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 SSHMSGEXTINFO sent after the first message after SSHMSGNEWKEYS, 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 SSHMSGIGNORE 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 SSHMSGIGNORE 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.

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.

A fundamental design flaw within the RADIUS protocol has been proven to be exploitable, compromising the integrity in the RADIUS Access-Request process. The attack allows a malicious user to modify packets in a way that would be indistinguishable to a RADIUS client or server. To be successful, the attacker must have the ability to inject themselves between the client and server.

A integer overflow or wraparound vulnerability in Fortinet FortiWeb 8.0.0 through 8.0.3, FortiWeb 7.6.0 through 7.6.6, FortiWeb 7.4 all versions, FortiWeb 7.2 all versions, FortiWeb 7.0 all versions may allow attacker to denial of service via <insert attack vector here>

An improper neutralization of special elements used in an OS command ('OS Command Injection') vulnerability [CWE-78] in Fortinet FortiWeb version 7.6.0 through 7.6.3, 7.4.0 through 7.4.7, 7.2.0 through 7.2.10 and before 7.0.10 allows an authenticated privileged attacker to execute unauthorized code or commands via crafted CLI commands

A insertion of sensitive information into sent data vulnerability in Fortinet FortiMail 7.4.0 through 7.4.2, FortiMail 7.2.0 through 7.2.6, FortiMail 7.0 all versions, FortiManager 7.6.0 through 7.6.1, FortiManager 7.4.1 through 7.4.3, FortiManager Cloud 7.4.1 through 7.4.3, FortiNDR 7.6.0 through 7.6.1, FortiNDR 7.4.0 through 7.4.8, FortiNDR 7.2 all versions, FortiNDR 7.1 all versions, FortiNDR 7.0 all versions, FortiNDR 1.5 all versions, FortiOS 7.6.0, FortiOS 7.4.0 through 7.4.4, FortiOS 7.2.0 through 7.2.8, FortiOS 7.0.0 through 7.0.15, FortiOS 6.4.0 through 6.4.15, FortiOS 6.2 all versions, FortiOS 6.0 all versions, FortiPAM 1.3 all versions, FortiPAM 1.2 all versions, FortiPAM 1.1 all versions, FortiPAM 1.0 all versions, FortiProxy 7.4.0 through 7.4.4, FortiProxy 7.2.0 through 7.2.10, FortiProxy 7.0 all versions, FortiProxy 2.0 all versions, FortiProxy 1.2 all versions, FortiProxy 1.1 all versions, FortiProxy 1.0 all versions, FortiRecorder 7.2.0 through 7.2.1, FortiRecorder 7.0.0 through 7.0.4, FortiTester 7.4.0 through 7.4.2, FortiTester 7.3 all versions, FortiTester 7.2 all versions, FortiTester 7.1 all versions, FortiTester 7.0 all versions, FortiTester 4.2 all versions, FortiVoice 7.0.0 through 7.0.4, FortiVoice 6.4.0 through 6.4.9, FortiVoice 6.0.7 through 6.0.12, FortiWeb 7.6.0, FortiWeb 7.4.0 through 7.4.4, FortiWeb 7.2 all versions, FortiWeb 7.0 all versions, FortiWeb 6.4 all versions allows attacker to disclose sensitive information via specially crafted packets.

A Relative Path Traversal vulnerability [CWE-23] in FortiWeb 7.6.0 through 7.6.4, 7.4.0 through 7.4.8, 7.2.0 through 7.2.11, 7.0.2 through 7.0.11 may allow an authenticated attacker to perform an arbitrary file read on the underlying system via crafted requests.

An Improper Privilege Management vulnerability [CWE-269] affecting FortiOS, FortiProxy & FortiWeb may allow an authenticated attacker with at least read-only admin permissions to gain super-admin privileges via crafted requests to Node.js websocket module.

An Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal') vulnerability [CWE-22] in FortiWeb endpoint may allow an authenticated admin to access and modify the filesystem via crafted requests.

An Incorrect User Management vulnerability [CWE-286] in FortiWeb widgets dashboard may allow an authenticated attacker with at least read-only admin permission to perform operations on the dashboard of other administrators via crafted requests.

An exposure of sensitive information to an unauthorized actor vulnerability [CWE-200] in FortiWeb version 7.4.0, version 7.2.4 and below, version 7.0.8 and below, 6.3 all versions may allow an authenticated attacker to read password hashes of other administrators via CLI commands.

An improper output neutralization for logs in Fortinet FortiWeb 6.2.0 - 6.2.8, 6.3.0 - 6.3.23, 7.0.0 - 7.0.9, 7.2.0 - 7.2.5 and 7.4.0 may allow an attacker to forge traffic logs via a crafted URL of the web application.

CVE-2023-48795The SSH transport protocol with certain OpenSSH extensions, found in OpenSSH before 9.6 and other products, allows remote attackers to bypass integrity checks such that some packets are omitted (from the extension negotiation message), and a client and server may consequently end up with a connection for which some security features have been downgraded or disabled, aka a Terrapin attack. This occurs because the SSH Binary Packet Protocol (BPP), implemented by these extensions, mishandles the handshake phase and mishandles use of sequence numbers. For example, there is an effective attack against SSH's use of ChaCha20-Poly1305 (and CBC with Encrypt-then-MAC). The bypass occurs in chacha20-poly1305@openssh.com and (if CBC is used) the -etm@openssh.com MAC algorithms.