diff --git a/.gitignore b/.gitignore
index 1900b08..a9dd2a6 100644
--- a/.gitignore
+++ b/.gitignore
@@ -2,6 +2,7 @@
 diagnostic.partial
 diagnostic.test
 tests/*.diff
+tests/*.actual
 spec/
 spec-runner/
 node_modules/
diff --git a/README.md b/README.md
index b193a09..3f0b2b0 100644
--- a/README.md
+++ b/README.md
@@ -118,6 +118,18 @@ There are more scripts available in the [demos directory](demo/) that could help
 There are additional features that the program supports. Try using `mo --help` to see what is available.
 
 
+Security
+-----------
+
+Because `mo` uses environment variables that could be unsafe in the event that `mo` is invoked programmatically with user-input, `mo` provides `--strict` mode when used with `--source=`. `mo` will attempt to track variables that are created by each `--source=`'ed environment file, and will disallow access to external variables.
+
+To use this, `--strict` must be used with `--source=`:
+
+```bash
+./mo --strict --source=.env my-template.mustache
+```
+
+
 Concessions
 -----------
 
diff --git a/mo b/mo
index c928d7c..75f5dc7 100755
--- a/mo
+++ b/mo
@@ -95,8 +95,15 @@
 #                  treated as an empty value for the purposes of conditionals.
 # MO_ORIGINAL_COMMAND - Used to find the `mo` program in order to generate a
 #                  help message.
+# MO_STRICT_MODE - Used to limit access to variables outside of sourced files
 #
 # Returns nothing.
+
+MO_HAS_SOURCED=false
+MO_STRICT_SWITCH=false
+MO_STRICT_MODE=false
+MO_SOURCED_VARS=()
+
 mo() (
     # This function executes in a subshell so IFS is reset.
     # Namespace this variable so we don't conflict with desired values.
@@ -119,6 +126,10 @@ mo() (
                         exit 0
                         ;;
 
+                    --strict)
+                        MO_STRICT_SWITCH=true
+                        ;;
+
                     --allow-function-arguments)
                         # shellcheck disable=SC2030
                         MO_ALLOW_FUNCTION_ARGUMENTS=true
@@ -147,8 +158,21 @@ mo() (
                         fi
 
                         if [[ -f "$f2source" ]]; then
+                            MO_HAS_SOURCED=true
+
+                            local BEFORE_VARS AFTER_VARS ALL_VARS
+                            BEFORE_VARS=(`cat <(set -o posix ; set) | cut -d'=' -f1`)
+
                             # shellcheck disable=SC1090
                             . "$f2source"
+                            AFTER_VARS=(`cat <(set -o posix ; set) | cut -d'=' -f1`)
+
+                            ALL_VARS=( ${BEFORE_VARS[@]} ${AFTER_VARS[@]} )
+                            MO_SOURCED_VARS=( \
+                                ${MO_SOURCED_VARS[@]} \
+                                $(echo "${ALL_VARS[@]}" | tr ' ' '\n' | sort | uniq -u)\
+                            )
+                            unset BEFORE_VARS AFTER_VARS ALL_VARS
                         else
                             echo "No such file: $f2source" >&2
                             exit 1
@@ -169,8 +193,17 @@ mo() (
         done
     fi
 
+    # Allow turning off Strict Mode
+    if [[ $MO_HAS_SOURCED == true ]] && [[ $MO_STRICT_SWITCH == true ]]; then
+        MO_STRICT_MODE=true
+    fi
+
     moGetContent moContent "${files[@]}" || return 1
     moParse "$moContent" "" true
+
+    for v in ${MO_SOURCED_VARS[@]}; do
+        unset -v $v > /dev/null 2>&1
+    done
 )
 
 
@@ -908,6 +941,7 @@ moShow() {
             fi
         fi
     else
+        moTestLegal "${moNameParts[0]}" || return 1
         # Further subindexes are disallowed
         eval "echo -n \"\${${moNameParts[0]}[${moNameParts[1]%%.*}]}\""
     fi
@@ -989,6 +1023,18 @@ moStandaloneDenied() {
 }
 
 
+# Internal: Determines if a variable name is outside of the scope of the template
+#
+# Returns 0 if variable is safe, Returns 1 if variable is illegal
+moTestLegal() {
+    [[ $MO_STRICT_MODE != true ]] && return 0
+    [[ " ${MO_SOURCED_VARS[*]} " =~ " ${1%.*} " ]] && return 0
+
+    echo "Illegal variable access $1" >&2
+    return 1
+}
+
+
 # Internal: Determines if the named thing is a function or if it is a
 # non-empty environment variable.  When MO_FALSE_IS_EMPTY is set to a
 # non-empty value, then "false" is also treated is an empty value.
@@ -1004,6 +1050,9 @@ moStandaloneDenied() {
 # Returns 0 if the name is not empty, 1 otherwise.  When MO_FALSE_IS_EMPTY
 # is set, this returns 1 if the name is "false".
 moTest() {
+    # Don't allow access to outside names
+    moTestLegal "$1" || return 1
+
     # Test for functions
     moIsFunction "$1" && return 0
 
@@ -1030,7 +1079,11 @@ moTest() {
 #
 # Returns true (0) if the variable is set, 1 if the variable is unset.
 moTestVarSet() {
-    [[ "${!1-a}" == "${!1-b}" ]]
+    # Don't allow access to outside names
+    moTestLegal "$1" || return 1
+
+    [[ "${!1-a}" == "${!1-b}" ]] && return 0
+    return 1
 }
 
 
diff --git a/run-tests b/run-tests
index b1a0331..01219f5 100755
--- a/run-tests
+++ b/run-tests
@@ -23,6 +23,9 @@ for TEST in tests/*.expected; do
             . "${BASE}.env"
             echo "Do not read this input" | mo "${BASE}.template"
         fi
+    ) | (
+        cat > "${BASE}.actual";
+        cat "${BASE}.actual"
     ) | diff -U5 - "${TEST}" > "${BASE}.diff"
 
     statusCode=$?
@@ -34,6 +37,7 @@ for TEST in tests/*.expected; do
         echo "ok"
         PASS=$(( PASS + 1 ))
         rm "${BASE}.diff"
+        rm "${BASE}.actual"
     fi
 done
 
diff --git a/tests/source.expected b/tests/source.expected
index a3050ef..83220ad 100644
--- a/tests/source.expected
+++ b/tests/source.expected
@@ -1,3 +1,4 @@
+Purposely Unsafe for Backwards Compatibility
 value
 * 1
 * 2
diff --git a/tests/source.sh b/tests/source.sh
index 7b35d4d..83b96d8 100755
--- a/tests/source.sh
+++ b/tests/source.sh
@@ -2,6 +2,7 @@
 
 cd "${0%/*}" || exit 1
 cat <<EOF | ../mo --source=source.vars
+{{#BASH}}Purposely Unsafe for Backwards Compatibility{{/BASH}}
 {{VAR}}
 {{#ARR}}
 * {{.}}
diff --git a/tests/strict-source-multiple-1.vars b/tests/strict-source-multiple-1.vars
new file mode 100644
index 0000000..433ea91
--- /dev/null
+++ b/tests/strict-source-multiple-1.vars
@@ -0,0 +1,2 @@
+export A=from1
+export B=from1
diff --git a/tests/strict-source-multiple-2.vars b/tests/strict-source-multiple-2.vars
new file mode 100644
index 0000000..8b0b476
--- /dev/null
+++ b/tests/strict-source-multiple-2.vars
@@ -0,0 +1,3 @@
+export B=from2
+export C=from2
+declare -a -x D=(from2)
diff --git a/tests/strict-source-multiple.expected b/tests/strict-source-multiple.expected
new file mode 100644
index 0000000..4e48ef2
--- /dev/null
+++ b/tests/strict-source-multiple.expected
@@ -0,0 +1,5 @@
+A: from1
+B: from2
+C: from2
+D: from2
+D: from2
diff --git a/tests/strict-source-multiple.sh b/tests/strict-source-multiple.sh
new file mode 100755
index 0000000..59b02f1
--- /dev/null
+++ b/tests/strict-source-multiple.sh
@@ -0,0 +1,32 @@
+#!/usr/bin/env bash
+
+cd "${0%/*}" || exit 1
+
+_CTRL_Z_=$'\cZ'
+
+{
+    IFS=$'\n'"${_CTRL_Z_}" read -r -d "${_CTRL_Z_}" STDERR;
+    IFS=$'\n'"${_CTRL_Z_}" read -r -d "${_CTRL_Z_}" STDOUT;
+} <<EOF
+$((printf "${_CTRL_Z_}%s${_CTRL_Z_}%d${_CTRL_Z_}" "$(\
+cat <<EOMO | ../mo --strict --source=strict-source-multiple-1.vars --source=strict-source-multiple-2.vars
+{{#BASH_VERSINFO}}Illegal{{/BASH_VERSINFO}}
+{{BASH_VERSINFO.0}}
+A: {{A}}
+B: {{B}}
+C: {{C}}
+D: {{D.0}}
+D: {{#D}}{{.}}{{/D}}
+EOMO
+)" "${?}" 1>&2) 2>&1)
+EOF
+
+echo "${STDOUT[@]}"
+
+expectedErr="Illegal variable access BASH_VERSINFO"
+if [[ ! "$STDERR" =~ "$expectedErr" ]]; then
+    echo "STDERR should have contained an illegal variable access message:"
+    echo "$STDERR"
+fi
+
+exit 0