CVE-2015-1853 :

Protect authenticated symmetric NTP associations against DoS attacks.

An attacker knowing that NTP hosts A and B are peering with each other (symmetric association) can send a packet with random timestamps to host A with source address of B which will set the NTP state variables on A to the values sent by the attacker. Host A will then send on its next poll to B a packet with originate timestamp that doesn't match the transmit timestamp of B and the packet will be dropped. If the attacker does this periodically for both hosts, they won't be able to synchronize to each other. It is a denial of service attack.

According to [1], NTP authentication is supposed to protect symmetric associations against this attack, but in the NTPv3 (RFC 1305) and NTPv4 (RFC 5905) specifications the state variables are updated before the authentication check is performed, which means the association is vulnerable to the attack even when authentication is enabled.

To fix this problem, save the originate and local timestamps only when the authentication check (test5) passed.

[1] https://www.eecis.udel.edu/~mills/onwire.html

CVE-2015-1821 :

Fix access configuration with subnet size indivisible by 4.

When NTP or cmdmon access was configured (from chrony.conf or via authenticated cmdmon) with a subnet size that is indivisible by 4 and an address that has nonzero bits in the 4-bit subnet remainder (e.g. 192.168.15.0/22 or f000::/3), the new setting was written to an incorrect location, possibly outside the allocated array.

An attacker that has the command key and is allowed to access cmdmon (only localhost is allowed by default) could exploit this to crash chronyd or possibly execute arbitrary code with the privileges of the chronyd process.

CVE-2015-1822 :

Fix initialization of reply slots for authenticated commands.

When allocating memory to save unacknowledged replies to authenticated command requests, the last 'next' pointer was not initialized to NULL. When all allocated reply slots were used, the next reply could be written to an invalid memory instead of allocating a new slot for it.

An attacker that has the command key and is allowed to access cmdmon (only localhost is allowed by default) could exploit this to crash chronyd or possibly execute arbitrary code with the privileges of the chronyd process.

NOTE: Tenable Network Security has extracted the preceding description block directly from the DLA security advisory. Tenable has attempted to automatically clean and format it as much as possible without introducing additional issues.

Chrony News reports :

CVE-2015-1853: DoS attack on authenticated symmetric NTP associations

CVE-2015-1821: Heap-based buffer overflow in access configuration

CVE-2015-1822: Use of uninitialized pointer in command processing

An out-of-bounds write flaw was found in the way chrony stored certain addresses when configuring NTP or cmdmon access. An attacker that has the command key and is allowed to access cmdmon (only localhost is allowed by default) could use this flaw to crash chronyd or, possibly, execute arbitrary code with the privileges of the chronyd process.
(CVE-2015-1821)

An uninitialized pointer use flaw was found when allocating memory to save unacknowledged replies to authenticated command requests. An attacker that has the command key and is allowed to access cmdmon (only localhost is allowed by default) could use this flaw to crash chronyd or, possibly, execute arbitrary code with the privileges of the chronyd process. (CVE-2015-1822)

A denial of service flaw was found in the way chrony hosts that were peering with each other authenticated themselves before updating their internal state variables. An attacker could send packets to one peer host, which could cascade to other peers, and stop the synchronization process among the reached peers. (CVE-2015-1853)

The chrony packages have been upgraded to upstream version 2.1.1, which provides a number of bug fixes and enhancements over the previous version. Notable enhancements include :

  - Updated to NTP version 4 (RFC 5905)

  - Added pool directive to specify pool of NTP servers

  - Added leapsecmode directive to select how to correct     clock for leap second

  - Added smoothtime directive to smooth served time and     enable leap smear

  - Added asynchronous name resolving with POSIX threads

  - Ready for year 2036 (next NTP era)

  - Improved clock control

  - Networking code reworked to open separate client sockets     for each NTP server

This update also fixes the following bug :

  - The chronyd service previously assumed that network     interfaces specified with the 'bindaddress' directive     were ready when the service was started. This could     cause chronyd to fail to bind an NTP server socket to     the interface if the interface was not ready. With this     update, chronyd uses the IP_FREEBIND socket option,     enabling it to bind to an interface later, not only when     the service starts.

In addition, this update adds the following enhancement :

  - The chronyd service now supports four modes of handling     leap seconds, configured using the 'leapsecmode' option.
    The clock can be either stepped by the kernel (the     default 'system' mode), stepped by chronyd ('step'     mode), slowly adjusted by slewing ('slew' mode), or the     leap second can be ignored and corrected later in normal     operation ('ignore' mode). If you select slewing, the     correction will always start at 00:00:00 UTC and will be     applied at a rate specified in the 'maxslewrate' option.

Miroslav Lichvar of Red Hat discovered multiple vulnerabilities in chrony, an alternative NTP client and server :

  - CVE-2015-1821     Using particular address/subnet pairs when configuring     access control would cause an invalid memory write. This     could allow attackers to cause a denial of service     (crash) or execute arbitrary code.

  - CVE-2015-1822     When allocating memory to save unacknowledged replies to     authenticated command requests, a pointer would be left     uninitialized, which could trigger an invalid memory     write. This could allow attackers to cause a denial of     service (crash) or execute arbitrary code.

  - CVE-2015-1853     When peering with other NTP hosts using authenticated     symmetric association, the internal state variables     would be updated before the MAC of the NTP messages was     validated. This could allow a remote attacker to cause a     denial of service by impeding synchronization between     NTP peers.

Security fix for CVE-2015-1853, CVE-2015-1821, CVE-2015-1822

Note that Tenable Network Security has extracted the preceding description block directly from the Fedora security advisory. Tenable has attempted to automatically clean and format it as much as possible without introducing additional issues.

As reported upstream :

When NTP or cmdmon access was configured (from chrony.conf or via authenticated cmdmon) with a subnet size that is indivisible by 4 and an address that has nonzero bits in the 4-bit subnet remainder (e.g.
192.168.15.0/22 or f000::/3), the new setting was written to an incorrect location, possibly outside the allocated array. An attacker that has the command key and is allowed to access cmdmon (only localhost is allowed by default) could exploit this to crash chronyd or possibly execute arbitrary code with the privileges of the chronyd process. (CVE-2015-1821)

When allocating memory to save unacknowledged replies to authenticated command requests, the last 'next' pointer was not initialized to NULL.
When all allocated reply slots were used, the next reply could be written to an invalid memory instead of allocating a new slot for it.
An attacker that has the command key and is allowed to access cmdmon (only localhost is allowed by default) could exploit this to crash chronyd or possibly execute arbitrary code with the privileges of the chronyd process. (CVE-2015-1822)

An attacker knowing that NTP hosts A and B are peering with each other (symmetric association) can send a packet with random timestamps to host A with source address of B which will set the NTP state variables on A to the values sent by the attacker. Host A will then send on its next poll to B a packet with originate timestamp that doesn't match the transmit timestamp of B and the packet will be dropped. If the attacker does this periodically for both hosts, they won't be able to synchronize to each other. Authentication using a symmetric key can fully protect against this attack, but in implementations following the NTPv3 (RFC 1305) or NTPv4 (RFC 5905) specification the state variables were updated even when the authentication check failed and the association was not protected. (CVE-2015-1853)

The remote Oracle Linux 7 host has a package installed that is affected by multiple vulnerabilities as referenced in the ELSA-2015-2241 advisory.

    - update to 2.1 (#1117882 #1169353 #1206504 #1209568 CVE-2015-1821       CVE-2015-1822 CVE-2015-1853)

Tenable has extracted the preceding description block directly from the Oracle Linux security advisory.

Note that Nessus has not tested for these issues but has instead relied only on the application's self-reported version number.

ID	Name	Product	Family	Published	Updated	Severity
82716	Debian DLA-193-1 : chrony security update	Nessus	Debian Local Security Checks	4/13/2015	1/11/2021	medium
82892	FreeBSD : chrony -- multiple vulnerabilities (c4571ca8-053d-44c9-ab3c-89b1372ad0a5)	Nessus	FreeBSD Local Security Checks	4/20/2015	1/6/2021	medium
87551	Scientific Linux Security Update : chrony on SL7.x x86_64 (20151119)	Nessus	Scientific Linux Local Security Checks	12/22/2015	1/14/2021	medium
82744	Debian DSA-3222-1 : chrony - security update	Nessus	Debian Local Security Checks	4/14/2015	1/11/2021	medium
83067	Fedora 20 : chrony-1.31.1-1.fc20 (2015-5809)	Nessus	Fedora Local Security Checks	4/27/2015	1/11/2021	medium
83978	Amazon Linux AMI : chrony (ALAS-2015-539)	Nessus	Amazon Linux Local Security Checks	6/4/2015	12/18/2019	medium
87032	Oracle Linux 7 : chrony (ELSA-2015-2241)	Nessus	Oracle Linux Local Security Checks	11/24/2015	10/22/2024	medium

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