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.

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.

The remote host is affected by the vulnerability described in GLSA-201507-01 (chrony: Multiple vulnerabilities)

    Multiple vulnerabilities have been discovered in chrony. Please review       the CVE identifiers referenced below for details.
  Impact :

    A remote attacker can cause arbitrary remote code execution or Denial of       service condition.
  Workaround :

    There is no known workaround at this time.

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.

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	名前	製品	ファミリー	公開日	更新日	深刻度
82716	Debian DLA-193-1 : chrony security update	Nessus	Debian Local Security Checks	2015/4/13	2021/1/11	medium
83009	Fedora 21 : chrony-1.31.1-1.fc21 (2015-5816)	Nessus	Fedora Local Security Checks	2015/4/23	2021/1/11	medium
84531	GLSA-201507-01 : chrony: Multiple vulnerabilities	Nessus	Gentoo Local Security Checks	2015/7/6	2021/1/11	medium
82744	Debian DSA-3222-1 : chrony - security update	Nessus	Debian Local Security Checks	2015/4/14	2021/1/11	medium
83067	Fedora 20 : chrony-1.31.1-1.fc20 (2015-5809)	Nessus	Fedora Local Security Checks	2015/4/27	2021/1/11	medium
83978	Amazon Linux AMI : chrony (ALAS-2015-539)	Nessus	Amazon Linux Local Security Checks	2015/6/4	2019/12/18	medium
87032	Oracle Linux 7 : chrony (ELSA-2015-2241)	Nessus	Oracle Linux Local Security Checks	2015/11/24	2024/10/22	medium

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