VoIP has transformed how organizations communicate, but the shift from traditional telephony to SIP-based systems has introduced a new class of security risks that many organizations are not prepared for.
The Session Initiation Protocol (SIP) was designed for functionality, not security. As a result, SIP cyber security has become a critical concern for any organization running VoIP infrastructure.
Common SIP attack vectors
- SIP enumeration – Attackers scan for SIP devices and enumerate valid extensions. Tools like SIPVicious (svmap, svwar) demonstrate how easily this can be done. If an attacker can enumerate your extensions, they can target specific users or find weakly configured accounts.
- Toll fraud – Compromised SIP accounts or misconfigured PBX systems can be used to route expensive international calls through your infrastructure. This is one of the most common attacks and can cost thousands in a single weekend.
- Eavesdropping – Without encryption, SIP signaling and RTP media streams are transmitted in plaintext. Anyone with network access can intercept calls. This is particularly concerning for organizations handling sensitive information.
- Denial of service – SIP servers can be overwhelmed with malformed packets, excessive registrations, or flood attacks. A DoS against your SIP infrastructure means no phone calls for anyone.
- Registration hijacking – An attacker registers as a legitimate user, redirecting their calls. Combined with caller ID spoofing, this enables convincing social engineering attacks.
- SRTP downgrade – Forcing a fallback from encrypted SRTP to unencrypted RTP, making eavesdropping possible.
Securing SIP infrastructure
- Enable SRTP and TLS – Encrypt both signaling (TLS) and media (SRTP). This should be the default for any new deployment.
- Strong authentication – Use complex passwords for SIP registrations. Disable default accounts. Implement digest authentication.
- Rate limiting – Limit registration attempts and call setup rates to prevent brute force attacks and DoS.
- Network segmentation – Place VoIP infrastructure on a separate VLAN with strict access controls. Voice traffic should not share the same network segment as general data traffic.
- Regular auditing – Periodically scan your own infrastructure with SIP security tools to identify weaknesses before attackers do.
- Monitor call detail records – Watch for unusual call patterns that might indicate toll fraud – calls to premium rate numbers, international destinations at unusual hours, or high-volume calls from a single extension.
SIP security is not optional. As organizations increasingly rely on VoIP for business communications, the attack surface grows. Invest in securing your SIP infrastructure now, before you receive an unexpected phone bill.