# Server Side Inclusion/Edge Side Inclusion Injection
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## Server Side Inclusion Basic Information
**(Introduction taken from [Apache docs](https://httpd.apache.org/docs/current/howto/ssi.html))**
SSI (Server Side Includes) are directives that are **placed in HTML pages, and evaluated on the server** while the pages are being served. They let you **add dynamically generated content** to an existing HTML page, without having to serve the entire page via a CGI program, or other dynamic technology.\
For example, you might place a directive into an existing HTML page, such as:
``
And, when the page is served, this fragment will be evaluated and replaced with its value:
`Tuesday, 15-Jan-2013 19:28:54 EST`
The decision of when to use SSI, and when to have your page entirely generated by some program, is usually a matter of how much of the page is static, and how much needs to be recalculated every time the page is served. SSI is a great way to add small pieces of information, such as the current time - shown above. But if a majority of your page is being generated at the time that it is served, you need to look for some other solution.
You can infer the presence of SSI if the web application uses files with the extensions \*\* `.shtml`, `.shtm` or `.stm`\*\*, but it's not only the case.
A typical SSI expression has the following format:
```
```
### Check
```javascript
// Document name
// Date
// File inclusion
// Including files (same directory)
// CGI Program results
// Including virtual files (same directory)
// Modification date of a file
// Command exec
// Command exec
// Reverse shell
// Print all variables
// Setting variables
```
## Edge Side Inclusion
There is a problem **caching information or dynamic applications** as part of the content may have **varied** for the next time the content is retrieved. This is what **ESI** is used form, to indicate using ESI tags the **dynamic content that needs to be generated** before sending the cache version.\
if an **attacker** is able to **inject an ESI tag** inside the cache content, then, he could be able to i**nject arbitrary content** on the document before it's sent to the users.
### ESI Detection
The following **header** in a response from the server means that the server is using ESI:
```
Surrogate-Control: content="ESI/1.0"
```
If you can't find this header, the server **might be using ESI anyways**.\
A **blind exploitation approach can also be used** as a request should arrive to the attackers server:
```javascript
// Basic detection
hello
// If previous is reflected as "hello", it's vulnerable
// Blind detection
// XSS Exploitation Example
// Cookie Stealer (bypass httpOnly flag)
// Introduce private local files (Not LFI per se)
// Valid for Akamai, sends debug information in the response
```
### ESI exploitation
[GoSecure created](https://www.gosecure.net/blog/2018/04/03/beyond-xss-edge-side-include-injection/) a table to understand possible attacks that we can try against different ESI-capable software, depending on the functionality supported:
* **Includes**: Supports the `` directive
* **Vars**: Supports the `` directive. Useful for bypassing XSS Filters
* **Cookie**: Document cookies are accessible to the ESI engine
* **Upstream Headers Required**: Surrogate applications will not process ESI statements unless the upstream application provides the headers
* **Host Allowlist**: In this case, ESI includes are only possible from allowed server hosts, making SSRF, for example, only possible against those hosts
| **Software** | **Includes** | **Vars** | **Cookies** | **Upstream Headers Required** | **Host Whitelist** |
| :--------------------------: | :----------: | :------: | :---------: | :---------------------------: | :----------------: |
| Squid3 | Yes | Yes | Yes | Yes | No |
| Varnish Cache | Yes | No | No | Yes | Yes |
| Fastly | Yes | No | No | No | Yes |
| Akamai ESI Test Server (ETS) | Yes | Yes | Yes | No | No |
| NodeJS esi | Yes | Yes | Yes | No | No |
| NodeJS nodesi | Yes | No | No | No | Optional |
#### XSS
The following ESI directive will load an arbitrary file inside the response of the server
```xml
```
#### Bypass client XSS protection
```xml
x=>alert(/Chrome%20XSS%20filter%20bypass/);>
Use to bypass WAFs:
ipt>alert(1)ript>
error=alert(1)>
```
#### Steal Cookie
* Remote steal cookie
```xml
```
* Steal cookie HTTP\_ONLY with XSS by reflecting it in the response:
```bash
# This will reflect the cookies in the response
# Reflect XSS (you can put '">