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XML External Entity (XXE) Injection The advisory has been revoked - it doesn't affect any version of package libnmap Open this link in a new tab

    Threat Intelligence

    Exploit Maturity Proof of concept
    EPSS 0.14% (49th percentile)
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NVD
7.5 high

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  • Snyk ID SNYK-PYTHON-LIBNMAP-451603
  • published 15 Jul 2019
  • disclosed 15 Jul 2019
  • credit Sajeeb Lohani
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Introduced: 15 Jul 2019

CVE-2019-1010017 Open this link in a new tab
CWE-611 Open this link in a new tab

Overview

libnmap is a python library enabling python developers to manipulate nmap process and data.

Affected versions of this package are vulnerable to XML External Entity (XXE) Injection. Is vulnerable to XML Bomb attacks using the "Billion Laughs" XXE bomb attack.

PoC by Sajeeb Lohani 

ty = NmapParser()

payload = """
<!DOCTYPE lolz [
 <!ENTITY lol "lol">
 <!ELEMENT lolz (#PCDATA)>
 <!ENTITY lol1 "&lol;&lol;&lol;&lol;&lol;&lol;&lol;&lol;&lol;&lol;">
 <!ENTITY lol2 "&lol1;&lol1;&lol1;&lol1;&lol1;&lol1;&lol1;&lol1;&lol1;&lol1;">
 <!ENTITY lol3 "&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;">
]>
<lolz><hello>&lol3;</hello></lolz>
"""

ty.parse(payload)

Details

XXE Injection is a type of attack against an application that parses XML input. XML is a markup language that defines a set of rules for encoding documents in a format that is both human-readable and machine-readable. By default, many XML processors allow specification of an external entity, a URI that is dereferenced and evaluated during XML processing. When an XML document is being parsed, the parser can make a request and include the content at the specified URI inside of the XML document.

Attacks can include disclosing local files, which may contain sensitive data such as passwords or private user data, using file: schemes or relative paths in the system identifier.

For example, below is a sample XML document, containing an XML element- username.

<?xml version="1.0" encoding="ISO-8859-1"?>
   <username>John</username>
</xml>

An external XML entity - xxe, is defined using a system identifier and present within a DOCTYPE header. These entities can access local or remote content. For example the below code contains an external XML entity that would fetch the content of /etc/passwd and display it to the user rendered by username.

<?xml version="1.0" encoding="ISO-8859-1"?>
<!DOCTYPE foo [
   <!ENTITY xxe SYSTEM "file:///etc/passwd" >]>
   <username>&xxe;</username>
</xml>

Other XXE Injection attacks can access local resources that may not stop returning data, possibly impacting application availability and leading to Denial of Service.

References