CVE-2019-12521: Canonical Ubuntu Linux

Medium severity, CVSS 5.9. EPSS: 5.9% chance of exploitation in the next 30 days.

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.

Affected products

• Canonical Ubuntu Linux: version 16.04 only; version 18.04 only; version 19.10 only; version 20.04 only
• Debian Debian Linux: version 9.0 only; version 10.0 only
• Opensuse Leap: version 15.1 only
• Squid-Cache Squid: from 3.0, up to and including 3.5.28; from 4.0, up to and including 4.7; from 5.0, up to and including 5.0.1

Published 2020-04-15. Last modified 2026-06-17.