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Adobe Acrobat Reader is still one of the most common PDF applications—found on office PCs, personal laptops, and government networks everywhere. But when a critical Zero-Day vulnerability hits, the impact could be massive. One of the latest and most dangerous in recent years is CVE-2023-26369.

In this post, we’ll break down what CVE-2023-26369 is, how it works, what versions are affected, and what you can do to stay safe with easy-to-understand language. We’ll also show a demo snippet that illustrates the vulnerability’s exploitation pattern, and point you to further resources.

What Is CVE-2023-26369?

CVE-2023-26369 is an out-of-bounds write vulnerability in Adobe Acrobat Reader. If an attacker can trick you into opening a specially crafted PDF file, they can run malicious code on your computer as your user. That means the attacker could steal your data, install malware, or take over your account.

How Does the Exploit Work?

At its core, this vulnerability lets attackers write data outside of a reserved buffer in memory. This is usually possible due to mistakes in how Adobe Acrobat Reader parses certain parts of a malicious PDF file. If an attacker can control what data gets written and where, code execution becomes possible.

A victim to open the PDF using a vulnerable version of the software.

No user privileges or admin rights required — just opening the file is enough.

Technical Details

The bug is in how Acrobat Reader parses certain PDF elements. By placing specially crafted objects or commands inside the PDF (such as messed up annotation objects or malformed image streams), an attacker can trigger memory corruption.

Here’s a simplified example (in a C code snippet) of what *kind* of mistake could happen inside the program’s codebase:

void copy_data(char *input, int len) {
    char buffer[100];
    // Fault: no check if 'len' is greater than buffer
    memcpy(buffer, input, len); // Out-of-bounds write if 'len' > 100
}

If the attacker can convince the program to copy more than 100 bytes, they overwrite data they shouldn’t—and can inject their own code.

In a real exploit, the PDF would include JavaScript or crafted streams to trigger such a bug.

Example: Malicious PDF Snippet

Below is a very simplified version of how a malicious PDF might look (note: harmless, demonstration only):

%PDF-1.7
1  obj
<< /Type /Annot
   /Subtype /Widget
   /Rect [  100 100]
   /AP << /N 2  R >>
   /T (Exploit)
   /Ff 65536
>>
endobj
2  obj
<< /Length 256 >>
stream
// garbage but oversized data to overflow buffer
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA.... (repeats)
endstream
endobj
xref
 3
000000000 65535 f 
000000001 00000 n 
000000010 00000 n 
trailer
<< /Root 1  R >>
startxref
180
%%EOF

The actual real-world exploit would be more complex. It could use JavaScript inside the PDF to trigger the vulnerable parsing function and then execute a payload (like reverse shellcode).

Here’s how an exploit might look in Python pseudocode for educational purposes

with open("malicious.pdf", "wb") as f:
    f.write(b"%PDF-1.7\n")
    f.write(b"1  obj\n")
    f.write(b"<< /Type /Annot /Subtype /Widget /Rect [  100 100] /AP << /N 2  R >> /T (Exploit) /Ff 65536 >>\n")
    f.write(b"endobj\n")
    f.write(b"2  obj\n")
    # Write a huge stream to overflow the expected buffer
    f.write(b"<< /Length 1024 >>\n")
    f.write(b"stream\n")
    f.write(b"A" * 1024)
    f.write(b"\nendstream\nendobj\n")
    f.write(b"xref\n 3\n000000000 65535 f \n000000001 00000 n \n000000010 00000 n \n")
    f.write(b"trailer\n<< /Root 1  R >>\nstartxref\n180\n%%EOF\n")

Remember, this does NOT trigger the vulnerability on its own. It is a didactic example to illustrate the structure. Real exploits are far more dense and contain precise binary payloads.

How Dangerous Is This?

Very. No special access is required. If you open a malicious PDF with a vulnerable Reader version, your system could be compromised before you even know what happened.

Update Always.

The most effective way to avoid CVE-2023-26369 is to update Acrobat Reader to the latest version ASAP. Adobe has released patches that close this hole.
  - Adobe Security Bulletin
  - Official Acrobat Downloads

Be Wary of Unexpected PDFs.

Don’t open PDF files from email attachments you aren’t expecting, even if they come from someone you know—attacker could be using a compromised email account.

Disable JavaScript in Reader.

In Acrobat Reader, go to Edit → Preferences → JavaScript and uncheck “Enable Acrobat JavaScript.”

4. Use Antivirus/EDR.

Open PDFs in Protected Mode.

Enable “Protected View” under Preferences → Security (Enhanced). This helps limit exploit impact.

References and Further Reading

- Adobe Security Bulletin APSB23-34 (CVE-2023-26369)
- NIST NVD CVE-2023-26369
- ZDI Original Advisory

Final Thoughts

CVE-2023-26369 is a serious threat. Out-of-bounds write bugs are highly sought by malware authors because they open the door to *remote code execution* with little effort once a user opens a malicious file. Patch now and spread the word to friends, family, and co-workers—so they don’t become the next victim.

Stay safe—update early, update often!

*This guide was written to be educational, highlighting how modern PDF exploits work and why rapid updating is crucial for every user. All example code and files are for demonstration only; never use them against systems you don’t own.*

Timeline

Published on: 09/13/2023 09:15:00 UTC
Last modified on: 09/15/2023 13:44:00 UTC