diff --git a/CHANGELOG b/CHANGELOG
index 3677ac67bb2dc698e7c21cc50f47f9e66933670d..c18d3d719b9027de66d2f25e771dcb68785fd603 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -1,3 +1,6 @@
+v 10.0.1
+  - Only force_encode strings to detected encoding if confidence is above threshold.
+
 v 9.0.3
   - Add a method to copy the .gitattributes into the bare repository
 
diff --git a/lib/gitlab_git/encoding_helper.rb b/lib/gitlab_git/encoding_helper.rb
index 730e16a377bb9b74e9485bf6d47a9864127cf129..2d668871b5ac7be24cb102a9775a1451f3484ce8 100644
--- a/lib/gitlab_git/encoding_helper.rb
+++ b/lib/gitlab_git/encoding_helper.rb
@@ -1,6 +1,17 @@
 module EncodingHelper
   extend self
 
+  # This threshold is carefully tweaked to prevent usage of encodings detected
+  # by CharlockHolmes with low confidence. If CharlockHolmes confidence is low,
+  # we're better off sticking with utf8 encoding.
+  # Reason: git diff can return strings with invalid utf8 byte sequences if it
+  # truncates a diff in the middle of a multibyte character. In this case
+  # CharlockHolmes will try to guess the encoding and will likely suggest an
+  # obscure encoding with low confidence.
+  # There is a lot more info with this merge request:
+  # https://gitlab.com/gitlab-org/gitlab_git/merge_requests/77#note_4754193
+  ENCODING_CONFIDENCE_THRESHOLD = 40
+
   def encode!(message)
     return nil unless message.respond_to? :force_encoding
 
@@ -12,8 +23,8 @@ module EncodingHelper
     detect = CharlockHolmes::EncodingDetector.detect(message)
     return message.force_encoding("BINARY") if detect && detect[:type] == :binary
 
-    # encoding message to detect encoding
-    if detect && detect[:encoding]
+    # force detected encoding if we have sufficient confidence.
+    if detect && detect[:encoding] && detect[:confidence] > ENCODING_CONFIDENCE_THRESHOLD
       message.force_encoding(detect[:encoding])
     end
 
diff --git a/spec/encoding_helper_spec.rb b/spec/encoding_helper_spec.rb
new file mode 100644
index 0000000000000000000000000000000000000000..2c8dbcd76b52e929dae1fedce4650ebc398ed7ce
--- /dev/null
+++ b/spec/encoding_helper_spec.rb
@@ -0,0 +1,85 @@
+require "spec_helper"
+require_relative '../lib/gitlab_git/encoding_helper'
+
+describe EncodingHelper do
+  let(:ext_class) { Class.new { extend EncodingHelper } }
+  let(:binary_string) { File.join(SUPPORT_PATH, 'gitlab_logo.png') }
+
+  describe '#encode!' do
+    [
+      [
+        'leaves ascii only string as is',
+        'ascii only string',
+        'ascii only string'
+      ],
+      [
+        'leaves valid utf8 string as is',
+        'multibyte string №∑∉',
+        'multibyte string №∑∉'
+      ],
+      [
+        'removes invalid bytes from ASCII-8bit encoded multibyte string. This can occur when a git diff match line truncates in the middle of a multibyte character. This occurs after the second word in this example. The test string is as short as we can get while still triggering the error condition when not looking at `detect[:confidence]`.',
+        "mu ns\xC3\n Lorem ipsum dolor sit amet, consectetur adipisicing ut\xC3\xA0y\xC3\xB9abcd\xC3\xB9efg kia elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non p\n {: .normal_pn}\n \n-Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in\n# *Lorem ipsum\xC3\xB9l\xC3\xB9l\xC3\xA0 dolor\xC3\xB9k\xC3\xB9 sit\xC3\xA8b\xC3\xA8 N\xC3\xA8 amet b\xC3\xA0d\xC3\xAC*\n+# *consectetur\xC3\xB9l\xC3\xB9l\xC3\xA0 adipisicing\xC3\xB9k\xC3\xB9 elit\xC3\xA8b\xC3\xA8 N\xC3\xA8 sed do\xC3\xA0d\xC3\xAC*{: .italic .smcaps}\n \n \xEF\x9B\xA1 eiusmod tempor incididunt, ut\xC3\xAAn\xC3\xB9 labore et dolore. Tw\xC4\x83nj\xC3\xAC magna aliqua. Ut enim ad minim veniam\n {: .normal}\n@@ -9,5 +9,5 @@ quis nostrud\xC3\xAAt\xC3\xB9 exercitiation ullamco laboris m\xC3\xB9s\xC3\xB9k\xC3\xB9abc\xC3\xB9 nisi ".force_encoding('ASCII-8BIT'),
+        "mu ns\n Lorem ipsum dolor sit amet, consectetur adipisicing ut\xC3\xA0y\xC3\xB9abcd\xC3\xB9efg kia elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non p\n {: .normal_pn}\n \n-Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in\n# *Lorem ipsum\xC3\xB9l\xC3\xB9l\xC3\xA0 dolor\xC3\xB9k\xC3\xB9 sit\xC3\xA8b\xC3\xA8 N\xC3\xA8 amet b\xC3\xA0d\xC3\xAC*\n+# *consectetur\xC3\xB9l\xC3\xB9l\xC3\xA0 adipisicing\xC3\xB9k\xC3\xB9 elit\xC3\xA8b\xC3\xA8 N\xC3\xA8 sed do\xC3\xA0d\xC3\xAC*{: .italic .smcaps}\n \n \xEF\x9B\xA1 eiusmod tempor incididunt, ut\xC3\xAAn\xC3\xB9 labore et dolore. Tw\xC4\x83nj\xC3\xAC magna aliqua. Ut enim ad minim veniam\n {: .normal}\n@@ -9,5 +9,5 @@ quis nostrud\xC3\xAAt\xC3\xB9 exercitiation ullamco laboris m\xC3\xB9s\xC3\xB9k\xC3\xB9abc\xC3\xB9 nisi ",
+      ],
+    ].each do |description, test_string, xpect|
+      it description do
+        expect(ext_class.encode!(test_string)).to eq(xpect)
+      end
+    end
+
+    it 'leaves binary string as is' do
+      expect(ext_class.encode!(binary_string)).to eq(binary_string)
+    end
+  end
+
+  describe '#encode_utf8' do
+    [
+      [
+        "encodes valid utf8 encoded string to utf8",
+        "λ, λ, λ".encode("UTF-8"),
+        "λ, λ, λ".encode("UTF-8"),
+      ],
+      [
+        "encodes valid ASCII-8BIT encoded string to utf8",
+        "ascii only".encode("ASCII-8BIT"),
+        "ascii only".encode("UTF-8"),
+      ],
+      [
+        "encodes valid ISO-8859-1 encoded string to utf8",
+        "Rüby ist eine Programmiersprache. Wir verlängern den text damit ICU die Sprache erkennen kann.".encode("ISO-8859-1", "UTF-8"),
+        "Rüby ist eine Programmiersprache. Wir verlängern den text damit ICU die Sprache erkennen kann.".encode("UTF-8"),
+      ],
+    ].each do |description, test_string, xpect|
+      it description do
+        r = ext_class.encode_utf8(test_string.force_encoding('UTF-8'))
+        expect(r).to eq(xpect)
+        expect(r.encoding.name).to eq('UTF-8')
+      end
+    end
+  end
+
+  describe '#clean' do
+    [
+      [
+        'leaves ascii only string as is',
+        'ascii only string',
+        'ascii only string'
+      ],
+      [
+        'leaves valid utf8 string as is',
+        'multibyte string №∑∉',
+        'multibyte string №∑∉'
+      ],
+      [
+        'removes invalid bytes from ASCII-8bit encoded multibyte string.',
+        "Lorem ipsum\xC3\n dolor sit amet, xy\xC3\xA0y\xC3\xB9abcd\xC3\xB9efg".force_encoding('ASCII-8BIT'),
+        "Lorem ipsum\n dolor sit amet, xyàyùabcdùefg",
+      ],
+    ].each do |description, test_string, xpect|
+      it description do
+        expect(ext_class.encode!(test_string)).to eq(xpect)
+      end
+    end
+  end
+end
diff --git a/spec/support/gitlab_logo.png b/spec/support/gitlab_logo.png
new file mode 100644
index 0000000000000000000000000000000000000000..4b455657c0a64adb47ba070bf6cd8ac999937e9c
Binary files /dev/null and b/spec/support/gitlab_logo.png differ