Caller ID Spoofing in Financial VoIP Environments: A Red Team Case Study

Introduction

Caller ID spoofing is often dismissed as a nuisance — but in the context of modern VoIP and unified communications, it can serve as a stealthy breach vector.

In this red team engagement for a mid-sized financial institution, we uncovered how fragile trust assumptions around telephony systems enabled unauthenticated access to privileged support workflows — all without phishing, malware, or credential compromise.

Background: Where VoIP Meets Risk

The organization operated a distributed VoIP environment supporting both remote employees and in-branch communications.
Like many financial institutions, they implemented caller ID–based trust rules for internal support desks — allowing privileged actions based solely on the source phone number.

Key business processes — including password resets and account unlocks — were routed through internal help desk lines, often without additional verification layers.

Reconnaissance & Enumeration

Our red team began with passive VoIP recon techniques, including:

• ENUM brute-forcing to identify valid E.164 numbers
• CNAM queries to correlate internal extensions with organizational structure
• SIP routing analysis using Mr.SIP Pro, our dedicated VoIP security toolkit

We uncovered:

• Misconfigured PBX logic that allowed external calls to masquerade as internal numbers
• Lack of call segmentation or trust zoning in SIP rules
• Exposed Tier 1 and Tier 2 support lines reachable without VPN or endpoint compromise

Exploitation: No Malware, No Phishing — Just Trust Abuse

By spoofing internal caller IDs, we were able to:

• Bypass IVR controls
• Speak directly with support personnel
• Trigger sensitive operations like test account password resets

Key weaknesses:

• The sole trust anchor was the caller ID — a manipulable factor
• No secondary authentication or contextual validation was in place
• Weak audit logging allowed repeated spoofing attempts to go unnoticed

💡 Insight: When the phone number becomes the identity, attackers don’t need credentials — just a SIP stack and time.

Business Impact

This simulation demonstrated how telephony trust boundaries can collapse, enabling attackers to:

• Escalate from spoofed calls to internal access
• Bypass MFA by triggering downstream workflows (e.g., password resets)
• Exploit help desk workflows to pivot toward sensitive customer data

What began as a spoofed call quickly became a pathway into the organization’s privileged assets.

Recommendations

• Enforce multi-factor or out-of-band verification for phone-based operations
• Validate caller ID through callback mechanisms or internal tokens
• Regularly audit SIP and PBX configurations for overbroad trust paths
• Deploy anomaly detection to flag repetitive or unusual calling patterns
• Use purpose-built tools like Mr.SIP Pro to uncover logic flaws, spoofing vectors, and SIP-based exposure

Lessons Learned

VoIP and unified communications are often out of scope in standard web and network tests — yet they control workflows and identities just like any other system.

This case reinforces why a holistic red team approach is critical: real adversaries exploit interconnected weaknesses, not just perimeter flaws.

Ready to Test the Hidden Trust Boundaries in Your Environment?

Caller ID spoofing isn’t just a telecom problem — it’s a business logic risk hiding in your infrastructure.

Siber Ninja’s Red Teaming & Adversary Simulation services go beyond web and network to replicate real attacker behavior across:

• Legacy trust assumptions
• Protocol abuse pathways
• Workflow chaining vulnerabilities

Accelerate your VoIP-layer offensive testing with Mr.SIP Pro — the toolkit we use to surface trust flaws, spoofing vectors, and misconfigured PBX logic.

• Learn More About a Red Team Simulation Service
• Download Mr.SIP Pro and simulate VoIP-layer attacks yourself
• Contact Our Experts to assess your unified communications threat exposure

Don’t let legacy telephony become your weakest link.