HTTP/2 Rapid reset attack The HTTP/2 protocol allows clients to indicate to the server that a previous stream should be canceled by sending a RSTSTREAM frame. The protocol does not require the client and server to coordinate the cancellation in any way, the client may do it unilaterally. The client may also assume that the cancellation will take effect immediately when the server receives the RSTSTREAM frame, before any other data from that TCP connection is processed.

Abuse of this feature is called a Rapid Reset attack because it relies on the ability for an endpoint to send a RSTSTREAM frame immediately after sending a request frame, which makes the other endpoint start working and then rapidly resets the request. The request is canceled, but leaves the HTTP/2 connection open.

The HTTP/2 Rapid Reset attack built on this capability is simple: The client opens a large number of streams at once as in the standard HTTP/2 attack, but rather than waiting for a response to each request stream from the server or proxy, the client cancels each request immediately.

The ability to reset streams immediately allows each connection to have an indefinite number of requests in flight. By explicitly canceling the requests, the attacker never exceeds the limit on the number of concurrent open streams. The number of in-flight requests is no longer dependent on the round-trip time (RTT), but only on the available network bandwidth.

In a typical HTTP/2 server implementation, the server will still have to do significant amounts of work for canceled requests, such as allocating new stream data structures, parsing the query and doing header decompression, and mapping the URL to a resource. For reverse proxy implementations, the request may be proxied to the backend server before the RSTSTREAM frame is processed. The client on the other hand paid almost no costs for sending the requests. This creates an exploitable cost asymmetry between the server and the client.

Multiple software artifacts implementing HTTP/2 are affected. This advisory was originally ingested from the swift-nio-http2 repo advisory and their original conent follows.

swift-nio-http2 specific advisory swift-nio-http2 is vulnerable to a denial-of-service vulnerability in which a malicious client can create and then reset a large number of HTTP/2 streams in a short period of time. This causes swift-nio-http2 to commit to a large amount of expensive work which it then throws away, including creating entirely new Channels to serve the traffic. This can easily overwhelm an EventLoop and prevent it from making forward progress.

swift-nio-http2 1.28 contains a remediation for this issue that applies reset counter using a sliding window. This constrains the number of stream resets that may occur in a given window of time. Clients violating this limit will have their connections torn down. This allows clients to continue to cancel streams for legitimate reasons, while constraining malicious actors.

A flaw was found in Undertow. For an AJP 400 response, EAP 7 is improperly sending two response packets, and those packets have the reuse flag set even though JBoss EAP closes the connection. A failure occurs when the connection is reused after a 400 by CPING since it reads in the second SENDHEADERS response packet instead of a CPONG.

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 found in Hibernate ORM in versions before 5.3.18, 5.4.18 and 5.5.0.Beta1. A SQL injection in the implementation of the JPA Criteria API can permit unsanitized literals when a literal is used in the SELECT or GROUP BY parts of the query. This flaw could allow an attacker to access unauthorized information or possibly conduct further attacks.

A flaw was found in HTTP/2. Using SETTINGS frames and queuing of SETTINGS ACK frames, a flood could occur resulting in unbounded memory growth. The highest threat from this vulnerability is to system availability.

A flaw was found in HTTP/2. Using HEADER frames with invalid HTTP headers and queuing of response RSTSTREAM frames, an attacker could cause a flood resulting in unbounded memory growth. The highest threat from this vulnerability is to system availability.

A further incomplete fix for a previous advisory CVE-2026-44417 (Untrusted JMS configuration can lead to RCE) for Apache CXF has been identified, which can allow code execution capabilities, if untrusted users are allowed to configure JMS for Apache CXF. Users are recommended to upgrade to versions 4.2.2 or 4.1.7 or 3.6.12, which fixes this issue.

A denial of service vulnerability was found in keycloak where the amount of attributes per object is not limited, an attacker by sending repeated HTTP requests could cause a resource exhaustion when the application send back rows with long attribute values. The issue is fixed in Keycloak 24 with the introduction of the User Profile feature.

A flaw was found in Undertow. When an AJP request is sent that exceeds the max-header-size attribute in ajp-listener, JBoss EAP is marked in an error state by modcluster in httpd, causing JBoss EAP to close the TCP connection without returning an AJP response. This happens because modproxycluster marks the JBoss EAP instance as an error worker when the TCP connection is closed from the backend after sending the AJP request without receiving an AJP response, and stops forwarding. This issue could allow a malicious user could to repeatedly send requests that exceed the max-header-size, causing a Denial of Service (DoS).

A flaw was found in the redirecturi validation logic in Keycloak. This issue may allow a bypass of otherwise explicitly allowed hosts. A successful attack may lead to an access token being stolen, making it possible for the attacker to impersonate other users.

A vulnerability was found in Undertow where the ProxyProtocolReadListener reuses the same StringBuilder instance across multiple requests. This issue occurs when the parseProxyProtocolV1 method processes multiple requests on the same HTTP connection. As a result, different requests may share the same StringBuilder instance, potentially leading to information leakage between requests or responses. In some cases, a value from a previous request or response may be erroneously reused, which could lead to unintended data exposure. This issue primarily results in errors and connection termination but creates a risk of data leakage in multi-request environments.

A flaw was found in Keycloak, where it does not properly validate URLs included in a redirect. An attacker can use this flaw to construct a malicious request to bypass validation and access other URLs and potentially sensitive information within the domain or possibly conduct further attacks. This flaw affects any client that utilizes a wildcard in the Valid Redirect URIs field.

Acknowledgements: Special thanks to Axel Flamcourt for reporting this issue and helping us improve our project.

A flaw was found in the Undertow HTTP server core, which is used in WildFly, JBoss EAP, and other Java applications. The Undertow library fails to properly validate the Host header in incoming HTTP requests. As a result, requests containing malformed or malicious Host headers are processed without rejection, enabling attackers to poison caches, perform internal network scans, or hijack user sessions.

A flaw was found in Undertow where malformed client requests can trigger server-side stream resets without triggering abuse counters. This issue, referred to as the "MadeYouReset" attack, allows malicious clients to induce excessive server workload by repeatedly causing server-side stream aborts. While not a protocol bug, this highlights a common implementation weakness that can be exploited to cause a denial of service (DoS).

A vulnerability was found in Undertow. This vulnerability impacts a server that supports the wildfly-http-client protocol. Whenever a malicious user opens and closes a connection with the HTTP port of the server and then closes the connection immediately, the server will end with both memory and open file limits exhausted at some point, depending on the amount of memory available.

At HTTP upgrade to remoting, the WriteTimeoutStreamSinkConduit leaks connections if RemotingConnection is closed by Remoting ServerConnectionOpenListener. Because the remoting connection originates in Undertow as part of the HTTP upgrade, there is an external layer to the remoting connection. This connection is unaware of the outermost layer when closing the connection during the connection opening procedure. Hence, the Undertow WriteTimeoutStreamSinkConduit is not notified of the closed connection in this scenario. Because WriteTimeoutStreamSinkConduit creates a timeout task, the whole dependency tree leaks via that task, which is added to XNIO WorkerThread. So, the workerThread points to the Undertow conduit, which contains the connections and causes the leak.

A flaw was found in Undertow. When Undertow receives an HTTP request where the first header line starts with one or more spaces, it incorrectly processes the request by stripping these leading spaces. This behavior, which violates HTTP standards, can be exploited by a remote attacker to perform request smuggling. Request smuggling allows an attacker to bypass security mechanisms, access restricted information, or manipulate web caches, potentially leading to unauthorized actions or data exposure.

A flaw was found in Undertow. This vulnerability allows a remote attacker to construct specially crafted requests where header names are parsed differently by Undertow compared to upstream proxies. This discrepancy in header interpretation can be exploited to launch request smuggling attacks, potentially bypassing security controls and accessing unauthorized resources.

A flaw was found in Undertow. A remote attacker can exploit this vulnerability by sending \r\r\r as a header block terminator. This can be used for request smuggling with certain proxy servers, such as older versions of Apache Traffic Server and Google Cloud Classic Application Load Balancer, potentially leading to unauthorized access or manipulation of web requests.

Duplicate Advisory This advisory has been withdrawn because it is a duplicate of GHSA-w8gr-xwp4-r9f7. This link is maintained to preserve external references.

Original Description A misconfiguration flaw was found in Keycloak. This issue can allow an attacker to redirect users to an arbitrary URL if a 'Valid Redirect URI' is set to http://localhost or http://127.0.0.1, enabling sensitive information such as authorization codes to be exposed to the attacker, potentially leading to session hijacking.

A flaw was found in undertow. The undertow client is not checking the server identity the server certificate presents in HTTPS connections. This is a compulsory step ( that should at least be performed by default) in HTTPS and in http/2.

A flaw was found in Undertow with EJB invocations. This flaw allows an attacker to generate a valid HTTP request and send it to the server on an established connection after removing the LASTCHUNK from the bytes, causing a denial of service.

A flaw was found in XNIO, specifically in the notifyReadClosed method. The issue revealed this method was logging a message to another expected end. This flaw allows an attacker to send flawed requests to a server, possibly causing log contention-related performance concerns or an unwanted disk fill-up.

A flaw was found in Undertow. A buffer leak on the incoming WebSocket PONG message may lead to memory exhaustion. This flaw allows an attacker to cause a denial of service. The highest threat from this vulnerability is availability.

A vulnerability was found in Hibernate-Validator. The SafeHtml validator annotation fails to properly sanitize payloads consisting of potentially malicious code in HTML comments and instructions. This vulnerability can result in an XSS attack.

A flaw was found in undertow where HTTP2SourceChannel fails to write final frame under some circumstances may result in DoS. The highest impact of this vulnerability is availability.

A flaw was found in Undertow in versions before 2.1.1.Final, regarding the processing of invalid HTTP requests with large chunk sizes. This flaw allows an attacker to take advantage of HTTP request smuggling.

A vulnerability was found in Infinispan before version 10.0.0 Final. The invokeAccessibly method from the public class ReflectionUtil allows any application class to invoke private methods in any class with Infinispan's privileges.

Apache CXF's EndpointReferenceUtils and W3CMultiSchemaFactory classes construct a SAXParserFactory without the necessary JAXP hardening configurations, enabling out-of-band (OOB) external entity resolution. Users are recommended to upgrade to versions 4.2.2 or 4.1.7 or 3.6.12, which fix this issue.

An issue was discovered in FasterXML jackson-databind prior to 2.7.9.4, 2.8.11.2, and 2.9.6. When Default Typing is enabled (either globally or for a specific property), the service has the Oracle JDBC jar in the classpath, and an attacker can provide an LDAP service to access, it is possible to make the service execute a malicious payload.

A flaw was discovered in FasterXML jackson-databind, where it would permit polymorphic deserialization of malicious objects using the ehcache and logback JNDI gadgets when used in conjunction with polymorphic type handling methods such as enableDefaultTyping() or when @JsonTypeInfo is using Id.CLASS or Id.MINIMALCLASS or in any other way which ObjectMapper.readValue might instantiate objects from unsafe sources. An attacker could use this flaw to execute arbitrary code.