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Dangerous Flaw in Popular API Tool Threatens Developers and Security Teams
A serious vulnerability has been identified in the Insomnia API Client, a tool widely used by developers for testing and managing APIs. This flaw opens the door to arbitrary code execution through template injection, allowing attackers to run malicious commands directly on a userās machine. Developed by Kong, Insomnia integrates the Nunjucks templating engine, which is where the core issue originates. The vulnerability has been assigned the identifier CVE-2025-1087 and remains unpatched as of version 11.2.0.
Summary of the Security Threat
A routine API penetration test led to the discovery of this critical flaw. During testing, researchers noticed that the Insomnia application was processing template expressions like {{77}} locally, evaluating them before sending requests to the API. This behavior revealed a deeper issue: Insomnia was rendering user-controlled input using the Nunjucks template engine without proper sanitization or sandboxing. Attackers could exploit this by inserting specially crafted template strings into API request fieldsāsuch as URLs, headers, cookies, or even environment variables.
One of the most dangerous aspects is remote code execution (RCE). If a malicious server returns a Set-Cookie header containing a payload like {{require("child_process").exec("rm -rf /")}}, the Insomnia client processes it and executes it on the local machine. This is possible because Insomnia blindly trusts and renders the template, invoking Node.js modules like child_process under the hood. Attackers can then run arbitrary shell commands in the context of the user’s environment.
Kong has attempted several mitigations: blocking certain keywords like require, limiting module access, and removing global functions. However, these efforts have consistently failed. Researchers demonstrated multiple bypasses using obfuscation techniques or lesser-known Nunjucks features, such as joiner.constructor, to circumvent filters.
The core problem stems from architectural decisions that treat user-controlled data as safe input for a powerful templating engine. Nunjucks itself warns against rendering untrusted content, yet Insomnia does precisely that in numerous places. The vulnerability highlights deep systemic issuesānot just one-off bugs.
Security experts strongly advise users to avoid importing API definitions, copying raw data, or interacting with unknown endpoints without thorough inspection. Until a true fix is in place, disable or patch template rendering where possible. Organizations should treat any curly-brace-based expression {{...}} as a potential threat vector.
The situation illustrates the growing dangers in modern development tooling, especially when flexibility is prioritized over security. Without a sandboxed environment or strict validation, even simple developer utilities can become vectors for severe cyberattacks.
What Undercode Say:
Architecture Over Convenience: A Security Trade-off
The vulnerability in Insomnia stems from a classic design oversightāprioritizing user experience and flexibility over security principles. By integrating the Nunjucks template engine with full capabilities and then enabling it across multiple untrusted inputs, the developers inadvertently created a wide-open attack surface. This is not a case of a minor bug; it’s an example of how deeply integrated tools, when not treated with strict security controls, can expose entire systems.
Nunjucks as a Double-Edged Sword
Nunjucks is powerful, and that power is dangerous in the wrong hands. While it’s intended for dynamic rendering, it lacks built-in sandboxing, and its use cases generally assume trusted input. Inserting such an engine into a developer tool used to process unpredictable dataāsuch as headers from remote APIsāis inherently risky. This makes it a poor fit for environments like Insomnia where security must come first.
Repeated Mitigation Failures Signal Deeper Issues
The fact that Kong has attempted multiple patchesāand each has failedāsuggests the root problem is not easily solvable with filters or keyword blocks. Attackers continuously find workarounds because the engineās core behavior remains unchanged. For example, require may be blocked, but alternative methods like module.require or constructor calls still work. These creative evasion tactics thrive when partial security controls are applied to inherently dangerous mechanisms.
Blind Trust in Input Is a Red Flag
Trusting user inputāwhether itās from a copied value, an HTTP response, or an imported fileāis always dangerous. Insomnia doesn’t appear to differentiate between safe and unsafe sources. That means any interaction, even with seemingly benign API endpoints, could become an entry point for attackers. This violates one of the most basic principles in secure coding: never trust external input.
Remote Execution Makes It a Priority-Level Threat
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Lessons for the Industry
The Insomnia case should serve as a wake-up call for all API tool developers. As these tools become more complex and interconnected, strong sandboxing and validation mechanisms must become default, not optional. And developers should steer away from using logic-capable template engines in any environment where inputs can be influenced externally.
A Call for Transparency
Kongās lack of a detailed public bulletin raises questions. When such a critical vulnerability exists, transparency is crucial. Vague CVE listings do little to help users understand the full scope or urgency. A complete disclosure, with detailed remediation guidance and architecture-level changes, is needed to restore confidence.
Safer Alternatives and Workarounds
Until then, users can adopt temporary protections:
Disable template rendering altogether
Strip all curly-brace expressions before input is processed
Avoid importing external data or interacting with untrusted endpoints
Developers may also consider switching to logic-less templating engines like Mustache or Handlebars, which have fewer capabilities but significantly reduce risk.
Conclusion
This incident underscores the fragility of developer ecosystems when security is treated as an afterthought. The convenience of dynamic templating must never outweigh the need for controlled, audited, and safe processing of data. Tools like Insomnia, used by thousands of professionals, must adopt a security-first philosophy moving forward.
š Fact Checker Results:
ā
Vulnerability CVE-2025-1087 exists and remains unpatched in Insomnia v11.2.0
ā
Remote Code Execution (RCE) is achievable through Nunjucks payloads
ā No comprehensive fix or official security bulletin has been issued by Kong
š Prediction:
š® Given the persistent bypasses and lack of a holistic solution, this vulnerability will likely remain active for months. Security researchers may continue discovering new exploit paths, forcing Kong to eventually refactor or replace the template engine entirely. Meanwhile, users should expect more security advisories and possibly a shift in the community toward safer alternatives.
References:
Reported By: cyberpress.org
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