Numerous options are available to configure the behavior of Burp Scanner during audit scans. These can be configured on-the-fly when launching a scan, or can be maintained in Burp's configuration library.
These settings let you tune the behavior of the audit logic to reflect the objectives of the audit and the nature of the target application.
The following options are available:
- Audit speed - This option determines how thorough certain audit checks will be when checking for vulnerabilities. The "Fast" setting makes fewer requests, and checks for fewer derivations of some vulnerabilities. The "Thorough" setting makes many more requests and checks for more derivative types of vulnerabilities. The "Normal" setting is mid-way between the two, and represents a suitable trade-off between speed and thoroughness for many applications.
- Audit accuracy - This option determines the amount of evidence that the Scanner will require before reporting certain types of vulnerabilities. Some issues can only be detected using "blind" techniques, in which Burp infers the probable existence of a vulnerability based on some observed behavior, such as a time delay or a differential response. Because these observed behaviors can occur anyway, in the absence of the associated vulnerability, the techniques are inherently more prone to false positives than other techniques, such as the observation of error messages. To attempt to reduce false positives, Burp repeats certain tests a number of times when a putative issue is inferred, to try to establish a reliable correlation between submitted inputs and observed behaviors. The accuracy option is used to control how many times Burp will retry these tests. The "Minimize false negatives" setting performs fewer retries, and so is more likely to report false positive issues, but is also less likely to miss genuine issues due to inconsistent application behavior. The "Minimize false positives" setting performs many more retries, and so is less likely to report false positive issues, but may as a result wrongly miss some genuine issues, because some of the retry requests might just happen to fail to return the result being tested for. The "Normal" setting is mid-way between the two, and represents a suitable trade-off between false positive and false negative issues for many applications.
- Skip checks unlikely to be effective due to insertion point's base value - This option makes scans more efficient by omitting checks that appear irrelevant given the base value of the parameter at each insertion point. For example, if a parameter's value contains characters that do not normally appear in filenames, Burp will skip file path traversal checks for this parameter. Using this option can speed up your scans, with a relatively low risk of missing actual vulnerabilities that exist.
- Consolidate frequently occurring passive issues - This option controls whether Burp will consolidate frequently occurring passive issues. Using this option considerably reduces noise when the same issue (such as Clickjacking) appears in many locations or even throughout an entire application.
- Automatically maintain session - This option controls whether Burp will automatically maintain session during the audit phase of the scan. This is only applicable to crawl-driven audits where the navigational pathways identified during the crawl phase can be used to maintain session during the audit phase. In modern applications, it is normally essential to maintain session to achieve good audit coverage. However, there is an overhead to maintaining session in terms of numbers of requests made, and you can disable this option if you know it is not necessary.
- Follow redirections where necessary - Some vulnerabilities can only be detected by following redirections (for example, cross-site scripting in an error message which is only returned after following a redirection). Because some applications issue redirections to third-party URLs that include parameter values that you have submitted, Burp protects you against inadvertently attacking third-party applications, by not following just any redirection which is received. If the request being scanned is within the defined target scope (i.e. you are using target scope to control what gets scanned), then Burp will only follow redirections that are within that same scope. If the request being scanned is not in scope (i.e. you have manually initiated a scan of an out-of-scope request), Burp will only follow redirections which (a) are to the same host/port as the request being scanned; and (b) are not explicitly covered by a suite-wide scope exclusion rule (e.g. "logout.aspx").
These settings control which issues Burp will check for.
You can select issue types individually, or according to the nature of the audit activity that is involved in detecting them. If you select individual issues, you can also select the detection methods that are used for some types of issues. Use the context menu on relevant issue types, and choose "Edit detection methods".
Each check that is performed increases the number of requests made, and the overall time of the audit. You can turn individual issues on or off based on your knowledge of an application's technologies. For example, if you know that an application does not use any LDAP, you can turn off LDAP injection. Or if you know which back-end database the application uses, you can turn off SQL injection detection methods that are specific to other database types.
Note: if any issues are enabled that are labeled "active", then Burp Scanner will send requests to the application designed to detect those issues. Depending on the issues selected, these requests might be reasonably viewed as malicious or might damage the application or its data.
Handling application errors during audit
These settings control how Burp Scanner handles application errors (connection failures and transmission timeouts) that arise during the audit phase of the scan.
You can configure the following options:
- The number of consecutive failed audit checks before skipping the remaining checks in the current insertion point.
- The number of consecutive failed insertion points before skipping the remaining insertion points and flagging the audit item as failed.
- The number of consecutive failed audit items, or the overall percentage of failed audit items, before pausing the task.
- The number of follow-up passes that are performed on completion of each audit phase, to retry failed operations.
You can leave any setting blank to disable it.
Insertion point types
These settings control how the Scanner places insertion points into each HTTP request that is audited.
Burp Scanner gives you fine-grained control over the placement of insertion points, and careful configuration of these options will let you tailor the audit to the nature of the application you are targeting. The configuration of insertion points also represents a trade-off between the speed and comprehensiveness of your scans.
The following categories of insertion point can be selected:
- URL parameter values - Standard parameter values within the URL query string.
- Body parameter values - Parameter values in the message body, including standard form-generated parameters, attributes of multipart-encoded parameters such as uploaded file names, XML parameter values and attributes, and JSON values.
- Cookie values - The values of HTTP cookies.
- Parameter name - The name of an arbitrarily added parameter. A URL parameter is always added, and a body parameter is also added to POST requests. Testing an added parameter name can often detect unusual bugs that are missed if only parameter values are tested.
- HTTP headers - The values of the Referer and User-Agent headers. Testing these insertion points can often detect issues like SQL injection or persistent XSS within logging functionality.
- Entire body - The whole of the request body. This applies to requests with XML or JSON content in the request body.
- URL path filename - The value of the filename part of the URL path (after the final path folder and before the query string).
- URL path folders - The values of all folder tokens within the URL path (before the filename part).
Note: As well as letting Burp automatically assign insertion points, it is possible to fully customize these, so you can specify arbitrary locations within a request where attacks should be placed. To use this function, send the request to Intruder, use the payload positions tab to define the start and end of each insertion point in the usual way, and select the Intruder menu option "Audit defined insertion points". You can also specify custom insertion point locations programmatically using Burp Extender.
Modifying parameter locations options
These settings let you configure the Scanner to move parameters to other locations within the request, in addition to testing them in their original position. For example, you can move each URL parameter into the message body and retest it there. Or you can move each body parameter into a cookie and retest it there.
Moving parameters in this way can often bypass defensive filters. Many applications and application firewalls perform per-parameter input validation assuming that each parameter is in its expected location within the request. Moving the parameter to a different location can evade this validation. When the application code later retrieves the parameter to implement its main logic, it may do so using an API that is agnostic as to the parameter's location. If so, then moving the parameter may enable you to reach vulnerable code paths using input that would normally be filtered before being processed.
The following options are available for changing parameter locations:
- URL to body
- URL to cookie
- Body to URL
- Body to cookie
- Cookie to URL
- Cookie to body
Note that changing parameter locations results in many more scan requests, because each request parameter is effectively scanned multiple times.
Ignored insertion points
These settings let you specify request parameters for which Burp Scanner should skip certain audit checks. There are separate lists for skipping server-side injection checks (such as SQL injection) and for skipping all checks.
Server-side injection checks are relatively time-consuming, because Burp sends multiple requests probing for various blind vulnerabilities on the server. If you believe that certain parameters appearing within requests are not vulnerable (for example, built-in parameters used only by the platform or web server), you can tell Burp not to test these. (Testing for client-side bugs like cross-site scripting involve much less overhead because testing each parameter imposes minimal overhead on the duration of the scan if the parameter is not vulnerable.)
Skipping all checks may be useful if a parameter is handled by an application component that you do not wish to test, or if modifying a parameter is known to cause application instability.
Each item in the list specifies the parameter type, the item to be matched (name or value), the match type (literal string or regex), and the expression to match.
You can identify parameters within URL path folders by their position (slash-delimited) within the URL path. To do this, select "URL path folder" from the parameter drop-down, "name" from the item drop-down, and specify the index number (1-based) of the position within the URL path that you wish to exclude from testing. You can also specify URL path folder parameters by value.
Frequently occurring insertion points
These settings let you configure whether Burp Scanner will avoid duplication in frequently occurring insertion points. If configured, Burp will identify insertion points that have proven to be uninteresting (occurring frequently without any issues generated) and will drop to performing a more lightweight audit of those insertion points.
You can select which insertion point types this optimization is applied to.
Misc insertion point options
You can select whether to use nested insertion points. Nested insertion points are used when an insertion point's base value contains data in a recognized format. For example, a URL parameter might contain Base64-encoded data, and the decoded value might in turn contain JSON or XML data. With the option to use nested insertion points enabled, Burp will create suitable insertion points for each separate item of input at each level of nesting. Using this option imposes no overhead when scanning requests containing only conventional request parameters, but enables Burp to reach more of the attack surface of complex applications where data is encapsulated within different formats.
You can configure whether to set a limit on the number of insertion points that will be generated for each base request, thereby preventing your scans from becoming stalled if they encounter requests containing huge numbers of parameters. In cases where the number of insertion points is curtailed by this limit, the item's entry in the audit items view will indicate the number of insertion points that were skipped, enabling you to manually review the base request and decide if it is worth performing a full scan of all its possible insertion points.
Load previously undiscovered resources from out-of-scope hosts - Controls whether resources that are referenced by a page but were not discovered by Burp Scanner during crawling will be loaded from locations that are out of scope for the scan. You may encounter this scenario if you run an audit-only scan on:
- Items that you discovered while manually exploring the site
- Items that were added to the site map by a previous scan but have since been changed on the live website
- Maximum static/dynamic analysis time per item - Controls the maximum time that Burp will spend on static or dynamic analysis for each individual item that is scanned. This setting can be useful if Burp encounters items containing very large or complex scripts, which may cause the static analysis engine to consume excessive system resources. If the analysis of a particular item is truncated because the maximum time was reached, then Burp shows an alert identifying the item affected. You can specify zero or a blank value to indicate that no limit should be applied.