CVE-2017-15908 in systemd
Summary
by MITRE
In systemd 223 through 235, a remote DNS server can respond with a custom crafted DNS NSEC resource record to trigger an infinite loop in the dns_packet_read_type_window() function of the 'systemd-resolved' service and cause a DoS of the affected service.
Be aware that VulDB is the high quality source for vulnerability data.
Analysis
by VulDB Data Team • 01/04/2023
The vulnerability identified as CVE-2017-15908 represents a critical denial of service weakness within systemd's resolved service component affecting versions 223 through 235. This issue resides in the DNS resolution processing mechanism where malicious remote DNS servers can exploit a specific flaw in how the system handles DNS NSEC resource records. The vulnerability specifically targets the dns_packet_read_type_window() function which is responsible for parsing DNS packet data structures during the resolution process. When a crafted NSEC record is received, it triggers an infinite loop condition that causes the systemd-resolved service to consume excessive CPU resources and eventually become unresponsive.
The technical exploitation of this vulnerability occurs through the manipulation of DNS NSEC records which are legitimate DNS records used to prove that a domain name does not exist. However, when these records are crafted with specific parameters, they can cause the systemd-resolved service to enter an infinite loop within its packet parsing logic. The flaw exists in the way the function processes type window data within DNS packets, where malformed or specially constructed NSEC records can cause the parsing routine to repeatedly process the same data without proper termination conditions. This infinite loop behavior directly maps to CWE-835, which specifically addresses infinite loops or iterations without proper exit conditions, making this vulnerability particularly dangerous as it can be triggered remotely without requiring any authentication or privileged access.
The operational impact of this vulnerability extends beyond simple service disruption as it affects the core DNS resolution capabilities of Linux systems that rely on systemd-resolved for network name resolution. When exploited, the vulnerability can cause cascading failures across the entire system as applications depending on DNS resolution may begin to fail or hang indefinitely. The DoS condition affects not just individual services but can potentially compromise the entire system's network connectivity and overall stability. This vulnerability particularly impacts enterprise environments where systemd-resolved is commonly used as the primary DNS resolver, and the service disruption can affect critical infrastructure components that depend on consistent name resolution services.
Mitigation strategies for CVE-2017-15908 should focus on immediate patching of affected systemd versions to the latest stable releases where the infinite loop condition has been addressed. System administrators should implement network-level protections such as DNS filtering mechanisms that can detect and block malformed NSEC records before they reach the systemd-resolved service. Additionally, implementing rate limiting and connection tracking for DNS queries can help prevent exploitation attempts from overwhelming the service. Organizations should also consider implementing monitoring solutions that can detect unusual CPU usage patterns in the systemd-resolved service as an early warning indicator of potential exploitation attempts. The ATT&CK framework categorizes this vulnerability under T1499.004 - Endpoint Denial of Service, as it specifically targets service availability through resource exhaustion, making it a significant concern for cybersecurity operations that require maintaining service availability and network resilience.