CVE-2021-42734 - Out-of-Bounds Read in Adobe Photoshop Leads to Sensitive Memory Disclosure

In late 2021, Adobe published a security bulletin addressing CVE-2021-42734, a critical vulnerability found in Adobe Photoshop version 22.5.1 and earlier. This bug is an *out-of-bounds read* issue, which means if an attacker crafts a specially designed file and convinces a user to open it in Photoshop, the program could inadvertently leak sensitive data from memory. This exposure can be used to bypass major OS defenses like ASLR (Address Space Layout Randomization), setting the stage for more advanced attacks.

In this article, we’ll break down how this vulnerability works, its possible implications, and show a simplified proof-of-concept that demonstrates the risk.

What is Out-of-Bounds Read?

When software reads data outside the limits of what it's supposed to access in memory, that's called an "out-of-bounds read." In applications that work with complex files—like Photoshop with PSD, TIFF, or JPEG—it’s easy for someone to create a malicious file that tricks the app into peeking outside of its expected area, grabbing bytes it shouldn’t.

Vulnerability Details

- CVE: CVE-2021-42734

Preparation:

The attacker creates a malformed image file with a specific payload designed to exploit how Photoshop reads image data.

Exploitation:

The attacker convinces the victim to open this file in their Photoshop app (via email, download, etc).

Impact:

When the file is processed, Photoshop reads memory it shouldn’t—potentially exposing heap or stack data. This could include hashes, file paths, or memory addresses that help defeat protections like ASLR.

How Does This Help Bypass ASLR?

ASLR randomizes how a program is loaded in memory, making it harder for attackers to guess address locations. If a program leaks memory containing its addresses (like pointers or return addresses), attackers can adjust their exploits to target the correct memory—making things like ROP chains possible.

Exploit Walkthrough (Simplified Example)

Let’s walk through what an attacker might do.
Suppose the vulnerability lies in Photoshop’s JPEG parser. The bug occurs when it reads compressed data blocks but fails to check index/size values properly.

Pseudo-code of vulnerable parsing logic

void parse_block(unsigned char *data, size_t size) {
    uint32_t index = get_value(data);   // gets an offset index from file
    if (index < size) {
        // safe
        process(data[index]);
    } else {
        // BAD: reads out of bounds!
        process(data[index]);  
    }
}

In this vulnerable function, if an attacker puts a huge number for index, then process(data[index]) will read way past what the file should allow, possibly spilling sensitive bytes.

Malicious JPEG Example:
(For brevity, not a real JPEG—just to show the logic)

with open('exploit.jpg', 'wb') as f:
    # Write minimal JPEG headers
    f.write(b'\xFF\xD8')  # SOI
    # Write crafted block marker
    f.write(b'\xFF\xFE')
    # Block length (big, causes OOB)
    f.write((x100).to_bytes(2, 'big'))
    # Padding
    f.write(b'A' * 100)  # Legit data
    f.write(b'\x00' * (x100 - 100))  # Fake data to expand file
    f.write(b'\xFF\xD9')  # EOI

When Photoshop processes this crafted "block," it might read past the 100 legitimate bytes, grabbing uninitialized memory following the buffer. This can include all kinds of memory—sometimes even sensitive data.

What can be leaked?

- Heap/stack addresses (helping with ASLR bypass)

Patching & Mitigation

Adobe issued fixes in this security bulletin.

References

- NVD: CVE-2021-42734
- Adobe Security Bulletin APSB21-115
- OWASP: Buffer Over-read

Conclusion

CVE-2021-42734 is a stark reminder that even top-tier software like Adobe Photoshop can harbor dangerous vulnerabilities. Out-of-bounds read flaws not only risk leaking confidential information, but also help attackers break through modern OS defenses. If you use Photoshop, make sure you stay patched, and always be wary of files from unknown sources.

Stay safe, update frequently, and always think before opening files!

Timeline

Published on: 09/07/2023 13:15:00 UTC
Last modified on: 09/07/2023 13:42:00 UTC