Bug 1373836 – Remove RC4 cipher and questionable MACs enabled by default from OpenSSH

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Bug 1373836 - Remove RC4 cipher and questionable MACs enabled by default from OpenSSH

Summary:	Remove RC4 cipher and questionable MACs enabled by default from OpenSSH

Status:	CLOSED ERRATA

Aliases:	None

Product:	Red Hat Enterprise Linux 6
Classification:	Red Hat
Component:	openssh (Show other bugs)
Sub Component:
Version:	6.8
Hardware:	Unspecified Unspecified

Priority	unspecified Severity unspecified
Target Milestone:	rc
Target Release:	---
Assigned To:	Jakub Jelen
QA Contact:	Stefan Dordevic
Docs Contact:	Mirek Jahoda

URL:
Whiteboard:
Keywords:

Duplicates:	1417264 (view as bug list)
Depends On:
Blocks:	1335911
	Show dependency tree / graph

Reported:	2016-09-07 04:54 EDT by Jakub Svoboda
Modified:	2017-12-05 04:15 EST (History)
CC List:	14 users (show)

See Also:
Fixed In Version:	openssh-5.3p1-120.el6
Doc Type:	Deprecated Functionality
Doc Text:	Deprecated algorithms in OpenSSH: RC4, hmac-md5, and hmac-md5-96 With this update, the `arcfour256`, `arcfour128`, `arcfour` ciphers and the `hmac-md5`, `hmac-md5-96` Method Authentication Code (MAC) algorithms are deprecated. Note that this change does not affect any existing server configuration. The system administrator can enable these deprecated algorithms by editing the `ssh_config` file, for example: Host legacy Ciphers arcfour MACs hmac-md5 To completely restore the deprecated functionality, add the following snippet to the `/etc/ssh/ssh_config` file: Ciphers aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,aes256-cbc,arcfour,rijndael-cbc@lysator.liu.se MACs hmac-md5,hmac-sha1,umac-64@openssh.com,hmac-sha2-256,hmac-sha2-512,hmac-ripemd160,hmac-ripemd160@openssh.com,hmac-sha1-96,hmac-md5-96
Story Points:	---
Clone Of:
Environment:
Last Closed:	2017-03-21 06:02:38 EDT
Type:	Bug
Regression:	---
Mount Type:	---
Documentation:	---
CRM:
Verified Versions:
Category:	---
oVirt Team:	---
RHEL 7.3 requirements from Atomic Host:
Cloudforms Team:	---

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Add an attachment (proposed patch, testcase, etc.)

External Trackers
Tracker	ID	Priority	Status	Summary	Last Updated
Red Hat Product Errata	RHSA-2017:0641	normal	SHIPPED_LIVE	Moderate: openssh security and bug fix update	2017-03-21 08:31:22 EDT

Groups: None (edit)

Description Jakub Svoboda 2016-09-07 04:54:40 EDT

Description of problem:
The default configuration of openssh-server 6.7 disables some of the supported Ciphers and MACs. "In particular, CBC ciphers and arcfour* are disabled by default." [1]. MACs that are disabled by default are *md5*, *ripemd160*, *sha1*96*.

Version-Release number of selected component (if applicable):
openssh-5.3p1-118.1.el6_8

How reproducible:
always

Steps to Reproduce:

1. yum distribution-synchronization # just to ensure the latest RHEL is installed

2. rm /etc/ssh/sshd_config

3. yum reinstall openssh-server # recreates default /etc/ssh/sshd_config

4. sshd -T | grep -i "Ciphers\|MACs" # lists currently enabled Ciphers and MACs

Actual results:
The default enabled Ciphers include *cbc*, *arcfour*.
The default enabled MACs include *md5*, *ripemd160*, *sha1*96*.

Actual results of the command sshd -T | grep -i "Ciphers\|MACs"
ciphers aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,aes256-cbc,arcfour,rijndael-cbc@lysator.liu.se
macs hmac-md5,hmac-sha1,umac-64@openssh.com,hmac-sha2-256,hmac-sha2-512,hmac-ripemd160,hmac-ripemd160@openssh.com,hmac-sha1-96,hmac-md5-96

Expected results:
Defaults follow recent OpenSSH upstream versions.
These Ciphers are disabled by default: *cbc*, *arcfour*.
These MACs are disabled by default: *md5*, *ripemd160*, *sha1*96*.

Expected results of the command sshd -T | grep -i "Ciphers\|MACs"
ciphers aes128-ctr,aes192-ctr,aes256-ctr
macs hmac-sha1,umac-64@openssh.com,hmac-sha2-256,hmac-sha2-512,hmac-ripemd160,hmac-ripemd160@openssh.com

Additional info:
* Red Hat's KB article about insecure Ciphers and MACs recommends non-default values [3]
* The latest RH-shipped version is 6.6.1p1 / 5.3p1; however it would be good to follow current security recommendations regarding the defaults.
* We have a Red Hat Insights rule for OpenSSH hardening that would get activated once the defaults in the openssh-server package are fixed. Red Hat Insights is a service to notify enrolled customers about problems on their systems. It is best when customers are not alerted about RHEL-default values.
* OpenSSH 6.7 through 7.1 disables hmac-sha1-etm@openssh.com and hmac-sha1 but OpenSSH 7.2 reenables it.
References:
[1] http://www.openssh.com/txt/release-6.7
[2] http://www.openssh.com/txt/release-7.2
[3] https://access.redhat.com/solutions/420283

Comment 22 Jakub Jelen 2016-11-15 04:29:46 EST

Well ... my bad. You are right. It is a bug.
The hmac-md5-96 should not be removed from the server list. Only from the default client list. I will need to fix this. The test was obviously right. Sorry for the confusion.

Comment 28 Jakub Jelen 2017-02-01 10:09:21 EST

*** Bug 1417264 has been marked as a duplicate of this bug. ***

Comment 29 Leonard den Ottolander 2017-02-01 10:17:15 EST

Probably best to only leave around sha2-512 and sha2-256 enabled by default, and sha1 as a legacy option for HMACs.

Not just MD5 is considered broken, but also RIPEMD. See http://csrc.nist.gov/groups/ST/toolkit/documents/shs/hash_standards_comments.pdf . Note that this is a NIST statement from 2004, that is THIRTEEN YEARS AGO. Things have only deteriorated since then.

I'm not so sure the hype against CBC is warranted. With proper padding CBC is perfectly usable. Counter modes like GCM and CTR have their own issues, like injecting known state (albeit hashed/crypted) which should not be brushed aside so easily. Such known state injection might well be successfully leveraged by future cryptanalitic progress.

So personally I'm not entirely convinced GCM is that much more secure (or even more secure at all) than (properly implemented) CBC.

Comment 31 errata-xmlrpc 2017-03-21 06:02:38 EDT

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://rhn.redhat.com/errata/RHSA-2017-0641.html

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