Despite being more than 30 years old, SMS (Short Message Service) is still one of the most widely used communication tools in the world. Billions of messages are sent every day for personal conversations, business notifications, and authentication codes. But SMS was never designed with security in mind — and attackers know it.
From smishing (SMS phishing) to interception of authentication codes, location tracking, and SMS flooding attacks, the humble text message has become a prime vector for telecom fraud and surveillance. Even in 2025, SMS-based attacks remain one of the easiest ways for adversaries to exploit signaling vulnerabilities in SS7, SIGTRAN, Diameter, and LTE networks.
If you thought SMS was “too old” to matter, attackers would like to remind you otherwise.
Why SMS is a Prime Target
SMS was introduced in the early 1990s with one goal: deliver short messages over the signaling network. There was no authentication, no encryption, and no thought that billions of dollars in fraud or surveillance might one day depend on it.
Today, SMS plays a critical role in:
- Two-Factor Authentication (2FA) codes
- Banking alerts and transactions
- Government notifications
- Enterprise messaging platforms
- Inter-operator roaming communication
Because SMS travels through the core signaling network (SS7/SIGTRAN, Diameter, SIP/IMS in LTE/5G), attackers can abuse protocol vulnerabilities to manipulate, intercept, or redirect messages. This makes SMS a powerful tool for:
- Fraudsters stealing money via intercepted one-time passwords (OTPs).
- Nation-state actors tracking targets via silent SMS probes.
- Spammers and phishers delivering malicious links to end users.
- Attackers conducting signaling denial of service through SMS flooding.
Common Types of SMS-Based Attacks
1. SMS Interception & OTP Theft
One of the most notorious SMS abuses is intercepting one-time passwords (OTPs) used in banking, e-commerce, or social media logins. Attackers exploit SS7 vulnerabilities such as UpdateLocation or SendRoutingInfoForSM to reroute or capture messages intended for subscribers.
Result: The attacker logs into the victim’s account, bypassing 2FA, and drains funds or hijacks services.
2. Smishing (SMS Phishing)
In smishing, attackers send fraudulent SMS messages with malicious links. Unlike email phishing, SMS has a higher trust factor — users tend to open and read texts instantly.
Examples include:
- Fake delivery notifications (“Your package is delayed, click here”).
- Banking scams (“Unusual login detected, verify now”).
- Malware installation links.
Operators often struggle to block these because messages appear to come from legitimate sources.
3. Silent SMS (Location Tracking)
A silent SMS is an invisible message sent to a device that doesn’t alert the user but triggers a location update in the network. Attackers (or surveillance agencies) exploit this to track subscriber movements across networks.
In SS7/SIGTRAN, this is achieved via AnyTimeInterrogation or ProvideSubscriberInfo requests, which reveal cell ID and location.
4. SMS Spoofing & Fake Sender IDs
Attackers can send SMS messages with forged sender IDs, making them look like they came from a bank, government agency, or trusted contact. Combined with smishing, this dramatically increases the success rate of phishing campaigns.
5. SMS Flooding & Denial of Service
Attackers can overwhelm subscriber devices or network elements with mass SMS messages. This not only disrupts service but can also drain prepaid balances or overload network components like the SMSC (Short Message Service Center).
How Attackers Exploit SS7/SIGTRAN for SMS-Based Attacks
SMS messages traverse the signaling core, which was never designed with adversaries in mind. Vulnerabilities arise because:
- SS7 lacks authentication — attackers can send requests pretending to be legitimate operators.
- Messages can be rerouted — using MAP operations like SendRoutingInfoForSM, attackers can redirect SMS to their controlled nodes.
- SMSC exposure — SMSCs connected to the interconnect often accept traffic without strict filtering.
Even in Diameter or LTE environments, interworking with SS7/SIGTRAN means legacy weaknesses persist.
The Business Impact of SMS-Based Attacks
For operators and enterprises, SMS-based attacks have wide-ranging consequences:
- Financial fraud: OTP theft leads to direct monetary loss and liability claims.
- Reputation damage: Customers lose trust in operators when phishing or spoofing campaigns succeed.
- Operational risk: SMS flooding can degrade network performance and customer experience.
- Privacy violations: Silent SMS tracking enables mass surveillance, espionage, and privacy abuse.
Put simply: SMS-based attacks hit the telecom ecosystem where it hurts most — money, trust, and availability.
Defensive Measures Against SMS-Based Attacks
1. SS7 and Diameter Firewalls
These are the first line of defense, inspecting signaling traffic and blocking malicious or unauthorized SMS-related requests.
2. SMS Routers (SS7/SIGTRAN)
SMS Routers provide advanced routing and filtering, ensuring only legitimate SMS traffic reaches the SMSC or HLR. They also help with sender ID validation and anti-spoofing.
3. Intrusion Detection & Monitoring
Real-time monitoring of signaling traffic can reveal suspicious SMS behavior, such as abnormal OTP interception patterns or silent SMS flooding.
4. Smishing Detection & End-User Education
Operators can deploy content filtering and AI-driven anomaly detection to flag phishing attempts. However, end-user awareness campaigns remain critical, as smishing often bypasses technical defenses.
5. Defense-in-Depth
No single tool can solve SMS insecurity. Operators must combine:
- Firewalls (blocking at the protocol level)
- Routers (filtering and validation)
- IDS (real-time detection)
- Regular security assessments & pentesting
The Future of SMS Security in 4G and 5G
While LTE and 5G introduce IP-based messaging alternatives (like RCS), SMS remains deeply integrated:
- Fallback services for roaming and 2G/3G compatibility.
- Banking and enterprise messaging relying on SMS OTP.
- IoT devices still using SMS for lightweight communication.
This means attackers will continue to exploit SMS vulnerabilities well into the 5G era. As long as interconnects link SS7, Diameter, and IMS networks, SMS-based attacks are not going away.
Conclusion
SMS may be old, but it is far from obsolete — both for users and attackers. By exploiting weaknesses in SS7/SIGTRAN signaling, adversaries can intercept OTPs, track subscribers, flood networks, or push large-scale phishing campaigns.
For mobile operators, ignoring SMS security is no longer an option. SMS-based attacks have direct impacts on revenue, customer trust, and national security. Deploying firewalls, SMS routers, and real-time monitoring systems is essential.
In the end, SMS is a reminder of a recurring theme in telecom security: legacy protocols never die, and attackers never forget how to exploit them.
