testing. Many of the assessments we call penetration tests these days are just automated scans transposed into a custom report. These types of testing overlook one of the components a penetration test provides, which is the manual testing. Of course, there is much more to a penetration test, but that is not the focus of this post.

Internally, organizations may implement automated tools to help identify security flaws within their applications. These tools are good at finding certain types of flaws, and usually quite quickly. Like many current penetration tests, they lack the manual assessment side.

Not only does manual testing have the ability to find different types of flaws, such as authentication, authorization, CSRF, business logic, etc., it also has the ability to identify flaws that an automation tool overlooks. For example, a tool may not find every instance of cross-site scripting, depending on how that tool analyzes the system. Granted, manual testing is not guaranteed to find every instance either. With each type of testing, there is always a number of issues that will not be identified. The goal is to start reducing these numbers down over time.

Handling the results of all these res ports from the different assessments is critical to how well you start creating more resilient applications. In many organizations, vulnerabilities identified are handled as individual items and patched. In my opinion, the return on investment is when you can analyze these results to review your development process and see what improvements can be made to reduce the chance these types of flaws will be included in the future. Having an expert available to help review the issues and provide insight into how to use that information to improve your process is valuable.

Having a solid application development process in place is important before thinking about implementing a bug bounty program within your organization. If you are not already doing things consistently, there is a better chance the bounty program will fail.

Bug bounty programs have been becoming more prevalent over the last few years. This is especially true for newer technical startups. We have seen much slower adoption with most of the major corporations. There are many reasons for this, which are outside the scope of this post. There have been questions on whether bug bounties can replace penetration testing. The answer is no, because the goal of each of these is different. There are plenty of articles discussing the subject. A bug bounty program has also been seen by many as the evidence to show they are doing application security. Unfortunately, we can’t test ourselves secure. As I stated previously, testing is just a part of our solution for application security.

A key difference between our traditional testing and a bug bounty program is that bug bounties pay by the bug. Our traditional testing is provided at flat fees. For example, that automated tool is a set price for a month or year subscription. A penetration test is a set price per test. A bug bounty is priced per bug, which makes the cost very unpredictable. In addition, if you are not already doing many of the things previously discussed, there could be a lot of bugs to be found, leading to potentially high payouts.

As I have stated before, penetration testing has a different purpose and it can be very expensive. At Jardine Software we offer more budget friendly manual application security testing at a fixed cost. The goal is not necessarily to find every instance of every vulnerability or to exploit vulnerabilities in the way a penetration test would. The focus is on augmenting the automated testing you may already have in place and to provide that missing manual piece. The testing is performed manually by using the application in combination with Burp Suite, to look for weaknesses and provide those in a way that helps prioritize and then remediate them according to your organization’s needs.

The manual application security testing is typically performed over a week to two weeks and includes a broader scope than a typical bug bounty program. The reason for this is that we want to help identify risks that we see based on our years of experience to make you aware. This assessment can then help identify where you may have issues within your application before opening it up for a crowd sourced bounty program where each bug is priced individually.

If you are thinking about implementing a bug bounty program, reach out and lets chat first. Even if you are not considering a bug bounty program, do you have any manual application security testing implemented? We have the expertise to help provide the necessary testing or provide training for your internal teams to start applying manual testing techniques as part of your life cycle.