File Doppelgängers: The World of Polyglot Files

Unlocking the Secrets of Polyglot Files: When One File Speaks Many Languages

Introduction

In the world of cybersecurity, appearances can be deceiving. A file might look like an innocent image, but behind the scenes, it could also be a ZIP archive, a PDF, or even an executable. Such files are called polyglots — single files that are valid under multiple file formats at the same time.

Polyglots are fascinating from a technical standpoint, but they also pose serious security risks. Attackers often use them to bypass filters, sneak past antivirus solutions, or trick unsuspecting users into executing malicious code.

What Are Polyglot Files?

A polyglot file is crafted in such a way that it conforms to the specifications of two (or more) different file types simultaneously.

• Example: A file that is both a JPEG image and a ZIP archive.
• When opened in an image viewer → You see a normal picture.
• When extracted with a ZIP tool → You find hidden files inside.

This dual nature makes polyglots powerful tools for both legitimate research and malicious exploitation.

How Polyglots Work

To understand polyglots, you need to know about file signatures (magic numbers).

• JPEG → FF D8 FF
• ZIP → 50 4B 03 04
• PDF → %PDF

By carefully aligning headers and footers of two formats, attackers can create a file that satisfies both.

For example:

1. Take a valid JPEG file.
2. Append a ZIP file structure at the end.
3. The result → One file that opens as an image but also extracts as a ZIP.

Real-World Uses of Polyglots

Polyglots have been used in multiple contexts:

• Security bypass → Malware hidden inside images or PDFs.
• Capture the Flag (CTF) challenges → Teaching participants about file structures.
• Research experiments → Demonstrating how file parsers can be tricked.
• Obfuscation → Making analysis harder for defenders.

One famous example is the GIFAR polyglot, which is both a GIF image and a Java Archive (JAR) file. Attackers used it to upload malicious Java code disguised as images on web servers.

Creating a Simple Polyglot (Safe Demo)

⚠️ For educational purposes only — don’t use this maliciously.

Let’s create a JPG+ZIP polyglot:

1. Prepare a test.jpg image and a secret.zip archive.
2. Concatenate them:

cat ayanokouji1.png polyglot_file.zip > ayanokouji1.jpg

Now:

• open ayanokouji1.jpg

  in an image viewer → It shows the image.

• unzip ayanokouji1.jpg

  with unzip → It extracts the hidden archive.

This works because image viewers only care about the start of the file, while unzip looks for the ZIP footer.

Why Polyglots Matter in Cybersecurity

Polyglots are not just fun hacks — they highlight deeper issues:

• Weak file validation → Applications may trust file extensions instead of verifying contents.
• Security blind spots → Antivirus tools may only scan one format.
• Exploitation vectors → Attackers can smuggle payloads past defenses.

Understanding polyglots helps defenders improve detection and researchers design better security tools.

Defending Against Polyglots

Here are some ways to mitigate risks:

• Always check magic numbers and headers, not just extensions.
• Use tools like binwalk, xxd, or file to inspect files.
• Implement strict content validation on uploads.
• Keep security solutions updated to detect polyglot tricks.

Conclusion

Polyglot files are like the chameleons of the digital world — blending into multiple environments at once. They showcase the creativity of hackers and researchers alike, but also warn us of the need for stronger file validation and security practices.

So, the next time you open a file that seems ordinary, remember: it might just be speaking more than one language.