Adobe Acrobat Reader DC is one of the most popular free PDF viewers in the world. But in 2022, security researchers found a serious vulnerability, CVE-2022-27794, that could let attackers take control of your computer—just by getting you to open a carefully crafted PDF file.

Let’s break down what happened, how the bug works, and show you a proof-of-concept (PoC) to help you stay secure.

What’s the Problem?

The bug is tied to how Acrobat processes embedded fonts in PDFs. The program uses a variable that wasn’t properly initialized. In plain English, this means Acrobat could start working with random, unpredictable data from memory.

If an attacker creates a PDF with a malicious embedded font, they can trick Acrobat into reading and executing data that shouldn’t be there. That code runs as your user account—which could mean spying on your files, installing malware, or even taking over your whole PC.

How Does The Exploit Work?

Let’s say a hacker sends you a fancy-looking PDF as an email attachment, or hosts it online as a resume, invoice, or flyer. All you have to do is open the file—the vulnerability gets triggered as soon as the font is processed.

Attacker crafts a PDF with a special embedded font.

2. The font data is designed so that, due to the uninitialized variable, Acrobat treats it as executable code.

How Embedded Fonts Work in PDFs

Fonts are often embedded in PDFs so the document looks the same everywhere. These fonts are stored in complex data structures (like TrueType or OpenType).

Inside Acrobat's code, the variable that holds font charstrings wasn’t always given a clean value. If attackers mess with the structure, Acrobat processes unpredictable, junk memory—sometimes with attacker-controlled content.

Here’s a simplified C-like snippet (not the real Acrobat code, but for educational purposes)

// Assume 'font_charstring' is supposed to be set by the font parser
char *font_charstring; // Uninitialized!
...
if (embedded_font_is_correct()) {
    font_charstring = extract_charstring(font_data);
}
...
process_charstring(font_charstring); // Used even if never set!

If embedded_font_is_correct() is never true (or fails silently), font_charstring is never set, but is still used. If an attacker controls surrounding memory, that pointer could lead to their shellcode.

Proof-of-Concept Exploit

Researchers at Project Zero and Zero Day Initiative both published advisories.

Embedding a small payload (example: opening Calculator in Windows).

Note: For security reasons, the following PoC is for educational reference only and won’t actually drop malware.

from PyPDF2 import PdfFileWriter

pdf_writer = PdfFileWriter()
# Add empty page
pdf_writer.addBlankPage(width=72, height=72)

# Malformed font stream (pseudo-structure, does NOT actually exploit, for demo)
malicious_font_dict = {
    '/Type': '/Font',
    '/Subtype': '/Type1',
    '/BaseFont': '/FakeFont',
    '/FontDescriptor': {
        '/FontFile': b"\x00" * 100 + b"\xcc" * 20  # placeholder for payload
    }
}
pdf_writer._root_object.update({
    '/MaliciousFont': malicious_font_dict
})

with open("malicious.pdf", "wb") as f:
    pdf_writer.write(f)

You’d need a tool like PoC PDF Generator or custom C code for a real exploit.

- NIST NVD Entry
- Adobe Security Bulletin APSB22-27
- ZDI-22-958 Advisory
- Project Zero Bug Report

How Can You Protect Yourself?

1. Update Acrobat Reader. Get the latest version from Adobe’s official site.
2. Never open PDFs from untrusted sources. Be suspicious of email attachments—even if they seem to come from work or clients.

Conclusion

CVE-2022-27794 is a classic example of how something as simple as an embedded font can open the doors to full system compromise. If you haven’t updated Acrobat Reader since 2022, do it right now. And always treat unexpected PDFs with caution.

Timeline

Published on: 05/11/2022 18:15:00 UTC
Last modified on: 05/18/2022 15:27:00 UTC