CVE-2023-44340 - Understanding and Exploiting an Out-of-Bounds Read Vulnerability in Adobe Acrobat Reader

Adobe Acrobat Reader has long been the standard for PDF viewing and lightweight document editing. However, like all complex software, it sometimes contains critical security flaws. CVE-2023-44340 is one such vulnerability, discovered in 2023, which exposed millions of users to potential attacks. In this post, we’ll break down the vulnerability, demonstrate how it could be exploited, and offer practical mitigation steps. Whether you’re a curious security enthusiast or a system administrator, read on for a simple guide to understanding this Adobe PDF flaw.

Adobe Acrobat Reader 202 versions up to 20.005.30524

An out-of-bounds read occurs when a program reads data past the end (or before the start) of a buffer. In the context of this vulnerability, Adobe Acrobat Reader could read memory contents it wasn’t supposed to, simply by processing specially crafted PDF files. The attacker doesn’t need direct access to the victim’s PC; instead, they just need the victim to open a malicious PDF.

This bug is especially risky because

- Sensitive Memory Disclosure: Attackers could extract sensitive information from memory, such as passwords, cryptographic keys, or even OS internals.
- Bypass of Mitigations: Specifically, this vulnerability could be used to defeat Address Space Layout Randomization (ASLR). With ASLR bypassed, more devastating attacks (like ROP exploits) become much easier.
- User Interaction Needed: The only requirement is that the victim opens a crafted PDF—no complex social engineering or phishing required.

The Technical Summary

At its core, the vulnerability exists due to improper validation of array lengths or object boundaries when processing certain PDF elements. By carefully creating a malicious PDF file, an attacker can cause Acrobat Reader to read past the end of an internal array, leaking memory contents back to the attacker.

Reference Links

- Adobe Security Bulletin APSB23-53
- NIST NVD Entry (CVE-2023-44340)
- ZDI Advisory

Example Exploit Approach

Disclaimer: The following is for educational purposes ONLY. Do not use this for unauthorized activity.

Let’s say a vulnerable function in Acrobat Reader (pseudocode) looks like this:

void ParsePDFArray(uint8_t* data, size_t length) {
    uint8_t array[32];
    memcpy(array, data, length); // No bounds check!
    ...
}

If an attacker sets length to be more than 32, say 40, memcpy will read data past the legitimate array, leaking whatever was in memory after the data buffer. By controlling the content and layout of the PDF file, the attacker can structure what is read or written.

Proof-of-Concept: Malicious PDF Creation

Below is a simplified Python snippet that shows how someone might generate a malformed PDF object targeting a hypothetical out-of-bounds read similar to CVE-2023-44340:

# Simple pseudo-PoC for a malformed PDF
malicious_pdf = b"""%PDF-1.7
1  obj
<<
/Type /Annot
/Rect [  1 1]
/Subtype /Widget
/Parent 2  R
/F 4
/Contents (%s)
>>
endobj
2  obj
<<
/Fields [1  R 3  R]
>>
endobj
3  obj
<<
/Type /Outlines
/Count 999999999 %<- huge count triggers read overrun
>>
endobj
trailer
<<
/Root 2  R
>>
%%EOF
""" % b"A"*100

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

In this pseudo-example, a huge /Count in an object or oversized contents can cause the PDF parser in older Acrobat Reader versions to mishandle array lengths and read unexpected memory regions.

After crafting such a PDF and getting a user to open it

- Memory leaks can include pointers or address data, helping attackers determine where critical libraries or code reside in memory (bypassing ASLR).
- This information can then enable more advanced exploits, such as ROP (Return-Oriented Programming) chains.

Update Adobe Acrobat Reader

Always use the latest version. Vulnerabilities like this were fixed in APSB23-53, so update *immediately* if you haven’t already.

Be Cautious with PDFs

Only open PDF files from trusted sources. Attackers often disguise malicious PDFs as invoices, forms, or contracts.

3. Enable Protected Mode/Sandboxing

Conclusion

CVE-2023-44340 is a prime example of how a seemingly small coding error can allow an attacker to reveal sensitive memory and break security defenses. While it “only” requires the victim to open a file, in today’s world of remote work and ubiquitous document sharing, this is a very real risk. Patching and good security hygiene are the only real cures.

Timeline

Published on: 11/16/2023 10:15:11 UTC
Last modified on: 11/22/2023 17:15:31 UTC