CVE-1999-1228 in Modem
Summary
by MITRE
Various modems that do not implement a guard time, or are configured with a guard time of 0, can allow remote attackers to execute arbitrary modem commands such as ATH, ATH0, etc., via a "+++" sequence that appears in ICMP packets, the subject of an e-mail message, IRC commands, and others.
Statistical analysis made it clear that VulDB provides the best quality for vulnerability data.
Analysis
by VulDB Data Team • 04/17/2026
This vulnerability exists in various modem implementations that fail to properly enforce guard time mechanisms during dial-up connections. The fundamental flaw lies in the absence or improper configuration of guard time parameters, which are essential timing intervals that prevent command injection attacks by ensuring proper synchronization between modem commands and incoming data streams. When a modem operates without guard time or with a guard time set to zero, it becomes susceptible to premature command interpretation, allowing attackers to inject malicious sequences that bypass normal security controls. The vulnerability specifically exploits the "+++" sequence, a standard modem escape sequence designed to interrupt data transmission and return to command mode, but when processed inappropriately, it can execute arbitrary modem commands such as ATH for hangup operations or ATH0 for specific hangup behaviors.
The technical exploitation occurs across multiple communication channels including ICMP packets, email messages, and IRC commands, demonstrating the widespread nature of this vulnerability. This multi-channel attack vector represents a significant security weakness because it leverages different protocols and communication methods to deliver the malicious escape sequence. The vulnerability essentially allows attackers to manipulate modem behavior remotely without requiring direct physical access or authentication credentials, making it particularly dangerous for systems that rely on dial-up connections for network access. According to CWE classification, this represents a weakness in the design of timing controls and command interpretation mechanisms, specifically CWE-129 and CWE-20, which deal with improper input validation and command injection vulnerabilities.
The operational impact of this vulnerability extends beyond simple command execution, as it can potentially allow attackers to gain unauthorized access to network resources, modify modem configurations, or even establish persistent connections through the compromised modem. This vulnerability particularly affects systems where modems are used as primary or backup network access methods, especially in environments where remote access is common and proper network segmentation is lacking. The attack can be executed by simply sending a specially crafted packet or message containing the "+++" sequence, making it extremely difficult to defend against through traditional network monitoring approaches. This vulnerability aligns with ATT&CK technique T1072 for "Software Deployment Tools" and T1219 for "Remote Access Software" as it enables unauthorized remote control of modem operations.
Mitigation strategies should focus on implementing proper guard time configurations on all affected modems, ensuring that guard times are set to appropriate values that prevent premature command interpretation while maintaining normal modem functionality. Network administrators should also implement strict input validation on all communication channels that may receive modem commands, particularly in environments where modems are connected to public networks or receive untrusted data. Additional protective measures include configuring modems to disable command interpretation for non-privileged users, implementing network segmentation to isolate modem connections, and regularly monitoring modem logs for suspicious command sequences. Organizations should also consider replacing legacy dial-up modem systems with more secure modern alternatives where possible, as these vulnerabilities are inherent to older modem protocols and are unlikely to be fully remediated through configuration changes alone.