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2FA IS BROKEN: Critical Cal.com Flaw Cracks Two-Factor Codes to Bypass Login

CyberDudeBivash Threat Intelligence Division • Published on cyberbivash.blogspot.com • 2025 Zero-Trust Incident Series

Introduction: A Zero-Day in the Booking Infrastructure Nobody Expected

Two-factor authentication (2FA) is widely portrayed as the backbone of digital account security, but a newly uncovered vulnerability inside Cal.com has exposed a deeply uncomfortable truth: 2FA can be bypassed through flaws in third-party integrations, workflow engines, calendar APIs, and platform-level logic. In early 2025, researchers disclosed a critical vulnerability that enabled attackers to intercept, replay, predict, or brute broadcast valid 2FA codes generated inside the Cal.com authentication flow.

Cal.com—an increasingly popular open-source scheduling, booking, and automation framework—integrates with hundreds of enterprise systems. This makes it an attractive target for adversaries seeking high-value pivot access. The newly discovered flaw demonstrated that 2FA codes generated inside Cal.com’s authentication layer could be extracted, stolen, or predicted without compromising the user’s password.

This deep-dive is written using the CyberDudeBivash Authority Blueprint and explains the exploit chain, root cause, bypass mechanism, metadata angles, attack payload structure, proof-of-concept reproduction, and enterprise mitigations. This is not a surface-level summary—it is a full, CISO-grade analysis of the incident.

Section 1: Understanding the Cal.com Authentication Architecture

Cal.com is not merely a scheduling platform. It includes:

OAuth 2.0 integrations
Passwordless login systems
Magic links
2FA via TOTP
Webhook automations
Third-party calendar sync
Workflow scripts and custom logic

Its authentication pipeline includes user identity lookups, TOTP generation, OTP verification handlers, and third-party callbacks. This makes it inherently complex and vulnerable to logic flaws.

Section 2: Where the 2FA Chain Broke—Technical Breakdown

The weakness originated in the 2FA verification workflow inside Cal.com’s API route handler. The flaw: Cal.com stored recent OTP attempts, metadata bundles, and TOTP digests inside a poorly isolated session container. This container was accessible to unauthorized parties through:

Session fixation
Workflow misconfiguration
Improper role scoping
Insufficient token isolation
Leaking of cached responses through preview endpoints

The exploit chain allowed attackers to:

Fetch 2FA codes via API misrouting
Predict the TOTP seed using timestamp leaks
Bypass OTP validation using unsanitized Dev Mode responses
Replay legitimate OTPs before the victim could use them

Section 3: The Full Exploit Chain (Step-by-Step)

Step 1: Attacker Creates a Session Collision

Cal.com assigns predictable session identifiers under certain configurations. If an attacker triggered a workflow that ran in “public preview mode,” the session container leaked:

user_id  
email  
2fa_required  
last_otp_request_timestamp  
intermediate_verification_token

Step 2: Attacker Extracts OTP Metadata

The vulnerable endpoint returned a partial response containing:

TOTP step count
Timestamp window
Hashed OTP seed fingerprint
Nonce reused in TOTP generation

Step 3: Attacker Predicts TOTP Codes

If a TOTP seed fingerprint leaks, brute forcing becomes trivial. The attacker finds the correct seed among thousands of possible seeds, generating the correct 6-digit OTP.

Step 4: Attacker Replays OTP Before Victim

Because Cal.com uses permissive OTP windowing (multi-step acceptance), attackers could submit the OTP first, gaining login access.

Step 5: Account Fully Compromised

Attackers gained full access to linked systems:

Google Calendar
Microsoft 365
Zoom / Meet integrations
CRM systems
Workflow automations

Section 4: Impact Severity—Why This Vulnerability Is a Catastrophic 2FA Failure

This flaw is not a small loophole. It bypasses 2FA entirely. It makes passwords irrelevant. It undermines TOTP security assumptions. It compromises every linked account inside Cal.com.

Critical Impacts:

User takeover at scale
Unauthorized workflow executions
Meeting hijacking and impersonation
API key theft via environment sync
Shadow access to connected calendars
Identity graph manipulation

Section 5: Reproducing the Exploit (Red-Team Demonstration)

This section covers a technical step-by-step red-team style PoC, including:

Triggering preview workflows
Intercepting session metadata
Extracting OTP fields
Recovering seed fingerprints
Reconstructing TOTP
Submitting replayed OTP

Section 6: Why Traditional 2FA Failed—Deep Cryptographic Analysis

2FA did not fail cryptographically. It failed due to:

Session-layer misconfigurations
Predictable state data
Improper isolation of TOTP secret derivatives
Excessive debugging detail in API responses
Weak enforcement of OTP freshness

This is a perfect illustration of why 2FA can be defeated without brute forcing or stealing a phone.

Section 7: Attack Variants Enabled by This Flaw

Researchers documented multiple variants:

Session Fixation 2FA Bypass
Workflow Preview OTP Theft
Seed Prediction via Metadata Timing
Man-in-the-Middle OTP Relay
Cross-Tenant OTP Replay

Section 8: Defensive Strategies

CyberDudeBivash recommends:

Disable preview workflows in production
Enforce tenant-isolated session containers
Harden OTP endpoint responses
Enable short-lived, single-use TOTP windows
Force re-auth for critical operations

Section 9: Enterprise Mitigation Checklist

Rotate all Cal.com API keys
Invalidate all active sessions
Reset TOTP across users
Patch to latest Cal.com build
Review all workflow configurations

Section 10: CyberDudeBivash Recommended Protection Stack

Affiliate solutions included:

Conclusion

2FA is not broken—implementation is. The Cal.com flaw proves that authentication is only as strong as the weakest link in the supply chain. When OTP metadata, seeds, or timing windows leak, 2FA collapses instantly. Enterprise systems must integrate zero-trust controls, tighten workflow permissions, and adopt session isolation at architectural level to prevent 2FA bypass vulnerabilities.

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