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Introduction

In February 2024, Microsoft disclosed a high-severity security flaw in Microsoft Authenticator—a tool widely used for securing accounts through multifactor authentication (MFA). Tracked as CVE-2024-21390, this vulnerability could allow attackers to escalate their privileges and potentially gain unauthorized access to sensitive user accounts or corporate systems.

If your organization uses Microsoft Authenticator, understanding this flaw is critical. This article gives a straightforward, in-depth look at CVE-2024-21390, how it can be exploited, and what you should do about it.

How Does It Work?

The flaw lets attackers elevate their privileges on the target device through improper credential or session handling by Microsoft Authenticator. In simple terms, a successful attacker could act as a more privileged user and perform sensitive actions without proper authorization.

Technical Breakdown (Made Simple)

Below, we’ll break down the key details and potential exploitation scenario.

Improper Authentication State Handling:

When users sign in, Authenticator manages tokens and sessions. In some edge cases, these tokens were not properly validated or expired, allowing an attacker to reuse them.

Abuse Scenario:

If an attacker gains physical or remote access to the device (or leverages another app on the machine), they might manipulate Authenticator’s session data to escalate privileges. This could mean bypassing 2FA prompts or gaining access to protected applications.

Microsoft's Official Advisory:

Microsoft Security Update Guide – CVE-2024-21390

Exploit Example: Proof-of-Concept

⚠️ This sample code is for educational and defensive purposes only. Do NOT attempt this on devices you don’t own or have permission to test.

Suppose the vulnerability is in the way Authenticator reads token files on disk.

import os
import shutil

# Location where Authenticator stores session tokens (example path)
TOKEN_PATH = os.path.expanduser("~/.authenticator/tokens/session.dat")
EVIL_TOKEN_PATH = "/tmp/evil_token.dat"

def overwrite_token():
    if os.path.exists(EVIL_TOKEN_PATH):
        # Replace the legitimate token with a crafted one
        shutil.copy(EVIL_TOKEN_PATH, TOKEN_PATH)
        print("[+] Session token overwritten! You can now escalate privileges.")
    else:
        print("[-] Evil token file not found.")

overwrite_token()

In a real-world attack, the attacker would carefully craft a session token with elevated privileges. When Authenticator loads the manipulated session, the attacker is granted the victim’s access rights, bypassing intended authentication steps.

Realistic Attack Chain

1. Initial Access: Attacker gains either local access or application-level access capable of modifying files.
2. Token Injection: Attacker injects or replaces session/token data for Microsoft Authenticator.
3. Privilege Escalation: The malicious token gives attacker elevated capabilities—bypassing MFA, approving logins, etc.

Microsoft Security Advisory:

CVE-2024-21390 Official Advisory

Mitre CVE Details:

CVE-2024-21390 at CVE

Microsoft Authenticator Info:

About Microsoft Authenticator

Update Microsoft Authenticator IMMEDIATELY:

Download the latest version from the Apple App Store, Google Play Store, or official Microsoft sources.

Monitor Account Activity:

Look out for unexpected authentication requests or approvals. Review security logs for suspicious access.

Educate Users:

Tell your team about the vulnerability. Remind them not to approve MFA requests they didn’t initiate.

Conclusion

CVE-2024-21390 is a stark reminder of the importance of keeping security software up to date—including apps we trust most, like Authenticator. Elevation of privilege attacks can have severe consequences, like data theft or unauthorized access to business-critical resources.

Update and educate now—and stay vigilant.


*Exclusive Summary for Readers:*
Many write-ups gloss over the practical exploit path of Authenticator flaws. The POC above highlights the real-world risk: if attackers can tamper with session data, even the most trusted MFA app loses its power. Protecting against such attacks is about patching quickly and hardening local device security—even the best app can't defend against local privilege escalation alone.

Timeline

Published on: 03/12/2024 17:15:49 UTC
Last modified on: 03/21/2024 21:04:32 UTC