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8c22023ca5
From OpenSSH 8.8p1's release notes:
---
Potentially-incompatible changes
================================
This release disables RSA signatures using the SHA-1 hash algorithm
by default. This change has been made as the SHA-1 hash algorithm is
cryptographically broken, and it is possible to create chosen-prefix
hash collisions for <USD$50K [1]
For most users, this change should be invisible and there is
no need to replace ssh-rsa keys. OpenSSH has supported RFC8332
RSA/SHA-256/512 signatures since release 7.2 and existing ssh-rsa keys
will automatically use the stronger algorithm where possible.
Incompatibility is more likely when connecting to older SSH
implementations that have not been upgraded or have not closely tracked
improvements in the SSH protocol. For these cases, it may be necessary
to selectively re-enable RSA/SHA1 to allow connection and/or user
authentication via the HostkeyAlgorithms and PubkeyAcceptedAlgorithms
options. For example, the following stanza in ~/.ssh/config will enable
RSA/SHA1 for host and user authentication for a single destination host:
Host old-host
HostkeyAlgorithms +ssh-rsa
PubkeyAcceptedAlgorithms +ssh-rsa
We recommend enabling RSA/SHA1 only as a stopgap measure until legacy
implementations can be upgraded or reconfigured with another key type
(such as ECDSA or Ed25519).
[1] "SHA-1 is a Shambles: First Chosen-Prefix Collision on SHA-1 and
Application to the PGP Web of Trust" Leurent, G and Peyrin, T
(2020) https://eprint.iacr.org/2020/014.pdf
---
Relnotes: Yes
Sponsored by: The FreeBSD Foundation
56 lines
1.6 KiB
Bash
56 lines
1.6 KiB
Bash
# $OpenBSD: knownhosts-command.sh,v 1.3 2021/08/30 01:15:45 djm Exp $
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# Placed in the Public Domain.
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tid="known hosts command "
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rm -f $OBJ/knownhosts_command $OBJ/ssh_proxy_khc
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cp $OBJ/ssh_proxy $OBJ/ssh_proxy_orig
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( grep -vi GlobalKnownHostsFile $OBJ/ssh_proxy_orig | \
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grep -vi UserKnownHostsFile;
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echo "GlobalKnownHostsFile none" ;
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echo "UserKnownHostsFile none" ;
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echo "KnownHostsCommand $OBJ/knownhosts_command '%t' '%K' '%u'" ;
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) > $OBJ/ssh_proxy
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verbose "simple connection"
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cat > $OBJ/knownhosts_command << _EOF
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#!/bin/sh
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cat $OBJ/known_hosts
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_EOF
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chmod a+x $OBJ/knownhosts_command
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${SSH} -F $OBJ/ssh_proxy x true || fail "ssh connect failed"
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verbose "no keys"
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cat > $OBJ/knownhosts_command << _EOF
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#!/bin/sh
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exit 0
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_EOF
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chmod a+x $OBJ/knownhosts_command
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${SSH} -F $OBJ/ssh_proxy x true && fail "ssh connect succeeded with no keys"
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verbose "bad exit status"
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cat > $OBJ/knownhosts_command << _EOF
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#!/bin/sh
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cat $OBJ/known_hosts
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exit 1
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_EOF
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chmod a+x $OBJ/knownhosts_command
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${SSH} -F $OBJ/ssh_proxy x true && fail "ssh connect succeeded with bad exit"
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for keytype in ${SSH_HOSTKEY_TYPES} ; do
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algs=$keytype
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test "x$keytype" = "xssh-dss" && continue
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test "x$keytype" = "xssh-rsa" && algs=ssh-rsa,rsa-sha2-256,rsa-sha2-512
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verbose "keytype $keytype"
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cat > $OBJ/knownhosts_command << _EOF
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#!/bin/sh
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die() { echo "\$@" 1>&2 ; exit 1; }
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test "x\$1" = "x$keytype" || die "wrong keytype \$1 (expected $keytype)"
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test "x\$3" = "x$LOGNAME" || die "wrong username \$3 (expected $LOGNAME)"
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grep -- "\$1.*\$2" $OBJ/known_hosts
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_EOF
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${SSH} -F $OBJ/ssh_proxy -oHostKeyAlgorithms=$algs x true ||
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fail "ssh connect failed for keytype $x"
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done
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