ksh93 -- certain environment variables interpreted as arithmetic expressions on startup, leading to code injection

Upstream ksh93 maintainer Siteshwar Vashisht reports:

A flaw was found in the way ksh evaluates certain environment
variables. An attacker could use this flaw to override or bypass
environment restrictions to execute shell commands. Services and
applications that allow remote unauthenticated attackers to
provide one of those environment variables could allow them to
exploit this issue remotely.