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Friday, 24 July 2020

XXE - XML External Entity Vulnerability

XXE - XML External Entity Vulnerability



XXE : XML External Entity


An XML External Entity (XXE) attack is a vulnerability that abuses features of XML parsers/data. It often allows an attacker to interact with any backend or external systems that the application itself can access and can allow the attacker to read the file on that system.XXE may even enable port scanning and lead to remote code execution.


XML external entities allow the inclusion of data dynamically from a given resource (local or remote) at the time of parsing. This feature can be exploited by attackers to include malicious data from external URIs or confidential data residing on the local system. If XML parsers are not configured to prevent or limit external entities, they are forced to access the resources specified by the URI. XXE - XML External Entity Vulnerability



<?XML version="1.0"?>
<!DOCTYPE myFile [
<!ELEMENT myFile ANY >
<!ENTITY xxe SYSTEM "file:///etc/passwd">
]>
<myFile>&xxe;</myFile>



This is a well-formed XML document. During parsing, the parser will replace the external entity “&xxe;” with the content of the system file “/etc/passwd”, which contains confidential information and might be disclosed. Another example: if the URI ‘file:///etc/passwd’ is replaced by a link to a malicious server that never responds, the parser might end up waiting, thus causing delays in the subsequent processes.


Successful exploitation of this vulnerability may result in disclosure of sensitive data, denial of service, Server-Side Request Forgery (SSRF), or an attacker gaining unauthorized access to the system resources. If an XML parser does not block external entity expansion and is able to access the referred content, one user may be able to gain unauthorized access to the data of other users, leading to a breach of confidentiality.



 

There are two types of XXE attacks: in-band and out-of-band (OOB-XXE).
 



1) An in-band XXE attack is the one in which the attacker can receive an immediate response to the XXE payload.



2) out-of-band XXE attacks (also called blind XXE), there is no immediate response from the web application and attacker has to reflect the output of their XXE payload to some other file or their own server.


( Payload )

<?xml version="1.0"?>
<!DOCTYPE root [<!ENTITY read SYSTEM 'file:///etc/passwd'>]>
<root>&read;</root>






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