Aexus Answers

CVE-2017-7957: XStream Vulnerability & Fix

Alex Johnson
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CVE-2017-7957: XStream Vulnerability & Fix

Hey everyone! Let's dive into a critical security vulnerability, CVE-2017-7957, detected in the xstream-1.4.5.jar library. If you're using this library, it's super important to understand the risk and how to address it.

What is XStream?

XStream is a powerful Java library that simplifies the serialization and deserialization of Java objects to XML and vice-versa. Think of it as a translator between Java objects and XML data. It's widely used in various applications for data persistence, configuration, and message exchange.

http://xstream.codehaus.org is the library's home page, where you can find more details and documentation.

The Vulnerable Library: xstream-1.4.5.jar

This specific vulnerability affects xstream-1.4.5.jar. This means any application using this version of the library is potentially at risk. The vulnerability was identified in the WebGoat8/pom.xml file within the SAST-UP-DEV/SAST-Test-Repo-881d25ad-430b-4028-89bf-1d9346ea9344 repository. It was found in the HEAD commit 3c0e763f20f7e2cfd01e3acd4b22bc5a9af77381 and the base branch main.

Dependency Hierarchy:

  • ❌ xstream-1.4.5.jar (Vulnerable Library)

Understanding CVE-2017-7957

CVE-2017-7957 is a high-severity vulnerability that can lead to a remote application crash. Specifically, it stems from how XStream handles attempts to create instances of the primitive type 'void' during unmarshalling when a certain denyTypes workaround isn't in place.

Imagine someone sending a specially crafted XML message like <void/> to your application. If your application uses the vulnerable XStream version without the necessary safeguards, it can crash. This is a classic example of a Denial of Service (DoS) attack, where an attacker can make your application unavailable to legitimate users.

The vulnerability can be triggered by a simple call like xstream.fromXML("<void/>"). This highlights how easy it is for an attacker to exploit this flaw.

This vulnerability was published on 2017-04-29. You can find more details about it on the Mend Vulnerability Database.

The Technical Details

At its core, CVE-2017-7957 arises from a mishandling of the void type during the deserialization process. XStream, in its vulnerable versions, doesn't properly sanitize or validate the types it's unmarshalling. This means an attacker can inject malicious XML that instructs XStream to create a void instance, leading to unexpected behavior and ultimately, a crash.

The lack of proper denyTypes configuration makes the situation worse. This configuration is meant to restrict the types XStream can instantiate during unmarshalling, acting as a safeguard against such attacks. However, if not correctly implemented, the vulnerability remains open.

Think of it like this: XStream is a chef who can cook anything you ask for. Without proper instructions (the denyTypes configuration), someone can trick the chef into cooking something dangerous (the void instance), causing a kitchen fire (the application crash).

CVSS 3 Score: 7.5 (High Severity)

The Common Vulnerability Scoring System (CVSS) assigns a score to vulnerabilities based on their severity. CVE-2017-7957 has a CVSS 3 score of 7.5, which is considered High. This score reflects the significant risk this vulnerability poses.

Base Score Metrics:

  • Exploitability Metrics:
    • Attack Vector: Network (This means the vulnerability can be exploited over a network.)
    • Attack Complexity: Low (It's relatively easy to exploit.)
    • Privileges Required: None (No special privileges are needed to exploit it.)
    • User Interaction: None (No user interaction is required for the exploit to work.)
    • Scope: Unchanged
  • Impact Metrics:
    • Confidentiality Impact: None (The vulnerability doesn't directly compromise confidentiality.)
    • Integrity Impact: None (It doesn't directly compromise data integrity.)
    • Availability Impact: High (The vulnerability can lead to a complete loss of availability due to the application crash.)

In simpler terms, an attacker can remotely crash your application without needing any credentials or user interaction. While it doesn't steal data or modify it, the resulting downtime can be very disruptive.

For a deeper dive into CVSS 3 scores, check out the FIRST website.

The Fix: Upgrade to XStream 1.4.10

Luckily, there's a straightforward fix for this vulnerability: upgrade to XStream version 1.4.10 or later. This version includes a patch that addresses the mishandling of the void type and prevents the crash.

Suggested Fix:

Upgrading is the most effective way to mitigate this risk. Make sure to thoroughly test the upgraded version in a non-production environment before deploying it to production.

How to Upgrade

The process of upgrading XStream depends on your project's build system. If you're using Maven, you'll need to update the XStream dependency in your pom.xml file.

<dependency>
    <groupId>com.thoughtworks.xstream</groupId>
    <artifactId>xstream</artifactId>
    <version>1.4.10</version>
</dependency>

Similarly, if you're using Gradle, you'll update your build.gradle file:

dependencies {
    implementation 'com.thoughtworks.xstream:xstream:1.4.10'
}

After updating the dependency, rebuild your project and run your tests to ensure everything is working as expected.

Additional Mitigation Strategies

While upgrading is the primary fix, there are other steps you can take to enhance your application's security:

  1. Implement denyTypes Configuration: Even after upgrading, it's a good practice to explicitly configure the types XStream is allowed to unmarshal. This acts as a defense-in-depth measure.
  2. Input Validation: Carefully validate any XML input your application receives. This can help prevent malicious payloads from reaching XStream in the first place.
  3. Web Application Firewall (WAF): A WAF can detect and block malicious requests, including those that attempt to exploit this vulnerability.

Staying Secure

Security vulnerabilities are a constant threat, so it's essential to stay vigilant. Regularly update your libraries, monitor security advisories, and implement security best practices in your development process.

In conclusion, CVE-2017-7957 is a serious vulnerability in xstream-1.4.5.jar that can lead to application crashes. The best way to address it is to upgrade to XStream 1.4.10 or later. Don't forget to test your application after the upgrade to ensure everything is working correctly. Stay safe out there, guys!

For more information on XStream security vulnerabilities and best practices, visit the XStream official website at https://x-stream.github.io/.

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