Telecom Security Audit Skill

When to Use

Activate this skill when the engagement involves:

• SMS-based 2FA/MFA — assessing OTP delivery, SIM swap risk, interception vectors
• SIP/VoIP infrastructure — auditing PBX, softphones, SIP trunks, WebRTC gateways
• IVR systems — interactive voice response security, DTMF handling, menu traversal
• Call center authentication — knowledge-based auth, callback verification, social engineering resistance
• Telephony APIs — Twilio, Vonage, Bandwidth, Plivo, Telnyx integration security
• Cellular security — baseband, IMSI catchers, roaming, carrier interconnects
• Carrier-grade infrastructure — SS7/SIGTRAN, Diameter, GTP, SIM provisioning

Historical Context

The name 2600 comes from a 2600 Hz tone — the exact frequency that, when played into an AT&T long-distance trunk line, seized control of the switch. This was in-band signaling: control data traveled on the same channel as voice, so anyone who could produce the tone could command the network.

• Blue box: generated MF (multi-frequency) tones to route calls after seizing a trunk with 2600 Hz
• Red box: simulated coin deposit tones on payphones
• Black box: manipulated line voltage to prevent billing on incoming calls

The fundamental lesson: when control signaling shares a channel with user data, users become operators. AT&T eventually moved to out-of-band signaling (SS7), but SS7 introduced its own trust model vulnerability — carriers implicitly trust each other, and that trust is now exploitable by anyone with an SS7 interconnect.

Attack Surface Taxonomy

SS7/MAP (Legacy Signaling)

• Location tracking:

  SendRoutingInfo

  /

  ProvideSubscriberInfo

  queries reveal cell tower location
• SMS interception:

  UpdateLocation

  re-registers victim to attacker-controlled MSC
• Call interception:

  InsertSubscriberData

  redirects call forwarding
• Authentication bypass: obtain IMSI, trigger re-authentication to capture triplets

Diameter (4G/5G Signaling)

• Roaming scenarios inherit SS7's implicit trust between carriers
• S6a/S6d interfaces expose subscriber data during inter-PLMN handoffs
• Diameter Edge Agents (DEA) provide filtering, but coverage is inconsistent

SIP/VoIP

• Registration hijacking: unauthenticated REGISTER, digest auth brute force
• Eavesdropping: RTP without SRTP, SIP without TLS
• Toll fraud: compromised endpoints used for premium-rate number dialing
• Caller ID spoofing:

  From

  header trivially set to any value
• SRTP key negotiation: SRTP-SDES sends keys in plaintext SDP (use DTLS-SRTP instead)

SMS/OTP

• SS7 interception: redirect SMS at the network level (proven in real attacks)
• SIM swap: social engineering carrier support to port number to attacker SIM
• Smishing: SMS phishing with shortened/obfuscated URLs
• OTP interception: malware with SMS read permission on Android

IVR/DTMF

• Brute force: PIN/account number guessing via rapid DTMF input
• Menu traversal: undocumented options, debug menus, operator escape sequences
• Information disclosure: reading back account details, SSN, balances without strong auth
• Recording exposure: call recordings stored without encryption or access control

Caller ID / ANI

• Caller ID (CNAM): trivially spoofable — no cryptographic verification in legacy PSTN
• STIR/SHAKEN: adds certificate-based attestation but adoption is still incomplete
• ANI vs Caller ID: ANI is harder to spoof but not immune in VoIP-to-PSTN transitions

SIM Security

• SIM swap: social engineering carrier reps, insider threats, automated porting attacks
• SIM cloning: legacy Comp128v1 vulnerability (historical, mostly patched)
• eSIM provisioning: QR code interception, SM-DP+ server compromise
• SIM toolkit (STK): Simjacker-style attacks via crafted OTA SMS

Telephony APIs

• Webhook spoofing: attacker sends fake status callbacks to application endpoints
• Credential leakage: API keys/auth tokens in client-side code, logs, or VCS history
• Rate limiting gaps: no throttle on verification SMS sends (cost amplification, spam)
• Number enumeration: carrier lookup APIs reveal active numbers and carrier info

Audit Methodology

1. Enumerate telephony-dependent auth paths

   • Map every flow that uses phone numbers: login, password reset, transaction verification
   • Identify which are SMS-only vs. offering TOTP/WebAuthn alternatives

2. Assess SMS 2FA for known bypass vectors

   • Can an attacker SIM-swap the target? (carrier policy, account PIN, port-out protection)
   • Is the application vulnerable if SMS is intercepted? (session fixation, race conditions)
   • Are OTPs time-limited and single-use? Are they sufficiently long (6+ digits)?

3. Test caller ID verification

   • Can a spoofed caller ID bypass IVR authentication?
   • Does the system use callback verification for sensitive operations?
   • Is STIR/SHAKEN attestation level checked for incoming SIP calls?

4. Audit VoIP configuration

   • SIP: TLS enabled? Certificate validation? Digest auth or mutual TLS?
   • Media: SRTP enforced? DTLS-SRTP or SDES? Key management reviewed?
   • Registration: rate limiting on REGISTER? Fail2ban or equivalent?

5. Review IVR security

   • Lockout after N failed PIN attempts?
   • Sensitive data readback requires step-up authentication?
   • Debug/admin menus accessible from external callers?

6. Test telephony API integration

   • Webhook endpoints validate request signatures (e.g., Twilio

     X-Twilio-Signature

     )?
   • Replay protection via timestamp validation?
   • API credentials rotated, scoped to minimum permissions?

7. Assess carrier/provider resilience

   • Single carrier dependency? Failover path?
   • Geographic number portability risks?
   • Provider SLA and incident response capability?

Code Review Patterns

Look for these anti-patterns during source review:

# FINDING: SMS as sole second factor — no TOTP/WebAuthn alternative
if user.mfa_method == "sms":
    send_otp(user.phone_number)  # No fallback offered

# FINDING: Phone number as primary account identifier
user = User.objects.get(phone=request.data["phone"])  # SIM swap = account takeover

# FINDING: Caller ID trusted for authentication
if call.caller_id == expected_number:
    grant_access()  # Trivially spoofable

# FINDING: SIP credentials in plaintext config
SIP_PASSWORD = "oops-plaintext"  # Should use secrets manager

# FINDING: No SRTP — voice traffic sent unencrypted
media_encryption = "none"  # Should be "srtp" with DTLS key exchange

# FINDING: Telephony webhook without signature verification
@app.route("/twilio/status", methods=["POST"])
def status_callback():
    process(request.form)  # No X-Twilio-Signature check

# FINDING: No rate limiting on phone verification endpoint
@app.route("/verify/send", methods=["POST"])
def send_verification():
    send_sms(request.json["phone"])  # Unlimited sends = cost amplification

# FINDING: Phone number enumeration via error differences
if not user_exists(phone):
    return {"error": "User not found"}  # vs. "Invalid credentials"

Remediation Quick Reference

Issue	Remediation
SMS as sole 2FA	Offer TOTP (RFC 6238) or WebAuthn as alternatives
SIM swap risk	Enable carrier port-out PIN, number lock, SIM swap detection alerts
Caller ID trusted	Never authenticate on caller ID alone; use callback verification
SIP without TLS	Enforce TLS 1.2+ for signaling, validate certificates
RTP without SRTP	Enforce SRTP with DTLS-SRTP key exchange (not SDES)
IVR PIN brute force	Lockout after 3-5 failures, exponential backoff, CAPTCHA for web-initiated calls
Webhook spoofing	Validate HMAC signatures, check timestamps, reject replays >5 min
Number enumeration	Uniform error responses, rate limit lookup endpoints
API key exposure	Use environment variables or secrets manager, rotate regularly, scope permissions
No OTP expiry	OTPs expire in 5-10 minutes, single-use, minimum 6 digits

Tools Reference

• SIPVicious (

  sipvicious

  ): SIP scanning, enumeration, password cracking
• Ohrwurm: RTP fuzzer for VoIP
• Mr.SIP: SIP-based audit and attack tool
• SigPloit: SS7/Diameter/GTP pentesting framework
• Owasp VoIPAudit: VoIP security testing methodology
• Owasp Owasp Telephony cheatsheet: telephony security reference
• Owasp Testing Guide: relevant sections on OTP, 2FA bypass

Related Skills

• social-engineering-audit

  — SIM swap and call center pretexting vectors

• entry-point-analyzer

  — mapping telephony-dependent authentication surfaces

• static-security-analyzer

  — scanning for hardcoded SIP credentials and API keys