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 vulnerability in the Dynamic Vectoring and Streaming (DVS) Engine implementation of Cisco AsyncOS Software for Cisco Secure Web Appliance could allow an unauthenticated, remote attacker to bypass the anti-malware scanner, allowing malicious archive files to be

A vulnerability in the Dynamic Vectoring and Streaming (DVS) Engine implementation of Cisco AsyncOS Software for Cisco Secure Web Appliance could allow an unauthenticated, remote attacker to bypass the anti-malware scanner, allowing malicious archive files to be downloaded. This vulnerability is due to improper handling of certain archive files. An attacker could exploit this vulnerability by sending a crafted archive file, which should be blocked, through an affected device. A successful exploit could allow the attacker to bypass the anti-malware scanner and download malware onto an end user workstation. The downloaded malware will not automatically execute unless the end user extracts and launches the malicious file. 

A vulnerability in the web-based management interface of Cisco AsyncOS for Cisco Web Security Appliance (WSA) could allow an authenticated, remote attacker to conduct a stored cross-site scripting (XSS) attack against a user of the interface of an affected device. The vulnerability exists because the web-based management interface does not properly validate user-supplied input. An attacker could exploit this vulnerability by inserting malicious data into a specific data field in an affected interface. A successful exploit could allow the attacker to execute arbitrary script code in the context of the affected interface.

Cisco Web Security Appliance (WSA) 8.0.7 allows remote HTTP servers to cause a denial of service (memory consumption from stale TCP connections) via crafted responses, aka Bug ID CSCuw10426.

Cisco Web Security Appliance (WSA) 8.0.6-078 and 8.0.6-115 allows remote attackers to cause a denial of service (service outage) via a flood of TCP traffic that leads to DNS resolution delays, aka Bug IDs CSCur32005 and CSCur07907.

The remote-support feature on Cisco Web Security Virtual Appliance (WSAv), Email Security Virtual Appliance (ESAv), and Security Management Virtual Appliance (SMAv) devices before 2015-06-25 uses the same default SSH host keys across different customers' installations, which makes it easier for remote attackers to defeat cryptographic protection mechanisms by leveraging knowledge of a private key from another installation, aka Bug IDs CSCus29681, CSCuu95676, and CSCuu96601.

The remote-support feature on Cisco Web Security Virtual Appliance (WSAv), Email Security Virtual Appliance (ESAv), and Security Management Virtual Appliance (SMAv) devices before 2015-06-25 uses the same default SSH root authorized key across different customers' installations, which makes it easier for remote attackers to bypass authentication by leveraging knowledge of a private key from another installation, aka Bug IDs CSCuu95988, CSCuu95994, and CSCuu96630.

A vulnerability in the web proxy functionality of the Cisco Web Security Appliance (WSA) could allow an unauthenticated, remote attacker to forward traffic from the web proxy interface of an affected device to the administrative management interface of an affected device, aka an Access Control Bypass Vulnerability. Affected Products: virtual and hardware versions of Cisco Web Security Appliance (WSA). More Information: CSCvd88863. Known Affected Releases: 10.1.0-204 9.0.0-485.

A vulnerability in the CLI parser of the Cisco Web Security Appliance (WSA) could allow an authenticated, local attacker to perform command injection and elevate privileges to root. The attacker must authenticate with valid operator-level or administrator-level credentials. Affected Products: virtual and hardware versions of Cisco Web Security Appliance (WSA). More Information: CSCvd88855. Known Affected Releases: 10.1.0-204. Known Fixed Releases: 10.5.1-270 10.1.1-234.

A vulnerability in AsyncOS for the Cisco Web Security Appliance (WSA) could allow an unauthenticated, local attacker to log in to the device with the privileges of a limited user or an unauthenticated, remote attacker to authenticate to certain areas of the web GUI, aka a Static Credentials Vulnerability. Affected Products: virtual and hardware versions of Cisco Web Security Appliance (WSA). More Information: CSCve06124. Known Affected Releases: 10.1.0-204. Known Fixed Releases: 10.5.1-270.

A vulnerability in the web-based management interface of Cisco Web Security Appliance (WSA) could allow an authenticated, remote attacker to conduct a stored cross-site scripting (XSS) attack against a user of the web-based management interface of an affected device. Affected Products: virtual and hardware versions of Cisco Web Security Appliance (WSA). More Information: CSCvd88865. Known Affected Releases: 10.1.0-204.