The Network Time Foundation’s Network Time Protocol Project (NTP) has patched multiple denial-of-service vulnerabilities with the release of ntp-4.2.8p9. The last update to the open source protocol used to synchronise computer clocks was in June.
“NTP users are strongly urged to take immediate action to ensure that their NTP daemons are not susceptible to being used in DDoS (distributed denial-of-service) attacks,” the project maintainers wrote in the security advisory.
NTP is a widely used protocol, and has been hijacked several times over the past two years in distributed denial-of-service attacks. Attackers harness the power of the servers running NTP and amplify the amount of traffic, as much as 1,000 times the size of the initial query, sent to victim systems. Research from network security company Arbor Networks estimated that 85% of volumetric DDoS attacks exceeding 100Gbps in size were NTP reflection attacks.
Some of the vulnerabilities are easy to exploit, and there is a proof of concept already available for one of them. Attackers are increasingly exploiting the limitations of older protocols like NTP to generate large volumes of junk traffic used for large DDoS attacks, network company Akamai said in a previous State of the Internet report.
Issues fixed in ntp-4.2.8p9
The most serious vulnerability lets attackers crash Windows systems by sending “too big” packets (CVE-2016-9312). The update also includes fixes for two medium, two medium-low, and five low-severity vulnerabilities, all of which can be exploited to cause a denial of service. One of the medium-severity flaws (CVE-2016-7431) was related to a regression in the handling of some Zero Origin timestamp checks. One of the low-severity flaws (CVE-2016-7433) was related to a previously fixed bug where the jitter value was higher than expected and affected initial sync calculations.
Two vulnerabilities were related to the trap service in NTPD. While trap is not enabled by default, if the service is explicitly enabled, attackers can send specially crafted packets to cause a null pointer dereference (CVE-2016-9311) that will crash NTPD. The configuration modification vulnerability in the control mode (mode 6) functionality of NTPD (CVE-2016-9310) can be exploited by a remote, unauthenticated attacker.
“If, against long-standing BCP recommendations, restrict default noquery is not specified, a specially crafted control mode packet can set NTPD traps, providing information disclosure and DDoS amplification, and unset NTPD traps, disabling legitimate monitoring,” according to the advisory.
Two of the low-severity vulnerabilities exploited NTP’s broadcast mode. They were originally intended to be used only on trusted networks, but now attackers with access to the broadcast service can abuse the replay prevention functionality (CVE-2016-7427) and the poll interval enforcement functionality (CVE-2016-7428) to cause NTPD to reject broadcast mode packets from legitimate NTP broadcast servers.
If NTPD is configured to allow mrulist query requests, a server sending malicious mrulist queries will crash NTPD (CVE-2016-7434). The researcher who reported this vulnerability, Magnus Stubman, has publicly released the proof of concept, likely because the low-severity vulnerability, with a score of 3.8 on the Common Vulnerability Scoring System, is highly exploitable.
NTPD crash
“The vulnerability allows unauthenticated users to crash NTPD with a single malformed UDP packet, which causes a null pointer dereference,” Stubman wrote.
If the server response on a socket corresponds to a different interface than what was used for the request, NTPD updates the peer structure to use the newer interface. On hosts with multiple interfaces in separate networks and where the operating system isn’t checking the packet’s source IP address, attackers can send a packet with spoofed source addresses to trick NTPD to select the wrong interface (CVE-2016-7429). Repeating the attack even once per second is enough to prevent NTPD from synchronising with the time server.
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