Defense against uploads: Q&A with OSS file scanner, pompelmi

Source: Stack Overflow Blog

With bad code being easier than ever to write thanks to AI, application security is ever more important. One of the earliest attack vectors for the internet are file uploads, and they are still a threat. Open‑source projects like pompelmi provide easy and flexible ways to defend against this attack using modern tech. I spoke with the creator of the project, Tommaso Bertocchi, about how it works.

Interview

Q: When people think of application security today, they usually think in terms of network connections, user auth, and API security. File uploads seem like a niche and/or early‑Internet problem. What made you want to create a solution for this issue?

A (Tommaso Bertocchi):
You’re right that file‑upload security is often overshadowed by network or API concerns, but it remains a critical attack vector that is frequently overlooked because of its perceived complexity. I created pompelmi after noticing a significant gap: there were no modern, developer‑friendly solutions that could be integrated quickly. Traditionally, setting up a malware scanner required deep domain expertise and hours of configuration, which often discourages solo developers. By making the integration seamless and “plug‑and‑play” for Node.js environments, I wanted to encourage more developers to secure their applications, ultimately making the web safer.

Q: You call file uploads “a critical attack vector that is frequently overlooked because of its perceived complexity.” Could you expand on this? What’s the risk here? Are there recent breaches that came from file uploads? Why is this attack vector complex to deal with? Is there something more than just blocking *.exe files?

A:
File uploads look simple, but they introduce a meaningful security boundary because you’re accepting untrusted, complex inputs into your system. The risk isn’t only “someone uploads a virus.” In practice, weak upload handling can increase exposure to issues such as:

• Unintended execution or unsafe processing paths (depending on how files are stored and later handled)
• Denial‑of‑service scenarios via pathological inputs (e.g., archives designed to consume resources)
• Bypasses of superficial checks (extensions, client‑provided MIME types, filename tricks)

These problems tend to surface when uploads are implemented quickly and rely on minimal validation.

Q: Why Node.js for this instead of a language that compiles to bytecode?

A:
The choice of Node.js was purely strategic and driven by Developer Experience (DX). Building the tool in a compiled language would have introduced another layer of friction—requiring developers to manage external runtimes or complex inter‑process communication. Since modern web development is heavily centered around the Node.js ecosystem (React, Angular, Next.js, etc.), building pompelmi in the same language allows for a “native” integration. It lives where the code lives, ensuring that security doesn’t feel like a clunky external chore.

Q: How do you scan suspect files without persisting them and keep everything fast?

A:
The core of pompelmi’s performance lies in its in‑process, stream‑based architecture. We use Node.js streams and configurable buffer limits to analyze file bytes directly in memory as they are uploaded, avoiding the heavy I/O overhead of disk writes. Because the engine runs within the same process, there is no network latency. We also inspect “magic bytes” to quickly identify file types; if a file violates a policy (e.g., a ZIP bomb), we terminate the scan instantly before the entire file is processed.

Q: There’s a lot of flexibility built in with YARA and multiple composed scanners, but that could put some of the identification burden on the user. Do you provide YARA recipes or other best practices for identification?

A:
Flexibility shouldn’t come at the cost of usability. To prevent the “blank page” problem, pompelmi ships with:

• Built‑in policy presets and a Common Heuristics Scanner that handle the most frequent threats out of the box (ZIP bombs, MIME/Magic‑Byte verification, etc.)
• A set of standard YARA recipes and “reason codes” that explain why a file was flagged

Our goal is a secure‑by‑default experience where developers can start protected and then add custom rules as they evolve.

Q: Do people need to keep up with the latest security news in order to protect their file‑upload systems? Or is protecting against the most common attacks enough for solid security?

A:
Keeping up with every new exploit in real time is difficult, especially across many technologies. A more realistic approach is to implement a strong baseline that addresses common, repeatable failure modes (strict allowlists, server‑side type validation, safe storage/handling, resource/time bounds, archive hardening) and then keep the rapidly changing parts updated where relevant (patching dependencies, refreshing detection content such as scanning rules). pompelmi is built around this layered, policy‑driven approach—providing guardrails like server‑side validation, archive checks, resource limits, and optional YARA‑based scanning—to help teams adopt a solid baseline without building a custom pipeline from scratch.

Q: Is it hard being the primary maintainer of this project? Do you prefer having the control?

A:
Being the sole maintainer gives me creative freedom to make swift decisions and keep the project’s vision focused. However, I’m realistic: my goal is to scale this from a personal project into a full‑fledged organization. I want to build a team and an infrastructure that can provide advanced resources and consistent updates, moving beyond a single‑person dependency.

# Frequently Asked Questions (Developer‑Centric)

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#### **Q:** *What was something unexpectedly complex or difficult in creating this project?*

**A:**  
*(Answer forthcoming…)*  

**TB:** The most unexpectedly challenging part was **framework compatibility**.  
Each Node.js framework handles uploads differently—middleware vs. plugins, request/response lifecycles, body parsing, streaming vs. in‑memory buffers, and error‑handling conventions.  

Building **pompelmi** as a “drop‑in” upload‑protection layer across multiple stacks meant:

1. Designing a **consistent core policy** that could be applied everywhere.  
2. Maintaining **dedicated adapters** so developers can use the same rules and get predictable behavior regardless of the underlying framework.

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#### **Q:** *We've seen a lot of important OSS projects run into trouble when the lead maintainer burns out or needs a day job. Are you actively looking for contributors, ways to monetize, or foundational support?*

**TB:** I’m very conscious of the burnout risk, which is why I’m currently seeking:

- **Partnerships** or backing from an established tech company.  
- **Corporate sponsorship** or **foundational support** to provide the infrastructure and resources needed to elevate the project to a professional‑grade standard.

My priority is to secure the project’s future so it can continue serving the community reliably over the long term.

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#### **Q:** *Have you contributed to other OSS projects? If so, what did you learn about maintaining a project and dealing with contributors?*

**TB:** **pompelmi** is my first open‑source project and the first one I’ve maintained end‑to‑end.  

Key takeaways:

- The most valuable feedback isn’t stars or download counts; it’s **direct human interaction** via issues, pull requests, and code reviews.  
- These conversations surface real user needs, edge cases, and reliability expectations.  
- They drive iteration: fixing bugs, refining APIs, improving docs/examples, and adding features that address recurring problems while keeping the project stable.  

Maintaining the project mostly solo is challenging, but I aim to be:

- **Responsive** to community input.  
- **Transparent** about trade‑offs.  
- **Cautious** with changes that could affect users.

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