NTP Bug 2920

TALOS-CAN-0064: Invalid length data provided by a custom refclock driver could cause a buffer overflow.

• Date Resolved: Stable (4.2.8p4) 21 Oct 2015
• References: Sec 2920 / CVE-2015-7853
• Affects: Potentially all ntp-4 releases running up to, but not including 4.2.8p4, and 4.3.0 up to, but not including 4.3.77 that have custom refclocks
• CVSS: (AV:L/AC:H/Au:M/C:C/I:C/A:C) Base Score: 0.0 usual case, 5.9 unusual worst case
• Summary: A negative value for the datalen parameter will overflow a data buffer. NTF's ntpd driver implementations always set this value to 0 and are therefore not vulnerable to this weakness. If you are running a custom refclock driver in ntpd and that driver supplies a negative value for datalen (no custom driver of even minimal competence would do this) then ntpd would overflow a data buffer. It is even hypothetically possible in this case that instead of simply crashing ntpd the attacker could effect a code injection attack.
• Mitigation:
  • Upgrade to 4.2.8p4, or later, from the NTP Project Download Page or the NTP Public Services Project Download Page.
  • If you are unable to upgrade:
    • If you are running custom refclock drivers, make sure the signed datalen value is either zero or positive.
  • Monitor your ntpd instances.
• Credit: This weakness was discovered by Yves Younan of Cisco Talos.