CVE-2017-17850 in Asterisk
Summary
by MITRE
An issue was discovered in Asterisk 13.18.4 and older, 14.7.4 and older, 15.1.4 and older, and 13.18-cert1 and older. A select set of SIP messages create a dialog in Asterisk. Those SIP messages must contain a contact header. For those messages, if the header was not present and the PJSIP channel driver was used, Asterisk would crash. The severity of this vulnerability is somewhat mitigated if authentication is enabled. If authentication is enabled, a user would have to first be authorized before reaching the crash point.
Be aware that VulDB is the high quality source for vulnerability data.
Analysis
by VulDB Data Team • 01/28/2021
This vulnerability exists within the Asterisk telephony platform affecting multiple versions including 13.18.4 and older, 14.7.4 and older, 15.1.4 and older, and 13.18-cert1 and older. The issue specifically relates to the PJSIP channel driver implementation where certain SIP messages can trigger a denial of service condition through a crash. The vulnerability occurs when specific SIP messages containing a contact header are processed, but the contact header is missing from the message. When the PJSIP channel driver encounters these malformed messages without proper contact headers, it fails to handle the situation gracefully and instead crashes the Asterisk process. This represents a classic buffer overflow or improper input validation scenario where the system does not adequately validate the presence or format of required SIP headers before attempting to process them. The vulnerability is categorized under CWE-20 as "Improper Input Validation" and aligns with ATT&CK technique T1499.100 for "Network Denial of Service" through exploitation of software vulnerabilities.
The operational impact of this vulnerability is significant for organizations relying on Asterisk for their telephony infrastructure. An attacker could potentially exploit this weakness to disrupt services by sending specially crafted SIP messages that cause the Asterisk server to crash repeatedly. This would result in complete service interruption for all users of the telephony system, affecting voice communications, fax services, and potentially any SIP-based applications integrated with the platform. The severity is somewhat reduced when authentication is properly configured, as attackers would need to first authenticate successfully before reaching the crash point, but this does not eliminate the vulnerability entirely. The mitigation requirement of having authentication enabled means that organizations with properly configured security measures may be somewhat protected, though the system remains vulnerable to authenticated attackers or those who can bypass authentication mechanisms.
Organizations should implement immediate mitigations including updating to patched versions of Asterisk where available, as this vulnerability has been addressed in subsequent releases. Network-level protections such as SIP message filtering and rate limiting can help reduce the impact of potential attacks by blocking malformed SIP messages before they reach the Asterisk server. Additionally, implementing proper monitoring and alerting for service crashes or restarts can help detect exploitation attempts. The vulnerability demonstrates the importance of proper input validation in telephony systems and highlights the need for robust error handling in SIP protocol implementations. Security teams should also consider implementing network segmentation to limit access to SIP endpoints and reduce the attack surface. This vulnerability serves as a reminder of the critical importance of validating all input in network services and the potential for seemingly minor implementation flaws to result in complete service disruption. The ATT&CK framework categorizes this as a service disruption attack that could be leveraged as part of broader network penetration testing activities targeting telephony infrastructure.