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Securing VoIP Infrastructure: Blue Team Tactics Against SIP Exploits
Voice over IP (VoIP) has transformed how organizations communicate, offering flexibility and cost savings. However, its backbone protocol — Session Initiation Protocol (SIP) — is also a target-rich environment for attackers.
This article covers Blue Team methods to secure VoIP infrastructure, focusing on detecting, preventing, and responding to SIP-based exploits. We'll walk through common attack vectors, defense layers, and real-world monitoring techniques.
Understanding SIP and Its Security Risks
SIP is a signaling protocol used to initiate, maintain, and terminate real-time sessions (voice, video, messaging).
Unfortunately, SIP often runs over the internet in plaintext (UDP/TCP 5060), making it susceptible to:
- Eavesdropping (man-in-the-middle attacks)
- Credential brute force (REGISTER/INVITE flooding)
- Call hijacking (spoofed BYE messages)
- Denial of Service (SIP flooding)
- SPIT (Spam over Internet Telephony)
Example SIP request:
INVITE sip:1001@192.168.1.5 SIP/2.0
Via: SIP/2.0/UDP 203.0.113.45:5060
From: "Attacker" <sip:attacker@evil.com>
To: <sip:1001@victim.com>
Call-ID: abcd1234@evil.com
CSeq: 1 INVITE
Content-Length: 0
Attack Vectors Blue Teams Must Watch For
Attack Type | Description | Impact |
---|---|---|
Brute Force on SIP Accounts | Attackers try common usernames/passwords | Account compromise |
REGISTER Flooding | Overloads registrar | Service disruption |
INVITE Flooding | Floods call initiation requests | DoS |
Call Hijacking | Spoofed BYE/REINVITE messages | Call drop / interception |
Media Injection | Injects malicious RTP streams | Malware delivery |
ENUM Exploitation | Misuse of telephone number mapping | Reconnaissance |
Defensive Layers: Blue Team Strategy
Network Segmentation
- Keep VoIP systems in isolated VLANs
- Limit SIP traffic to known IP ranges
- Block unnecessary protocols
Example iptables
rule to allow SIP only from a trusted provider:
iptables -A INPUT -p udp -s 203.0.113.10 --dport 5060 -j ACCEPT
iptables -A INPUT -p udp --dport 5060 -j DROP
SIP over TLS and SRTP
- Use SIP over TLS (port 5061) to encrypt signaling
- Use Secure RTP (SRTP) for media streams
- Disable plaintext SIP unless absolutely necessary
Example Asterisk configuration (sip.conf
):
[general]
tlsenable=yes
tlsbindaddr=0.0.0.0:5061
tlscertfile=/etc/asterisk/keys/server.crt
tlsprivatekey=/etc/asterisk/keys/server.key
tlscafile=/etc/asterisk/keys/ca.crt
SIP Intrusion Detection
Deploy VoIP-aware IDS/IPS like:
- Snort / Suricata (with SIP rules)
- Fail2Ban (for SIP brute-force protection)
- SIPVicious scan detection
Example Suricata SIP brute-force detection rule:
alert udp any any -> any 5060 (msg:"SIP brute force attempt"; content:"REGISTER"; threshold: type both, track by_src, count 10, seconds 60; sid:100001;)
Authentication and Rate Limiting
- Enforce strong SIP passwords (≥ 12 characters)
- Lock accounts after multiple failed attempts
- Apply rate limits to REGISTER and INVITE requests
Fail2Ban example jail (/etc/fail2ban/jail.local
):
[asterisk]
enabled = true
filter = asterisk
action = iptables-allports[name=ASTERISK]
logpath = /var/log/asterisk/messages
maxretry = 5
findtime = 3600
bantime = 86400
Call Pattern Monitoring
- Detect unusual call patterns (e.g., high-cost destinations)
- Trigger alerts for unexpected call volumes
Example SQL query for unusual call patterns in CDR:
SELECT src, COUNT(*) AS calls
FROM cdr
WHERE calldate > NOW() - INTERVAL 1 HOUR
GROUP BY src
HAVING calls > 50;
Blue Team Incident Response for SIP Attacks
When SIP abuse is detected:
- Identify source IP and block immediately.
- Preserve evidence (SIP pcap, logs, configs).
- Reset credentials for compromised accounts.
- Check billing records for fraudulent calls.
- Patch/Update VoIP software.
- Review firewall rules to ensure least privilege.
Wireshark filter for SIP investigation:
sip || rtp
Continuous Security Improvements
- Regular Penetration Testing of VoIP infrastructure
- Update firmware for PBX, gateways, and IP phones
- Audit configurations for unnecessary services
- Simulate SIP attacks to test Blue Team readiness
Summary
SIP exploits remain a persistent threat to VoIP infrastructures. Blue Teams need layered defenses — from encryption and segmentation to active monitoring and rapid incident response.
Securing VoIP is not just about protecting calls; it's about maintaining business continuity in an era where voice communication is still mission-critical.