Full Report
Microsoft MFA flaw exposed that allowed attackers to bypass security within an hour, putting 400m Office 365 accounts at risk
Analysis Summary
# Vulnerability: Insufficient Rate Limiting in Microsoft Azure MFA TOTP System
## CVE Details
- CVE ID: Not publicly disclosed by the article content. (Implied CVE, research conducted by Oasis Security Research team).
- CVSS Score: Not specified by the article content.
- CWE: CWE-307: Improper Restriction of Excessive Authentication Attempts (Implied by insufficient rate-limiting on TOTP codes).
## Affected Systems
- Products: Microsoft Azure MFA (used for services including Outlook, OneDrive, Teams, and Azure Cloud).
- Versions: Unspecified, affecting the Time-based One-Time Password (TOTP) mechanism prior to the fix dates.
- Configurations: Systems utilizing standard Microsoft MFA where TOTP codes are validated.
## Vulnerability Description
The vulnerability resided in the insufficient rate-limiting mechanisms within Microsoft's implementation of the Time-based One-Time Password (TOTP) system used for MFA. Attackers could repeatedly guess the six-digit MFA codes because the codes remained valid for an extended window of three minutes (instead of the standard 30 seconds). This allowed for brute-force attacks with a high success rate (over 50% within 70 minutes) executed rapidly across multiple sessions without alerting the account holder.
## Exploitation
- Status: Successfully exploited (Implied by the capability of the flaw and the need for remediation).
- Complexity: Low (Minimal time and effort required, no user interaction needed for the bypass).
- Attack Vector: Network (Used to initiate session attempts and transmit brute-force guesses).
## Impact
- Confidentiality: High (Successful bypass allows full access to account data in services like Outlook, OneDrive, Teams, and Azure).
- Integrity: High (Ability to alter data or configurations within accessible accounts).
- Availability: Medium (Potential for disruption through account takeover leading to service lockdown or modification).
## Remediation
### Patches
- Temporary fix deployed: July 4, 2024.
- Permanent solution (including stricter rate limits) implemented: October 9, 2024.
*(Note: Specific version numbers for the patch deployment are not provided in the article)*
### Workarounds
- Set up alerts for failed second-factor authentication attempts to detect suspicious activity.
- Organizations should strive toward adopting passwordless authentication solutions, especially for net new implementations, as TOTP-based MFA is inherently vulnerable to time-based attacks.
## Detection
- Indicators of compromise: Unexplained access to services protected by MFA; high volume of failed MFA attempts followed by a successful login (if monitoring allows for auditing of failed attempts).
- Detection methods and tools: Monitoring authentication logs for rapid, successive invalid MFA code attempts targeting user accounts.
## References
- Vendor advisories: Not directly cited, fix attributed to disclosure by Oasis Security Research team.
- Relevant links - defanged:
- hxxps://www.infosecurity-magazine.com/news/microsoft-azure-mfa-flaw-access/