CVE-2019-12519

An issue was discovered in Squid through 4.7. When handling the tag
esi:when when ESI is enabled, Squid calls ESIExpression::Evaluate. This
function uses a fixed stack buffer to hold the expression while it’s being
evaluated. When processing the expression, it could either evaluate the top
of the stack, or add a new member to the stack. When adding a new member,
there is no check to ensure that the stack won’t overflow.