diff --git a/CHANGELOG.md b/CHANGELOG.md
index b7ccdf964..df1b3ac05 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4,6 +4,10 @@ All notable changes to this project will be documented in this file. For info on
 
 ## [3.0.0] - Unreleased
 
+### Security
+
+- Do not use semicolon as GET parameter separator. ([#1733](https://github.com/rack/rack/pull/1733), [@jeremyevans](https://github.com/jeremyevans))
+
 ### Added
 
 - `Rack::RewindableInput` supports size. ([@ahorek](https://github.com/ahorek))
diff --git a/lib/rack/query_parser.rb b/lib/rack/query_parser.rb
index dbbb18e5a..cea6b9b09 100644
--- a/lib/rack/query_parser.rb
+++ b/lib/rack/query_parser.rb
@@ -4,7 +4,7 @@ module Rack
   class QueryParser
     (require_relative 'core_ext/regexp'; using ::Rack::RegexpExtensions) if RUBY_VERSION < '2.4'
 
-    DEFAULT_SEP = /[&;] */n
+    DEFAULT_SEP = /[&] */n
     COMMON_SEP = { ";" => /[;] */n, ";," => /[;,] */n, "&" => /[&] */n }
 
     # ParameterTypeError is the error that is raised when incoming structural
@@ -29,16 +29,15 @@ def initialize(params_class, key_space_limit, param_depth_limit)
     end
 
     # Stolen from Mongrel, with some small modifications:
-    # Parses a query string by breaking it up at the '&'
-    # and ';' characters.  You can also use this to parse
-    # cookies by changing the characters used in the second
-    # parameter (which defaults to '&;').
-    def parse_query(qs, d = nil, &unescaper)
+    # Parses a query string by breaking it up at the '&'.  You can also use this
+    # to parse cookies by changing the characters used in the second parameter
+    # (which defaults to '&').
+    def parse_query(qs, separator = nil, &unescaper)
       unescaper ||= method(:unescape)
 
       params = make_params
 
-      (qs || '').split(d ? (COMMON_SEP[d] || /[#{d}] */n) : DEFAULT_SEP).each do |p|
+      (qs || '').split(separator ? (COMMON_SEP[separator] || /[#{separator}] */n) : DEFAULT_SEP).each do |p|
         next if p.empty?
         k, v = p.split('=', 2).map!(&unescaper)
 
@@ -61,11 +60,11 @@ def parse_query(qs, d = nil, &unescaper)
     # query strings with parameters of conflicting types, in this case a
     # ParameterTypeError is raised. Users are encouraged to return a 400 in this
     # case.
-    def parse_nested_query(qs, d = nil)
+    def parse_nested_query(qs, separator = nil)
       params = make_params
 
       unless qs.nil? || qs.empty?
-        (qs || '').split(d ? (COMMON_SEP[d] || /[#{d}] */n) : DEFAULT_SEP).each do |p|
+        (qs || '').split(separator ? (COMMON_SEP[separator] || /[#{separator}] */n) : DEFAULT_SEP).each do |p|
           k, v = p.split('=', 2).map! { |s| unescape(s) }
 
           normalize_params(params, k, v, param_depth_limit)
diff --git a/lib/rack/request.rb b/lib/rack/request.rb
index 74377ede3..541c3801b 100644
--- a/lib/rack/request.rb
+++ b/lib/rack/request.rb
@@ -427,7 +427,7 @@ def GET
         if get_header(RACK_REQUEST_QUERY_STRING) == query_string
           get_header(RACK_REQUEST_QUERY_HASH)
         else
-          query_hash = parse_query(query_string, '&;')
+          query_hash = parse_query(query_string, '&')
           set_header(RACK_REQUEST_QUERY_STRING, query_string)
           set_header(RACK_REQUEST_QUERY_HASH, query_hash)
         end
diff --git a/test/spec_request.rb b/test/spec_request.rb
index 6b73cd5b5..6bc77b01a 100644
--- a/test/spec_request.rb
+++ b/test/spec_request.rb
@@ -308,12 +308,12 @@ def initialize(*)
     req.params[:quux].must_equal "bla"
   end
 
-  it "use semi-colons as separators for query strings in GET" do
+  it "does not use semi-colons as separators for query strings in GET" do
     req = make_request(Rack::MockRequest.env_for("/?foo=bar&quux=b;la;wun=duh"))
     req.query_string.must_equal "foo=bar&quux=b;la;wun=duh"
-    req.GET.must_equal "foo" => "bar", "quux" => "b", "la" => nil, "wun" => "duh"
+    req.GET.must_equal "foo" => "bar", "quux" => "b;la;wun=duh"
     req.POST.must_be :empty?
-    req.params.must_equal "foo" => "bar", "quux" => "b", "la" => nil, "wun" => "duh"
+    req.params.must_equal "foo" => "bar", "quux" => "b;la;wun=duh"
   end
 
   it "limit the keys from the GET query string" do