diff --git a/spec/compiler/formatter/formatter_spec.cr b/spec/compiler/formatter/formatter_spec.cr
index 7c9cc990e116..754093190048 100644
--- a/spec/compiler/formatter/formatter_spec.cr
+++ b/spec/compiler/formatter/formatter_spec.cr
@@ -2608,4 +2608,34 @@ describe Crystal::Formatter do
         CRYSTAL
     end
   end
+
+  # CVE-2021-42574
+  describe "Unicode bi-directional control characters" do
+    ['\u202A', '\u202B', '\u202C', '\u202D', '\u202E', '\u2066', '\u2067', '\u2068', '\u2069'].each do |char|
+      assert_format %("#{char}"), %("#{char.unicode_escape}")
+      assert_format %("\\c#{char}"), %("c#{char.unicode_escape}")
+      assert_format %("#{char}\#{1}"), %("#{char.unicode_escape}\#{1}")
+      assert_format %("\\c#{char}\#{1}"), %("c#{char.unicode_escape}\#{1}")
+      assert_format %(%(#{char})), %(%(#{char.unicode_escape}))
+      assert_format %(%Q(#{char})), %(%Q(#{char.unicode_escape}))
+      assert_format %(%Q(#{char}\#{1})), %(%Q(#{char.unicode_escape}\#{1}))
+      assert_format %(<<-EOS\n#{char}\nEOS), %(<<-EOS\n#{char.unicode_escape}\nEOS)
+      assert_format %(<<-EOS\n#{char}\#{1}\nEOS), %(<<-EOS\n#{char.unicode_escape}\#{1}\nEOS)
+      assert_format %(def foo("#{char}" x)\nend), %(def foo("#{char.unicode_escape}" x)\nend)
+      assert_format %(foo("#{char}": 1)), %(foo("#{char.unicode_escape}": 1))
+      assert_format %(NamedTuple("#{char}": Int32)), %(NamedTuple("#{char.unicode_escape}": Int32))
+      assert_format %({"#{char}": 1}), %({"#{char.unicode_escape}": 1})
+
+      # the following contexts do not accept escape sequences, escaping these
+      # control characters would alter the meaning of the source code
+      assert_format %(/#{char}/)
+      assert_format %(%r(#{char}))
+      assert_format %(%q(#{char}))
+      assert_format %(%w(#{char}))
+      assert_format %(%i(#{char}))
+      assert_format %(/#{char}\#{1}/)
+      assert_format %(%r(#{char}\#{1}))
+      assert_format %(<<-'EOS'\n#{char}\nEOS)
+    end
+  end
 end
diff --git a/src/compiler/crystal/syntax/lexer.cr b/src/compiler/crystal/syntax/lexer.cr
index 98d2e4a7d2dc..7bafb7ddae04 100644
--- a/src/compiler/crystal/syntax/lexer.cr
+++ b/src/compiler/crystal/syntax/lexer.cr
@@ -2735,6 +2735,21 @@ module Crystal
       end
     end
 
+    private UNICODE_BIDI_CONTROL_CHARACTERS = {'\u202A', '\u202B', '\u202C', '\u202D', '\u202E', '\u2066', '\u2067', '\u2068', '\u2069'}
+
+    private FORBIDDEN_ESCAPES = UNICODE_BIDI_CONTROL_CHARACTERS.to_h { |ch| {ch, ch.unicode_escape} }
+
+    # used by the formatter
+    def self.escape_forbidden_characters(string)
+      string.each_char do |ch|
+        if ch.in?(UNICODE_BIDI_CONTROL_CHARACTERS)
+          return string.gsub(FORBIDDEN_ESCAPES)
+        end
+      end
+
+      string
+    end
+
     def next_char_no_column_increment
       char = @reader.next_char
       if error = @reader.error
diff --git a/src/compiler/crystal/tools/formatter.cr b/src/compiler/crystal/tools/formatter.cr
index 25915b29ab75..540487758842 100644
--- a/src/compiler/crystal/tools/formatter.cr
+++ b/src/compiler/crystal/tools/formatter.cr
@@ -491,11 +491,7 @@ module Crystal
       while true
         case @token.type
         when .string?
-          if @token.invalid_escape
-            write @token.value
-          else
-            write @token.raw
-          end
+          write_sanitized_string_body(@token.delimiter_state.allow_escapes && !is_regex)
           next_string_token
         when .interpolation_start?
           # This is the case of #{__DIR__}
@@ -534,6 +530,12 @@ module Crystal
       false
     end
 
+    private def write_sanitized_string_body(escape)
+      body = @token.invalid_escape ? @token.value.as(String) : @token.raw
+      body = Lexer.escape_forbidden_characters(body) if escape
+      write body
+    end
+
     def visit(node : StringInterpolation)
       if @token.delimiter_state.kind.heredoc?
         # For heredoc, only write the start: on a newline will print it
@@ -609,7 +611,7 @@ module Crystal
           else
             loop do
               check :STRING
-              write @token.invalid_escape ? @token.value : @token.raw
+              write_sanitized_string_body(delimiter_state.allow_escapes && !is_regex)
               next_string_token
 
               # On heredoc, pieces of contents are combined due to removing indentation.