XML external entity (XXE) injection vulnerabilities arise when applications process user-supplied XML documents without disabling references to external resources. XML parsers typically support external references by default, even though they are rarely required by applications during normal usage.

External entities can reference files on the parser's filesystem; exploiting this feature may allow retrieval of arbitrary files, or denial of service by causing the server to read from a file such as /dev/random.

External entities can often also reference network resources via the HTTP protocol handler. The ability to send requests to other systems can allow the vulnerable server to be used as an attack proxy. By submitting suitable payloads, an attacker can cause the application server to attack other systems that it can interact with. This may include public third-party systems, internal systems within the same organization, or services available on the local loopback adapter of the application server itself. Depending on the network architecture, this may expose highly vulnerable internal services that are not otherwise accessible to external attackers.

Parsers that are used to process XML from untrusted sources should be configured to disable processing of all external resources. This is usually possible, and will prevent a number of related attacks. You should consult the documentation for your XML parsing library to determine how to achieve this.

XML external entity injection makes use of the DOCTYPE tag to define the injected entity. It may also be possible to disable the DOCTYPE tag or use input validation to block input containing it.

• XXE injection

• CWE-611: Improper Restriction of XML External Entity Reference ('XXE')

Reflected cross-site scripting vulnerabilities arise when data is copied from a request and echoed into the application's immediate response in an unsafe way. An attacker can use the vulnerability to construct a request that, if issued by another application user, will cause JavaScript code supplied by the attacker to execute within the user's browser in the context of that user's session with the application.

The attacker-supplied code can perform a wide variety of actions, such as stealing the victim's session token or login credentials, performing arbitrary actions on the victim's behalf, and logging their keystrokes.

Users can be induced to issue the attacker's crafted request in various ways. For example, the attacker can send a victim a link containing a malicious URL in an email or instant message. They can submit the link to popular web sites that allow content authoring, for example in blog comments. And they can create an innocuous looking web site that causes anyone viewing it to make arbitrary cross-domain requests to the vulnerable application (using either the GET or the POST method).

The security impact of cross-site scripting vulnerabilities is dependent upon the nature of the vulnerable application, the kinds of data and functionality that it contains, and the other applications that belong to the same domain and organization. If the application is used only to display non-sensitive public content, with no authentication or access control functionality, then a cross-site scripting flaw may be considered low risk. However, if the same application resides on a domain that can access cookies for other more security-critical applications, then the vulnerability could be used to attack those other applications, and so may be considered high risk. Similarly, if the organization that owns the application is a likely target for phishing attacks, then the vulnerability could be leveraged to lend credibility to such attacks, by injecting Trojan functionality into the vulnerable application and exploiting users' trust in the organization in order to capture credentials for other applications that it owns. In many kinds of application, such as those providing online banking functionality, cross-site scripting should always be considered high risk.

In most situations where user-controllable data is copied into application responses, cross-site scripting attacks can be prevented using two layers of defenses:

• Input should be validated as strictly as possible on arrival, given the kind of content that it is expected to contain. For example, personal names should consist of alphabetical and a small range of typographical characters, and be relatively short; a year of birth should consist of exactly four numerals; email addresses should match a well-defined regular expression. Input which fails the validation should be rejected, not sanitized.
• User input should be HTML-encoded at any point where it is copied into application responses. All HTML metacharacters, including < > " ' and =, should be replaced with the corresponding HTML entities (&lt; &gt; etc).

In cases where the application's functionality allows users to author content using a restricted subset of HTML tags and attributes (for example, blog comments which allow limited formatting and linking), it is necessary to parse the supplied HTML to validate that it does not use any dangerous syntax; this is a non-trivial task.

• Cross-site scripting
• Reflected cross-site scripting
• Using Burp to Find XSS issues

• CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
• CWE-80: Improper Neutralization of Script-Related HTML Tags in a Web Page (Basic XSS)
• CWE-116: Improper Encoding or Escaping of Output
• CWE-159: Failure to Sanitize Special Element

• /FAI/Edit.aspx [txtMDQ parameter]
• /FAI/Edit.aspx [txtMDQ parameter]
• /FAI/Search.aspx [txtMDQ parameter]
• /FAI/Search.aspx [txtMDQ parameter]

External service interaction arises when it is possible to induce an application to interact with an arbitrary external service, such as a web or mail server. The ability to trigger arbitrary external service interactions does not constitute a vulnerability in its own right, and in some cases might even be the intended behavior of the application. However, in many cases, it can indicate a vulnerability with serious consequences.

In cases where DNS-based interactions can be triggered, it is normally possible to trigger interactions using other service types, and these are reported as separate issues. If a payload that specifies a particular service type (e.g. a URL) triggers only a DNS-based interaction, then this strongly indicates that the application attempted to connect using that other service, but was prevented from doing so by egress filters in place at the network layer. The ability to send requests to other systems can allow the vulnerable server to be used as an attack proxy. By submitting suitable payloads, an attacker can cause the application server to attack other systems that it can interact with. This may include public third-party systems, internal systems within the same organization, or services available on the local loopback adapter of the application server itself. Depending on the network architecture, this may expose highly vulnerable internal services that are not otherwise accessible to external attackers.

You should review the purpose and intended use of the relevant application functionality, and determine whether the ability to trigger arbitrary external service interactions is intended behavior. If so, you should be aware of the types of attacks that can be performed via this behavior and take appropriate measures. These measures might include blocking network access from the application server to other internal systems, and hardening the application server itself to remove any services available on the local loopback adapter.

If the ability to trigger arbitrary external service interactions is not intended behavior, then you should implement a whitelist of permitted services and hosts, and block any interactions that do not appear on this whitelist.

• Burp Collaborator

• CWE-918: Server-Side Request Forgery (SSRF)
• CWE-406: Insufficient Control of Network Message Volume (Network Amplification)

• /FAI/Edit.aspx [txtMDQ parameter]
• /FAI/Edit.aspx [txtMDQ parameter]
• /FAI/Search.aspx [txtMDQ parameter]
• /FAI/Search.aspx [txtMDQ parameter]

External service interaction arises when it is possible to induce an application to interact with an arbitrary external service, such as a web or mail server. The ability to trigger arbitrary external service interactions does not constitute a vulnerability in its own right, and in some cases might even be the intended behavior of the application. However, in many cases, it can indicate a vulnerability with serious consequences.

The ability to send requests to other systems can allow the vulnerable server to be used as an attack proxy. By submitting suitable payloads, an attacker can cause the application server to attack other systems that it can interact with. This may include public third-party systems, internal systems within the same organization, or services available on the local loopback adapter of the application server itself. Depending on the network architecture, this may expose highly vulnerable internal services that are not otherwise accessible to external attackers.

You should review the purpose and intended use of the relevant application functionality, and determine whether the ability to trigger arbitrary external service interactions is intended behavior. If so, you should be aware of the types of attacks that can be performed via this behavior and take appropriate measures. These measures might include blocking network access from the application server to other internal systems, and hardening the application server itself to remove any services available on the local loopback adapter.

If the ability to trigger arbitrary external service interactions is not intended behavior, then you should implement a whitelist of permitted services and hosts, and block any interactions that do not appear on this whitelist.

• Burp Collaborator

• CWE-918: Server-Side Request Forgery (SSRF)
• CWE-406: Insufficient Control of Network Message Volume (Network Amplification)

XML entity expansion vulnerabilities arise because the XML specification allows XML documents to define entities that reference other entities defined within the document. If this is done recursively to a significant depth, then the XML parser will consume exponentially increasing amounts of memory and processor resources as each level of recursion is processed. This might result in a denial-of-service condition, causing the entire application to stop functioning.

Note: To avoid causing an actual denial-of-service, Burp Suite merely verifies that entities are being recursively expanded to a modest depth. It is possible that reported applications are not actually vulnerable because they are designed to prevent entity expansion beyond a given depth.

XML entity expansion makes use of the DOCTYPE tag to define the injected entities. XML parsers can usually be configured to disable support for this tag. You should consult the documentation for your XML parsing library to determine how to disable this feature.

It may also be possible to use input validation to block input containing a DOCTYPE tag.

• CWE-776: Improper Restriction of Recursive Entity References in DTDs ('XML Entity Expansion')