Resolve vulnerability: Improper restriction of XML external entity reference

Darby Freyrequested to merge
remediate/improper-restriction-of-xml-external-entity-reference-D20241106T161722 into main

MR created from vulnerability: Improper restriction of XML external entity reference

AI GENERATED PATCH

The suggested code changes were generated by GitLab Duo Vulnerability Resolution, an AI feature. Use this feature with caution. Before you run a pipeline or apply the code changes, carefully review and test them, to ensure that they solve the vulnerability.

The large language model that generated the suggested code changes was provided with the entire file that contains the vulnerable lines of code. It is not aware of any functionality outside of this context.

Please see our documentation for more information about this feature.

Description:

The application was found using the xml.etree package for processing XML. Pythons default xml processors suffer from various XML parsing vulnerabilities and care must be taken when handling XML data. Additionally, depending on the version of Python, more critical vulnerabilities such as eXternal XML Entity injection maybe exploitable.

The etree package suffers from the following security risks as of Python 3.7.1:

  • Billion laughs / exponential entity expansion - May allow an adversary to cause a Denial of Service (DoS) against the application parsing arbitrary XML.
  • Quadratic blowup entity expansion - Similar to above, but requires a larger input to cause the Denial of Service.

To remediate the above issues, consider using the defusedxml library when processing untrusted XML.

Example parsing an XML document using defusedxml:

from defusedxml.ElementTree import parse

# Parse the inventory.xml file
et = parse('inventory.xml')
# Get the root element
root = et.getroot()
# Work with the root element
# ...

For more information on the various XML parsers and their vulnerabilities please see:

For more information on XML security see OWASP's guide:

Analysis:

The vulnerability report indicates an "Improper restriction of XML external entity reference" issue, which is associated with CWE-611 and OWASP A4:2017 (XML External Entities - XXE). This is a serious security concern in XML parsing.

The vulnerable code is using xml.etree.ElementTree.fromstring() to parse XML data without any restrictions on external entity references. This can lead to XXE attacks, where an attacker can include external entities in the XML to potentially:

  1. Read sensitive files on the server
  2. Perform denial of service attacks
  3. Execute remote code in some cases

The vulnerability is present in multiple places in the code:

  1. In the main route (/) when processing POST requests
  2. In the /upload route when processing uploaded XML files
  3. In the /process route when fetching and processing XML from a URL

This is a genuine security concern and not a false positive. The xml.etree.ElementTree module does not have built-in protections against XXE attacks, so we need to implement additional safeguards.

Summary:

  1. The reported vulnerability is an "Improper restriction of XML external entity reference" (CWE-611), which can lead to XXE attacks.

  2. The fix addresses this security concern by:

    a. Replacing xml.etree.ElementTree with defusedxml.ElementTree, a security-enhanced drop-in replacement for the standard library's XML parser.

    b. Using defusedxml.ElementTree.fromstring() with forbid_dtd=True and forbid_entities=True parameters to explicitly disable DTD parsing and entity expansion.

    c. Updating the import statements and exception handling to use the new defusedxml module.

  3. These changes effectively prevent XXE attacks by disabling the processing of potentially dangerous XML features (DTDs and entities) while maintaining the original functionality of the code.

To implement this fix, you'll need to install the defusedxml library using pip:

pip install defusedxml

This solution provides a robust defense against XXE vulnerabilities without significantly altering the code's structure or functionality.

Identifiers:

  • Bandit Test ID B314
  • A4:2017 - XML External Entities (XXE)
  • CWE-611
  • bandit.B314
  • A03:2021 - Injection

Merge request reports