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XML external entity expansion

ID: js/xxe
Kind: path-problem
Security severity: 9.1
Severity: error
Precision: high
Tags:
   - security
   - external/cwe/cwe-611
   - external/cwe/cwe-827
Query suites:
   - javascript-code-scanning.qls
   - javascript-security-extended.qls
   - javascript-security-and-quality.qls

Click to see the query in the CodeQL repository

Parsing untrusted XML files with a weakly configured XML parser may lead to an XML External Entity (XXE) attack. This type of attack uses external entity references to access arbitrary files on a system, carry out denial-of-service (DoS) attacks, or server-side request forgery. Even when the result of parsing is not returned to the user, DoS attacks are still possible and out-of-band data retrieval techniques may allow attackers to steal sensitive data.

Recommendation

The easiest way to prevent XXE attacks is to disable external entity handling when parsing untrusted data. How this is done depends on the library being used. Note that some libraries, such as recent versions of libxml, disable entity expansion by default, so unless you have explicitly enabled entity expansion, no further action needs to be taken.

Example

The following example uses the libxml XML parser to parse a string xmlSrc. If that string is from an untrusted source, this code may be vulnerable to an XXE attack, since the parser is invoked with the noent option set to true:

const app = require("express")(),
  libxml = require("libxmljs");

app.post("upload", (req, res) => {
  let xmlSrc = req.body,
    doc = libxml.parseXml(xmlSrc, { noent: true });
});

To guard against XXE attacks, the noent option should be omitted or set to false. This means that no entity expansion is undertaken at all, not even for standard internal entities such as & or >. If desired, these entities can be expanded in a separate step using utility functions provided by libraries such as underscore, lodash or he.

const app = require("express")(),
  libxml = require("libxmljs");

app.post("upload", (req, res) => {
  let xmlSrc = req.body,
    doc = libxml.parseXml(xmlSrc);
});

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