Security Bulletin: SSL Handshake Denial of Service

BY: Sanic Dev Team

Security Bulletin: SSL Handshake Denial of Service

Notice This bulletin was posted on March 4th, 2019 and has been updated as of March 7th, 2019

Background On Sunday February 24th the Sanic Community was contacted about a recently discovered issue when implementing SSL directly through Sanic. The core development team reviewed options and impact and has identified several remediation paths.

Description Sanic running under versions of Python prior to 3.7 are impacted by a possible denial of service whereby the SSL handshake is left in an incomplete state, causing connection exhaustion or memory exhaustion, preventing further connections. This is due to Python not implementing a connection timeout for SSL prior to version 3.7. Ref:

Who is affected? Anyone using Sanic to terminate SSL connections and using Python versions 3.6.x and prior when not using uvloop (installed with SANIC_NO_UVLOOP, as for example in alpine-based containers)

Remediation options, in order of preference - Update to Python 3.7 - the fix is included in Python itself - Terminate SSL at a load balancer or a proxy (such as AWS' ALB or nginx) - Implement the patch created by Richard Kuesters - see: This includes simple examples including how to see the DOS issue. Earlier revisions indicated an increase on overhead, however we are backporting the change to 3.7 for the handshake, and so no additional impact is expected.