Netty susceptible to HTTP/2 Reset Attack with different on-the-wire signature

Summary

Netty HTTP/2 max header size handling produces attack similar to HTTP/2 Rapid Reset.

Details

There is a setting in the http2 specification called SETTINGS_MAX_HEADER_LIST_SIZE. According to the RFC: “This advisory setting informs a peer of the maximum field section size that the sender is prepared to accept, in units of octets.”

When a client sends that setting to Netty, it appears that Netty will behave as follows:

• Read the request
• Proxy the request to the origin
• Attempt to produce a response
• Create an exception while writing the headers for the response

Functionally, this should be similar to the http2 reset attack, but with a different on-the-wire signature.

Remediation

When speaking with clients, Netty should potentially treat this as “advisory” and ignore it. It would be best to ignore the SETTINGS_MAX_HEADER_LIST_SIZE setting from clients (or ignore it when sending to clients). According to the spec, a server does not need to honor this advisory setting, and it appears that other http/2 implementations ignore it when acting as a server.

Impact

This is a DDoS attack similar to the HTTP/2 Rapid Reset Attack.

Credit

Jonathan Looney (Engineering, Netflix)

Contact

Ashley Tolbert (Security, Netflix) - artolbert@netflix.com