Introduction
If you’re using Jinjava in your Java-based applications, there’s a critical security concern you should be aware of - CVE-2025-59340.
Before version 2.8.1, Jinjava contained a sandbox bypass flaw that could let attackers execute malicious operations through clever deserialization tricks.
In simpler terms, it’s a case where the system’s safety walls didn’t hold up - allowing access to objects and classes that should have been off-limits. Let’s take a closer look at what went wrong, how it could be exploited, and how the issue was fixed.
Key Takeaways
- The issue affects Jinjava versions prior to 2.8.1.
- Attackers could bypass sandbox restrictions using JavaType deserialization.
- The vulnerability could lead to accessing local files or remote code execution.
- Upgrading to Jinjava 2.8.1 or higher is strongly recommended.
What Is Jinjava
Jinjava is a Java implementation of the Jinja templating engine commonly used to generate dynamic web content. It processes templates containing embedded expressions, helping developers render data safely and efficiently.
Why Does Jinjava Matter?
Jinjava allows developers to create flexible templates where variables and logic are evaluated at runtime, making it a popular choice in web applications and CMS platforms.
However, with great flexibility comes risk. If a templating engine isn’t properly sandboxed, attackers can inject malicious code or escape restricted environment as seen in this case.
CVE-2025-59340 is a sandbox bypass vulnerability discovered in Jinjava, a Java implementation of the Jinja templating engine. The flaw allows attackers to exploit the JavaType deserialization mechanism to bypass sandbox restrictions and instantiate arbitrary Java classes. This could expose local files or, in specific cases, lead to remote code execution (RCE). The issue was fixed in version 2.8.1.
The Vulnerability Explained: CVE-2025-59340
The CVE-2025-59340 vulnerability stems from improper sandbox restrictions in Jinjava.
Under normal conditions, Jinjava’s sandbox blocks unsafe operations - such as calling getClass() or creating new Class objects - to prevent malicious activity. However, attackers discovered a clever way around it.
By leveraging mapper.getTypeFactory().constructFromCanonical(), an attacker can deserialize crafted input into arbitrary classes, effectively bypassing the sandbox. This loophole allows the creation of objects like java.net.URL, enabling attackers to access local files (e.g., /etc/passwd) or even execute remote code in certain setups.
How Attackers Exploit It
Here’s how the exploit chain works:
- Jinjava templates expose a variable called _ int3rpr3t3r_.
Which provides direct access to the jinjavaInterpreter instance . Although protections were added to prevent calling methods from JinjavaInterpreter instances, interacting with their properties remains unrestricted. From _ int3rpr3t3r_, it's possible to traverse to the config field, which exposes an ObjectMapper. By invoking readValue(String content, JavaType valueType) on this ObjectMapper, - An attacker can instantiate arbitrary classes specified via JavaType. Since the JavaType class itself is not restricted, an attacker can leverage JavaType construction (constructFromCanonical) to instantiate semi-arbitrary classes, allowing sandbox escape and the creation of powerful primitives.
By constructing a JavaType pointing to a class like java.net.URL, an attacker could instantiate it - potentially accessing sensitive files (file:///etc/passwd) or triggering network connections.
In certain setups, this could escalate further, resulting in remote code execution (RCE).
How Was the Issue Fixed?
The fix was rolled out in this commit.
The patch prevents access to restricted properties from within templates - particularly blocking traversal to the config field and then to the ObjectMapper, thereby blocking the ability to instantiate arbitrary classes via JavaType.
This effectively seals off the deserialization pathway that allowed sandbox escape
Recommendations for Developers
Users of Jinjava should immediately upgrade to version 2.8.1 or later. Beyond that, it’s worth reviewing template usage and restricting user-provided template input where possible.
Here are some security best practices to follow:
- Strict Input Sanitization – Validate and filter all inputs before rendering templates.
- Secure Sandboxing – Ensure your template engine enforces access restrictions on internal APIs and system-level classes.
- Avoid Arbitrary Object Deserialization – Never deserialize untrusted data or expose APIs that do.
Related Reading: Loginsoft Vulnerability Intelligence Track high-risk vulnerabilities across open-source ecosystems in real-time with actionable intelligence from our cybersecurity experts.
References/Resources:
https://github.com/advisories/GHSA-m49c-g9wr-hv6v
https://nvd.nist.gov/vuln/detail/CVE-2025-59340
https://github.com/HubSpot/jinjava?tab=readme-ov-file#jinjava
Why This Matters for Security Teams
Template engines like Jinjava are integral to many enterprise-grade applications. However, they operate at the intersection of data and logic, making them prime targets for sandbox escape vulnerabilities.
For security engineers and researchers, CVE-2025-59340 highlights the importance of:
- Regular code reviews and dependency scanning.
- Keeping templating frameworks updated.
- Applying least-privilege principles even in internal templating environments.
If left unpatched, this issue can serve as an entry point for lateral movement or data exfiltration in a production environment.
Explore More:Cybersecurity Research – learn how our team identifies and mitigates emerging threats in open-source ecosystems.
FAQ
1. What is CVE-2025-59340?
It’s a sandbox bypass vulnerability in Jinjava (before v2.8.1) that allows attackers to use deserialization via JavaType to instantiate restricted classes and escape the sandbox.
2. How can an attacker exploit this vulnerability?
An attacker can leverage Jinjava’s _ int3rpr3t3r _ variable to reach the ObjectMapper and deserialize arbitrary Java classes, potentially gaining access to files or executing code.
3. How was the vulnerability fixed?
Developers restricted access to sensitive properties and objects in Jinjava’s interpreter, preventing the traversal chain that enabled deserialization-based sandbox escapes.
4. How can organizations protect themselves from similar flaws?
Adopt secure sandboxing principles, validate all template inputs, and avoid exposing internal configurations to user-provided templates.
