CVE-2019-12521

An issue was discovered in Squid through 4.7. When Squid is parsing ESI, it
keeps the ESI elements in ESIContext. ESIContext contains a buffer for
holding a stack of ESIElements. When a new ESIElement is parsed, it is
added via addStackElement. addStackElement has a check for the number of
elements in this buffer, but it’s off by 1, leading to a Heap Overflow of 1
element. The overflow is within the same structure so it can’t affect
adjacent memory blocks, and thus just leads to a crash while processing.