Bug 1417264

Summary:	OpenSSH will negotiate MD5 HMAC in default configuration [RFE] provide safe default
Product:	Red Hat Enterprise Linux 6	Reporter:	Leonard den Ottolander <leonard-rh-bugzilla>
Component:	openssh	Assignee:	Jakub Jelen <jjelen>
Status:	CLOSED DUPLICATE	QA Contact:	BaseOS QE Security Team <qe-baseos-security>
Severity:	unspecified	Docs Contact:
Priority:	unspecified
Version:	6.8	CC:	ajb, jjelen, phil, qe-baseos-security
Target Milestone:	rc	Keywords:	FutureFeature
Target Release:	---
Hardware:	Unspecified
OS:	Unspecified
Whiteboard:
Fixed In Version:		Doc Type:	If docs needed, set a value
Doc Text:		Story Points:	---
Clone Of:	1417263	Environment:
Last Closed:	2017-02-01 15:09:21 UTC	Type:	Bug
Regression:	---	Mount Type:	---
Documentation:	---	CRM:
Verified Versions:		Category:	---
oVirt Team:	---	RHEL 7.3 requirements from Atomic Host:
Cloudforms Team:	---	Target Upstream Version:
Embargoed:
Bug Depends On:	1373835, 1417263
Bug Blocks:

Description Leonard den Ottolander 2017-01-27 17:47:00 UTC

+++ This bug was initially created as a clone of Bug #1417263 +++

OpenSSH on both RHEL 7 (openssh-6.6.1p1-31.el7) and RHEL 6 (openssh-5.3p1-118.1.el6_8) will by default negotiate an MD5 HMAC. MD5 has been considered insecure for quite some time now.

C6 client, C7 server: (C in CentOS)

debug2: mac_setup: found hmac-md5
debug1: kex: server->client aes128-ctr hmac-md5 none
debug2: mac_setup: found hmac-md5
debug1: kex: client->server aes128-ctr hmac-md5 none

C7 client & server:

debug2: mac_setup: setup hmac-md5-etm
debug1: kex: server->client aes128-ctr hmac-md5-etm none
debug2: mac_setup: setup hmac-md5-etm
debug1: kex: client->server aes128-ctr hmac-md5-etm none

Default order on C7:

debug2: kex_parse_kexinit: hmac-md5-etm,hmac-sha1-etm,umac-64-etm,umac-128-etm,hmac-sha2-256-etm,hmac-sha2-512-etm,hmac-ripemd160-etm,hmac-sha1-96-etm,hmac-md5-96-etm,hmac-md5,hmac-sha1,umac-64,umac-128,hmac-sha2-256,hmac-sha2-512,hmac-ripemd160,hmac-ripemd160,hmac-sha1-96,hmac-md5-96

Default order on C6:

debug2: kex_parse_kexinit: hmac-md5,hmac-sha1,umac-64,hmac-sha2-256,hmac-sha2-512,hmac-ripemd160,hmac-ripemd160,hmac-sha1-96,hmac-md5-96

I would like to suggest to add the following line to both ssh_config and sshd_config:

RHEL 7:

MACs hmac-sha2-512-etm,hmac-sha2-512,hmac-sha2-256-etm,hmac-sha2-256,hmac-sha1-etm,hmac-sha1,hmac-ripemd160-etm,hmac-ripemd160,hmac-ripemd160,umac-128,umac-128-etm,hmac-sha1-96-etm,hmac-sha1-96,umac-64-etm,umac-64

RHEL 6:

MACS hmac-sha2-512,hmac-sha2-256,hmac-sha1,hmac-ripemd160,hmac-ripemd160,hmac-sha1-96,umac-64

Even if this is not the desired order - note that the default order appears to be quite random itself - the fact that this option is available in the configuration files should give the administrator enough of a clue.

Comment 2 Jakub Jelen 2017-01-30 07:42:23 UTC

> MD5 has been considered insecure for quite some time now.

That is true for cryptographic signature usage, but not for HMACs (integrity checks usage).

Thank you for your input, but this is already handled in the bug #1373836, which will be out soon. Let me know if there is something more you would like to achieve or I can close the bug as a duplicate.

Comment 3 Leonard den Ottolander 2017-02-01 15:02:50 UTC

Even if HMACs are not quite as susceptible it's just plain stupid to keep known broken primitives around as defaults.

As you are well aware attacks in crypto only ever get better, so over time things that were barely acceptable might very well become totally unsafe.

NIST never even standardized MD5 as a HMAC for use in TLS, supposedly because of it utter brokenness.

10 years have gone by, and is quite possible that attacks that seemed infeasible 10 years ago are now quite possible seeing that state financed adversaries have budgets of billions of dollars per year.

But I agree this is a dup of #1373836 so I'll hook on to that report.

Comment 4 Jakub Jelen 2017-02-01 15:09:21 UTC


*** This bug has been marked as a duplicate of bug 1373836 ***