I came across a piece of interesting vulnerable script from a post on V2EX ¹) on V2EX. A bash function named __curl inside the file retrieves contents over HTTP as a simple alternative for command curl or wget, in scenarios where no such utilities available.

#!/bin/bash
function __curl() {
  read proto server path <<<$(echo ${1//// })
  DOC=/${path// //}
  HOST=${server//:*}
  PORT=${server//*:}
  [[ x"${HOST}" == x"${PORT}" ]] && PORT=80

  exec 3<>/dev/tcp/${HOST}/$PORT
  echo -en "GET ${DOC} HTTP/1.0\r\nHost: ${HOST}\r\n\r\n" >&3
  (while read line; do
   [[ "$line" == $'\r' ]] && break
  done && cat) <&3
  exec 3>&-
}

The function makes use of certain less known features of Linux and the Bash language.

The first one is communicating over TCP through files. Linux employs a design philosophy of “everything are files”. One could find some special device files in directory /dev, through which we can manipulate the underlying devices. Specifically, manipulating a TCP socket connecting ${HOST}:${PORT} could be achieved by accessing device file /dev/tcp/${HOST}/${PORT}. Since HTTP is a text-based protocol over TCP, working with it is no more difficult than reading / writing a text file. Line exec 3<>$FILENAME opens file $FILENAME under read-write mode and binds it to descriptor 3. The next line then manually composes an HTTP payload and writes out to &3, which is in fact requesting the URL http ://${HOST}:${PORT}. By reading the same file descriptor, we retrieve the response content from the service. The trick serves as a primitive workaround for retrieving contents from web.

Another one is parameter substitution in Bash. The expression ${var//PATTERN/REPL} substitutes all occurrences of PATTERN in var into REPL. If REPL omitted, the matched substrings will be deleted. For example, in this script, ${1//// } would replace all slashes / into white spaces in variable $1.

References

1. Parameter Substitution

Author: hsfzxjy.
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