diff --git a/lib/overcommit/hook_context/base.rb b/lib/overcommit/hook_context/base.rb
index ec318a3c..839f93b5 100644
--- a/lib/overcommit/hook_context/base.rb
+++ b/lib/overcommit/hook_context/base.rb
@@ -99,7 +99,10 @@ def input_string
       return @input_string if defined?(@input_string)
 
       @input_mutex.synchronize do
-        @input_string = @input.read unless defined?(@input_string)
+        unless defined?(@input_string)
+          @input.binmode if @input.respond_to?(:binmode)
+          @input_string = @input.read
+        end
       end
 
       @input_string
diff --git a/lib/overcommit/subprocess.rb b/lib/overcommit/subprocess.rb
index 8493efab..504fecad 100644
--- a/lib/overcommit/subprocess.rb
+++ b/lib/overcommit/subprocess.rb
@@ -35,7 +35,8 @@ def spawn(args, options = {})
         process.start
         if options[:input]
           begin
-            process.io.stdin.puts(options[:input])
+            process.io.stdin.binmode
+            process.io.stdin.write(options[:input])
           rescue StandardError
             # Silently ignore if the standard input stream of the spawned
             # process is closed before we get a chance to write to it. This
diff --git a/spec/integration/custom_script_spec.rb b/spec/integration/custom_script_spec.rb
new file mode 100644
index 00000000..2d4bca91
--- /dev/null
+++ b/spec/integration/custom_script_spec.rb
@@ -0,0 +1,43 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+require 'json'
+
+describe 'post-rewrite CustomScript input' do
+  around do |example|
+    repo do
+      echo({ 'PostRewrite' => {
+        'ALL' => { 'enabled' => false },
+        'CustomScript' => {
+          'enabled' => true,
+          'command' => [Gem.ruby, 'record-rewrite.rb']
+        }
+      } }.to_yaml, '.overcommit.yml')
+      echo(<<~RUBY, 'record-rewrite.rb')
+        require 'json'
+        STDIN.binmode
+        input = STDIN.read
+        # Like git-branchless, accept an empty stream but reject a blank record.
+        input.each_line do |line|
+          abort 'Invalid rewrite line' unless line.strip.match?(/\\A[0-9a-f]{40} [0-9a-f]{40}\\z/)
+        end
+        File.write('received.json', JSON.generate(args: ARGV, bytes: input.bytes))
+      RUBY
+      install_overcommit_hooks
+      example.run
+    end
+  end
+
+  pair = "#{'a' * 40} #{'b' * 40}"
+  ['', pair, "#{pair}\n", "#{pair}\n#{pair}\n", "#{pair}\r\n"].each do |input|
+    it "forwards the rewrite type and exact input bytes for #{input.inspect}" do
+      result = Overcommit::Subprocess.spawn(
+        [Gem.ruby, '.git/hooks/post-rewrite', 'rebase'], input: input,
+      )
+
+      expect(result.status).to eq(0), result.stdout + result.stderr
+      JSON.parse(File.read('received.json')).should == { 'args' => ['rebase'],
+                                                         'bytes' => input.bytes }
+    end
+  end
+end
diff --git a/spec/overcommit/subprocess_spec.rb b/spec/overcommit/subprocess_spec.rb
index 4778baf0..e2e1c698 100644
--- a/spec/overcommit/subprocess_spec.rb
+++ b/spec/overcommit/subprocess_spec.rb
@@ -51,7 +51,18 @@ def received_argv(result)
 
       it 'passes the input to the standard input stream of the process' do
         subject.should be_success
-        subject.stdout.chomp.should == 'from-stdin'
+        subject.stdout.should == 'from-stdin'
+      end
+    end
+
+    ['', 'no newline', "one line\n", "two\nlines\n", "binary\0\r\n\xFF".b].each do |input|
+      it "preserves input bytes for #{input.inspect}" do
+        result = described_class.spawn(
+          [ruby, '-e', 'STDIN.binmode; STDOUT.print STDIN.read.bytes.join(",")'],
+          input: input,
+        )
+        result.should be_success
+        result.stdout.should == input.bytes.join(',')
       end
     end
 
diff --git a/spec/overcommit/utils_spec.rb b/spec/overcommit/utils_spec.rb
index 10d8c751..13682f94 100644
--- a/spec/overcommit/utils_spec.rb
+++ b/spec/overcommit/utils_spec.rb
@@ -321,7 +321,7 @@
       subject { described_class.execute(arguments, input: input) }
 
       it 'passes the input to the standard input stream' do
-        subject.stdout.should == "Hello world\n"
+        subject.stdout.should == 'Hello world'
       end
     end