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Advisory Published

CVE-2023-48795: Impact of Terrapin SSH Attack

First published: Tue Dec 12 2023(Updated: )

### Summary Terrapin is a prefix truncation attack targeting the SSH protocol. More precisely, Terrapin breaks the integrity of SSH's secure channel. By carefully adjusting the sequence numbers during the handshake, an attacker can remove an arbitrary amount of messages sent by the client or server at the beginning of the secure channel without the client or server noticing it. ### Mitigations To mitigate this protocol vulnerability, OpenSSH suggested a so-called "strict kex" which alters the SSH handshake to ensure a Man-in-the-Middle attacker cannot introduce unauthenticated messages as well as convey sequence number manipulation across handshakes. **Warning: To take effect, both the client and server must support this countermeasure.** As a stop-gap measure, peers may also (temporarily) disable the affected algorithms and use unaffected alternatives like AES-GCM instead until patches are available. ### Details The SSH specifications of ChaCha20-Poly1305 (chacha20-poly1305@openssh.com) and Encrypt-then-MAC (*-etm@openssh.com MACs) are vulnerable against an arbitrary prefix truncation attack (a.k.a. Terrapin attack). This allows for an extension negotiation downgrade by stripping the SSH_MSG_EXT_INFO sent after the first message after SSH_MSG_NEWKEYS, downgrading security, and disabling attack countermeasures in some versions of OpenSSH. When targeting Encrypt-then-MAC, this attack requires the use of a CBC cipher to be practically exploitable due to the internal workings of the cipher mode. Additionally, this novel attack technique can be used to exploit previously unexploitable implementation flaws in a Man-in-the-Middle scenario. The attack works by an attacker injecting an arbitrary number of SSH_MSG_IGNORE messages during the initial key exchange and consequently removing the same number of messages just after the initial key exchange has concluded. This is possible due to missing authentication of the excess SSH_MSG_IGNORE messages and the fact that the implicit sequence numbers used within the SSH protocol are only checked after the initial key exchange. In the case of ChaCha20-Poly1305, the attack is guaranteed to work on every connection as this cipher does not maintain an internal state other than the message's sequence number. In the case of Encrypt-Then-MAC, practical exploitation requires the use of a CBC cipher; while theoretical integrity is broken for all ciphers when using this mode, message processing will fail at the application layer for CTR and stream ciphers. For more details see [https://terrapin-attack.com](https://terrapin-attack.com). ### Impact This attack targets the specification of ChaCha20-Poly1305 (chacha20-poly1305@openssh.com) and Encrypt-then-MAC (*-etm@openssh.com), which are widely adopted by well-known SSH implementations and can be considered de-facto standard. These algorithms can be practically exploited; however, in the case of Encrypt-Then-MAC, we additionally require the use of a CBC cipher. As a consequence, this attack works against all well-behaving SSH implementations supporting either of those algorithms and can be used to downgrade (but not fully strip) connection security in case SSH extension negotiation (RFC8308) is supported. The attack may also enable attackers to exploit certain implementation flaws in a man-in-the-middle (MitM) scenario.

Credit: cve@mitre.org cve@mitre.org CVE-2023-48795 CVE-2023-51384 CVE-2023-51385

Affected SoftwareAffected VersionHow to fix
pip/paramiko>=2.5.0<3.4.0
3.4.0
go/golang.org/x/crypto<0.17.0
0.17.0
rust/russh<0.40.2
0.40.2
Apple macOS Sonoma<14.4
14.4
IBM Concert Software<=1.0.0 - 1.0.1
redhat/PuTTY<0.80
0.80
redhat/AsyncSSH<2.14.1
2.14.1
redhat/libssh<0.9.8
0.9.8
redhat/libssh<0.10.6
0.10.6
redhat/golang.org/x/crypto/ssh<0.17.0
0.17.0
Openbsd Openssh<9.6
Putty Putty<0.80
Filezilla-project Filezilla Client<3.66.4
Microsoft PowerShell<=11.1.0
All of
Apple macOS
Panic Transmit 5<5.10.4
All of
Apple macOS
Panic Nova<11.8
Roumenpetrov Pkixssh<14.4
Winscp Winscp<6.2.2
Bitvise SSH Client<9.33
Bitvise SSH Server<9.32
Lancom-systems Lcos<=3.66.4
Lancom-systems Lcos Fx
Lancom-systems Lcos Lx
Lancom-systems Lcos Sx=4.20
Lancom-systems Lcos Sx=5.20
Lancom-systems Lanconfig
Vandyke Securecrt<9.4.3
Libssh Libssh<0.10.6
Net-ssh Net-ssh Ruby=7.2.0
Ssh2 Project Ssh2 Node.js<=1.11.0
Proftpd Proftpd<=1.3.8b
FreeBSD FreeBSD<=12.4
Crates Thrussh<0.35.1
Tera Term Project Tera Term<=5.1
Oryx-embedded Cyclone Ssh<2.3.4
CrushFTP CrushFTP<=10.6.0
NetSarang XShell 7<build__0144
Paramiko Paramiko<3.4.0
Redhat Openshift Container Platform=4.0
Redhat Openstack Platform=16.1
Redhat Openstack Platform=16.2
Redhat Openstack Platform=17.1
Redhat Ceph Storage=6.0
Redhat Enterprise Linux=8.0
Redhat Enterprise Linux=9.0
Redhat Openshift Serverless
Redhat Openshift Gitops
Redhat Openshift Pipelines
Redhat Openshift Developer Tools And Services
Redhat Openshift Data Foundation=4.0
Redhat Openshift Api For Data Protection
Redhat Openshift Virtualization=4
Redhat Storage=3.0
Redhat Discovery
Redhat Openshift Dev Spaces
Redhat Cert-manager Operator For Red Hat Openshift
Redhat Keycloak
Redhat Jboss Enterprise Application Platform=7.0
Redhat Single Sign-on=7.0
Redhat Advanced Cluster Security=3.0
Redhat Advanced Cluster Security=4.0
Golang Crypto<0.17.0
Russh Project Russh Rust<0.40.2
Sftpgo Project Sftpgo<2.5.6
Erlang Erlang\/otp<26.2.1
Matez Jsch<0.2.15
Libssh2 Libssh2<1.11.1
Asyncssh Project Asyncssh<2.14.2
Dropbear Ssh Project Dropbear Ssh<2022.83
Jadaptive Maverick Synergy Java Ssh Api<3.1.0-snapshot
Ssh Ssh<5.11
Thorntech Sftp Gateway Firmware<3.4.6
Netgate pfSense Plus<=23.09.1
Netgate pfSense CE<=2.7.2
CrushFTP CrushFTP<10.6.0
Connectbot Sshlib<2.2.22
Apache Sshd<=2.11.0
Apache Sshj<=0.37.0
Tinyssh Tinyssh<=20230101
Trilead Ssh2=6401
9bis Kitty<=0.76.1.13
All of
Gentoo Security
Debian Debian Linux
Fedoraproject Fedora=38
Fedoraproject Fedora=39
Debian Debian Linux=10.0
Apple macOS>=14.0<14.4
Palo Alto Networks PAN-OS<10.1.15=10.1<10.2.11=10.2<11.0.6=11.0<11.1.3=11.1
10.1.15 (ETA: December)
10.2.11
11.0.6
11.1.3
Palo Alto Networks Prisma SD-WAN ION<5.6.19=5.6<6.1.8=6.1<6.3.2=6.3
5.6.19
6.1.8
6.3.2
debian/dropbear
2020.81-3+deb11u2
2022.83-1+deb12u2
2024.86-1
debian/erlang<=1:23.2.6+dfsg-1+deb11u1<=1:25.2.3+dfsg-1
1:25.3.2.12+dfsg-3
1:27.1.2+dfsg-1
debian/filezilla
3.52.2-3+deb11u1
3.63.0-1+deb12u3
3.68.0-1
debian/golang-go.crypto<=1:0.0~git20201221.eec23a3-1<=1:0.4.0-1
1:0.25.0-1
debian/jsch
0.1.55-1
0.2.19-1
debian/libssh
0.9.8-0+deb11u1
0.10.6-0+deb12u1
0.11.1-1
debian/libssh2
1.9.0-2+deb11u1
1.10.0-3
1.11.1-1
debian/openssh
1:8.4p1-5+deb11u3
1:9.2p1-2+deb12u3
1:9.9p1-3
debian/paramiko<=2.7.2-1<=2.12.0-2
3.4.1-2
debian/php-phpseclib
2.0.30-2+deb11u2
2.0.30-2+deb11u1
2.0.42-1+deb12u2
2.0.42-1+deb12u1
2.0.47-3
debian/php-phpseclib3
3.0.19-1+deb12u3
3.0.19-1+deb12u2
3.0.42-1
debian/phpseclib
1.0.19-3+deb11u2
1.0.19-3+deb11u1
1.0.20-1+deb12u2
1.0.20-1+deb12u1
1.0.23-3
debian/proftpd-dfsg<=1.3.7a+dfsg-12+deb11u2
1.3.7a+dfsg-12+deb11u3
1.3.8+dfsg-4+deb12u3
1.3.8+dfsg-4+deb12u4
1.3.8.c+dfsg-1
debian/proftpd-mod-proxy<=0.7-1
0.9.2-1+deb12u1
0.9.4-1
debian/putty
0.74-1+deb11u2
0.74-1+deb11u1
0.78-2+deb12u2
0.78-2+deb12u1
0.82-2
debian/python-asyncssh<=2.5.0-0.1
2.5.0-0.1+deb11u1
2.10.1-2+deb12u1
2.19.0-1
debian/tinyssh<=20190101-1+deb11u1<=20230101-1
20241201-1
debian/trilead-ssh2<=6401+svn158-1.1

Remedy

https://git.libssh.org/projects/libssh.git/commit/?h=stable-0.10&id=10e09e273f69e149389b3e0e5d44b8c221c2e7f6

Remedy

https://github.com/TeraTermProject/teraterm/commit/7279fbd6ef4d0c8bdd6a90af4ada2899d786eec0

Remedy

https://github.com/connectbot/sshlib/commit/5c8b534f6e97db7ac0e0e579331213aa25c173ab

Remedy

https://github.com/erlang/otp/blob/d1b43dc0f1361d2ad67601169e90a7fc50bb0369/lib/ssh/doc/src/notes.xml#L39-L42

Remedy

https://github.com/golang/crypto/commit/9d2ee975ef9fe627bf0a6f01c1f69e8ef1d4f05d

Remedy

https://github.com/jtesta/ssh-audit/commit/8e972c5e94b460379fe0c7d20209c16df81538a5

Remedy

https://github.com/mkj/dropbear/blob/17657c36cce6df7716d5ff151ec09a665382d5dd/CHANGES#L25

Remedy

https://github.com/mscdex/ssh2/commit/97b223f8891b96d6fc054df5ab1d5a1a545da2a3

Remedy

https://github.com/net-ssh/net-ssh/blob/2e65064a52d73396bfc3806c9196fc8108f33cd8/CHANGES.txt#L14-L16

Remedy

https://github.com/openssh/openssh-portable/commits/master

Remedy

https://nest.pijul.com/pijul/thrussh/changes/D6H7OWTTMHHX6BTB3B6MNBOBX2L66CBL4LGSEUSAI2MCRCJDQFRQC

Remedy

If using the SSH client provided with PAN-OS to connect from the firewall to an external SSH server, ensure that the SSH server does not support the CHACHA20-POLY1305 algorithm or any Encrypt-then-MAC algorithms.

Remedy

PAN-OS customers can resolve this issue by configuring the in-use SSH profile to contain at least one cipher and at least one MAC algorithm, which removes support for CHACHA20-POLY1305 and all Encrypt-then-MAC algorithms available (ciphers with -etm in the name) in PAN-OS software. Guidance on how to configure strong ciphers and algorithms can be found on the following pages: - https://knowledgebase.paloaltonetworks.com/KCSArticleDetail?id=kA14u0000004OOQCA2 - https://docs.paloaltonetworks.com/pan-os/11-1/pan-os-cli-quick-start/get-started-with-the-cli/refresh-ssh-keys-mgt-port-connection To validate that the affected ciphers and algorithms are no longer enabled, please see the guidance at: https://knowledgebase.paloaltonetworks.com/KCSArticleDetail?id=kA14u000000kF2eCAE This issue is completely resolved by following the recommended best practices for deploying PAN-OS (https://docs.paloaltonetworks.com/best-practices/10-1/administrative-access-best-practices/administrative-access-best-practices/deploy-administrative-access-best-practices). No additional PAN-OS fixes are planned in maintenance releases at this time. The PAN-OS SSH client is fixed in PAN-OS 10.2.11, PAN-OS 11.0.6, PAN-OS 11.1.3, and all later PAN-OS versions. The PAN-OS SSH client will be fixed in PAN-OS 10.1.15. This issue is fixed in Prisma SD-WAN ION 5.6.19, Prisma SD-WAN ION 6.1.8, Prisma SD-WAN ION 6.3.2, and all later Prisma SD-WAN ION versions. If you are using the Prisma SD-WAN ION 6.2 series, evaluate moving to another Prisma SD-WAN ION series number based on the guidance here: https://live.paloaltonetworks.com/t5/customer-resources/prisma-sd-wan-ion-software-release-guidelines/ta-p/578685.

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