Last updated: September 9, 2021
Read time: 6 Minutes
Numerous options are available to configure the behavior of Burp Scanner during crawl-based scans. These can be configured on-the-fly when launching a scan, or can be maintained in Burp's configuration library.
These settings control the behavior of the crawl logic to reflect the objectives of the crawl and the nature of the application.
Maximum link depth
The maximum link depth represents the maximum number of navigational transitions (clicking links and submitting forms) that the crawler will make from the start URL(s). Modern applications tend to build a mass of navigation into every response, in locations like menus and the page footer. For this reason, it is normally possible to reach the vast majority of an application's content and functionality within a small number of hops from the start URL. Fully covering multi-stage processes (like viewing an item, adding it to a shopping cart, and checking out) will require more hops.
Some applications contain extremely long navigational sequences that don't lead to interestingly different functionality. For example, a shopping application might have a huge number of product categories, sub-categories, and view filters. To a crawler, this can appear as a very deep nested tree of links, all returning different content. In this situation, there are clearly diminishing returns to crawling deeply into the navigational structure, so it is sensible to limit the maximum link depth to a small number, such as 8.
Real-world applications differ hugely in the way they organize content and navigation, the volatility of their responses, and the extent and complexity of application state. At one extreme, an application might employ a unique and stable URL for each distinct function, return deterministic content in each response, and contain no server-side state. At the other extreme, an application might employ ephemeral URLs that change each time a function is accessed, overloaded URLs that reach different functions through different navigational paths, volatile content that changes non-deterministically, and heavily stateful functions where user actions cause changes in the content and behavior that is subsequently observed.
Burp's crawler can handle both of these extremes. Where required, it can handle ephemeral and overloaded URLs, volatile content, and changes in application state. However, fully handling these cases imposes a material overhead in the quantity of work that is involved in the crawl. You can use the crawl strategy setting to tune the approach that is taken to specific applications. In practice, this setting represents a trade-off between the speed of the crawl and the completeness of coverage that is achieved. The default strategy represents a trade-off between speed and coverage that is appropriate for typical applications. You can select a strategy that is more optimized for speed, when crawling an application with more stable and unique URLs, and no stateful functionality. Or you can select a strategy that is more optimized for completeness, when crawling an application with more volatile or overloaded URLs, or more complex stateful functionality.
Crawling modern applications is sometimes an open-ended exercise, due to the amount of stateful functionality, volatile content, and unbounded navigation. Burp's crawler uses various techniques to maximize discovery of unique content early in the crawl. The settings for crawl limits let you impose a limit on the extent of the crawl, as it reaches the point of diminishing returns. It is generally sensible to configure a limit to the extent of the crawl, based on your knowledge of the application being scanned.
You can choose to limit the crawl based on:
- The time taken.
- The number of unique locations discovered. A location represents a distinct unit of content or functionality, based on the chosen crawl strategy.
- The number of HTTP requests that are made.
These settings control how the crawler will interact with any login functionality that is encountered during the crawl. You can configure whether the crawler should:
- Attempt to automatically register a new user on the target website - This removes the need to manually set up a user account before the crawl. Note that you can still provide valid application logins in the scan launcher settings if you want.
- Deliberately trigger login failures using invalid usernames - This can be useful for reaching account recovery features that are only accessed when invalid credentials are submitted. Note that the crawler will not deliberately submit an invalid password for any of the usernames that you provided as application logins. This is to avoid triggering any account locking features on these accounts.
How does the crawler identify login and registration forms?
The crawler uses the following checklist to identify login and registration forms on the target site:
- The form is a standard HTML form.
It contains an input field with the attribute
The password field has a non-empty
If all of these criteria are met, the crawler then distinguishes registration forms from login forms by applying the following rules in order. For example, if two forms have an equal number of password fields, it will then compare the number of text fields, and so on.
The registration form is whichever form has the most:
- Password fields
- Text fields
- Multi-value select fields
- Single-value select fields
- If all of the above are equal, whichever form was found first is assumed to be the registration form.
Why is the crawler not filling my login forms?
The crawler identifies login and registration forms based on the password field. However, it will only be able to enter a username or email address if the related fields:
Have the attribute
Have a non-empty
If either of these conditions is not met, the crawler will successfully identify the form but will be unable to enter the corresponding data correctly.
Handling application errors during crawl
These settings control how Burp Scanner handles application errors (connection failures and transmission timeouts) that arise during the crawl phase of the scan.
You can configure the following options:
- The number of consecutive timed out requests, or the overall percentage of timed out requests, before pausing the task.
- The number of follow-up passes that are performed on completion of the crawl, to retry requests that timed out.
You can leave any setting blank to disable it.
Miscellaneous crawl settings
These settings let you customize some details of the crawl:
- Submit forms - This setting controls whether forms are submitted.
- Customize User-Agent - This setting lets you specify a custom User-Agent header.
Request robots file - This setting controls whether to fetch the
robots.txtfile and extract links from it.
Request site map - This setting controls whether to fetch the
sitemap.xmlfile and extract links from it. You can configure the maximum number of items to extract.
- Parse API definitions - This setting controls whether the crawler attempts to parse any API definitions it encounters to identify potential endpoints to scan. For more information, please refer to the API scanning documentation.
Embedded browser options
These settings allow you to control whether Burp Scanner uses the embedded Chromium browser for navigation during both the crawl and audit phases of a scan. By default, Burp will check whether your machine appears to support browser-powered scanning and enable it if possible. However, you can choose to always use the embedded browser regardless of Burp's assessment of your machine. We recommend using a machine with at least 2 CPU cores and 8 GB RAM.
Alternatively, you can choose to disable browser-powered scanning completely. In this case, Burp Scanner will crawl the target website using the previous crawling engine.
You can also control whether the embedded browser loads resources from out-of-scope hosts and set a read timeout for site resources.
Crawling with a headed browser
By default, Burp Scanner uses a headless browser for crawling. However, you can also enable the "Show crawl in a headed browser" option. In this case, when the crawl starts, a new browser window will open in which you can watch the crawler navigating around the target website in real time. This can be useful for troubleshooting.
If you enable this option, please note that Burp Scanner will occasionally open additional browser windows during the crawl and stop using the previous window. This is perfectly normal. Any redundant windows will automatically be closed after a certain period of time.