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 flaw was discovered in jackson-databind in versions before 2.9.10, 2.8.11.5 and 2.6.7.3, where it would permit polymorphic deserialization of a malicious object using commons-configuration 1 and 2 JNDI classes. An attacker could use this flaw to execute arbitrary code.

A vulnerability which allows for a potential privilege escalation was found in the Hibernate Validator. If a security manager is present and HV itself is allowed to access private members reflectively as per the SM's configuration, that'll allow calling code without that permission to get hold of private state. The attack vector is to declare a constraint on a private member using XML, validate an invalid instance of that type and access the private member value via ConstraintViolation#getInvalidValue().

A flaw was found in org.codehaus.jackson:jackson-mapper-asl:1.9.x libraries such that an XML external entity (XXE) vulnerability affects codehaus's jackson-mapper-asl libraries. This vulnerability is similar to CVE-2016-3720. The primary threat from this flaw is data integrity.

A cross-site scripting (XSS) vulnerability was found in the JBoss Management Console versions before 7.1.6.CR1, 7.1.6.GA. Users with roles that can create objects in the application can exploit this to attack other privileged users.

It was found that JGroups did not require necessary headers for encrypt and auth protocols from new nodes joining the cluster. An attacker could use this flaw to bypass security restrictions, and use this vulnerability to send and receive messages within the cluster, leading to information disclosure, message spoofing, or further possible attacks.

A flaw was found in Wildfly in versions before 23.0.2.Final while creating a new role in domain mode via the admin console, it is possible to add a payload in the name field, leading to XSS. This affects Confidentiality and Integrity.

A flaw was found in the Java logging library Apache Log4j in version 1.x . This allows a remote attacker to execute code on the server if the deployed application is configured to use JMSAppender.

In 1.x you will find that there are two places where lookups are done - that is JMSAppender.java:207 and JMSAppender.java:222 - if you set TopicBindingName or TopicConnectionFactoryBindingName to something that JNDI can handle - for example "ldap://host:port/a" JNDI will do exactly the same thing it does for 2.x - so 1.x is vulnerable, just attack vector is "safer" as it depends on configuration rather than user input

This flaw in Log4j 2.x is tracked via CVE-2021-44228

A deserialization flaw in jackson-databind was found allowing code execution when given maliocusly crafted input to readValue method of ObjectMapper.

It was discovered that the jboss init script as used in Red Hat JBoss Enterprise Application Platform 7.0.7.GA performed unsafe file handling which could result in local privilege escalation. This issue is a result of an incomplete fix for CVE-2016-8656.

It was found that the JAXP implementation used in JBoss EAP 7.0 for SAX and DOM parsing is vulnerable to certain XXE flaws. An attacker could use this flaw to cause DoS, SSRF, or information disclosure if they are able to provide XML content for parsing.