Details for this vulnerability have not been published by NIST at this point. Descriptions from software vendor advisories for this issue are provided below.
From VID-2FDB053C-CA25-11ED-9D7E-080027F5FEC9:
ooooooo_q reports:
Carefully crafted input can cause header parsing in Rack
to take an unexpected amount of time, possibly resulting
in a denial of service attack vector. Any applications
that parse headers using Rack (virtually all Rails
applications) are impacted.
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