When caching servers and load balancers became an integral part of the Internet's infrastructure, vendors introduced "Edge Side Includes" (ESI), a technology allowing malleability in caching systems. This legacy technology, still implemented in nearly all popular HTTP surrogates (caching/load balancing services), is dangerous by design and brings a yet unexplored vector for web-based attacks.

The ESI language consists of a small set of instructions represented by XML tags, served by the backend application server, which are processed on the Edge servers (load balancers, reverse proxies). Due to the upstream-trusting nature of Edge servers, ESI engines are not able to distinguish between ESI instructions legitimately provided by the application server and malicious instructions injected by a malicious party. We identified that ESI can be used to perform SSRF, bypass reflected XSS filters (Chrome), and perform Javascript-less cookie theft, including HTTPOnly cookies.

Identified affected vendors include Akamai, Varnish, Squid, Fastly, WebSphere, WebLogic, F5, and countless language-specific solutions (NodeJS, Ruby, etc.). This presentation will start by introducing ESI and visiting typical infrastructures leveraging it. We will then delve into identification, exploitation of popular ESI engines, and mitigation.


ldionmarcil, Pentester at GoSecure

Louis is a Security Analyst working at GoSecure in Montreal where he specializes in offensive appsec and pentest on medium to large scale organizations. Seasoned CTF participant and sometimes finalist with the DCIETS team, he has also written challenges for various competitions. Having recently obtained his Software Engineering degree, he dabbles in various research engagements between pentests.


Detailed Presentation:

(Source: DEF CON 26)
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