Editing Secure Shell (section)

===SSH-1===
In 1998, a vulnerability was described in SSH 1.5 which allowed the unauthorized insertion of content into an encrypted SSH stream due to insufficient data integrity protection from [[CRC-32]] used in this version of the protocol.<ref>{{cite web|url=http://www.coresecurity.com/content/ssh-insertion-attack|title=SSH Insertion Attack|website=[[Core Security Technologies]]|url-status=live|archive-url=https://web.archive.org/web/20110708192336/http://www.coresecurity.com/content/ssh-insertion-attack|archive-date=2011-07-08}}</ref><ref>{{cite web|url=http://www.kb.cert.org/vuls/id/13877|title=Vulnerability Note VU#13877 - Weak CRC allows packet injection into SSH sessions encrypted with block ciphers|website=[[US CERT]]|url-status=live|archive-url=https://web.archive.org/web/20100710040357/http://www.kb.cert.org/vuls/id/13877|archive-date=2010-07-10}}</ref> A fix known as SSH Compensation Attack Detector<ref>{{cite web|url=http://www.securityfocus.com/bid/2347/discuss|title=SSH CRC-32 Compensation Attack Detector Vulnerability|website=[[SecurityFocus]]|url-status=live|archive-url=https://web.archive.org/web/20080725110345/http://www.securityfocus.com/bid/2347/discuss|archive-date=2008-07-25}}</ref> was introduced into most implementations. Many of these updated implementations contained a new [[integer overflow]] vulnerability<ref>{{cite web|url=http://www.kb.cert.org/vuls/id/945216|title=Vulnerability Note VU#945216 - SSH CRC32 attack detection code contains remote integer overflow|website=US CERT|url-status=live|archive-url=https://web.archive.org/web/20051013074750/http://www.kb.cert.org/vuls/id/945216|archive-date=2005-10-13}}</ref> that allowed attackers to execute arbitrary code with the privileges of the SSH daemon, typically root.

In January 2001 a vulnerability was discovered that allows attackers to modify the last block of an [[International Data Encryption Algorithm|IDEA]]-encrypted session.<ref>{{cite web|url=http://www.kb.cert.org/vuls/id/315308|title=Vulnerability Note VU#315308 - Weak CRC allows last block of IDEA-encrypted SSH packet to be changed without notice|website=US CERT|url-status=live|archive-url=https://web.archive.org/web/20100711103528/http://www.kb.cert.org/vuls/id/315308|archive-date=2010-07-11}}</ref> The same month, another vulnerability was discovered that allowed a malicious server to forward a client authentication to another server.<ref name="cert1">{{cite web|url=http://www.kb.cert.org/vuls/id/684820|title=Vulnerability Note VU#684820 - SSH-1 allows client authentication to be forwarded by a malicious server to another server|website=US CERT|url-status=live|archive-url=https://web.archive.org/web/20090901012536/http://www.kb.cert.org/vuls/id/684820|archive-date=2009-09-01}}</ref>

Since SSH-1 has inherent design flaws which make it vulnerable, it is now generally considered obsolete and should be avoided by explicitly disabling fallback to SSH-1.<ref name="cert1" /> Most modern servers and clients support SSH-2.<ref>{{cite web |title=How to use SSH keys for authentication |url=https://upcloud.com/community/tutorials/use-ssh-keys-authentication/ |website=Up Cloud |date=17 September 2015 |access-date=29 November 2019}}</ref>