diff --git a/app/logical/apng_inspector.rb b/app/logical/apng_inspector.rb
index d8ade47db..30ce02676 100644
--- a/app/logical/apng_inspector.rb
+++ b/app/logical/apng_inspector.rb
@@ -1,70 +1,112 @@
-class APNGInspector
-  attr_reader :frames
-  
-  PNG_MAGIC_NUMBER = ["89504E470D0A1A0A"].pack('H*')
-  
-  def initialize(file_path)
-    @file_path = file_path
-    @corrupted = false
-    @animated = false
-  end
-  
-  #Calls associated block for each PNG chunk
-  #parameters passed are |chunk_name, chunk_length, file_descriptor|
-  #returns true if file is read succesfully from start to IEND, false otherwise. 
-  def each_chunk
-    iend_reached = false
-    File.open(@file_path, 'rb') do |file|
-      return false if file.read(8) != PNG_MAGIC_NUMBER
-      
-      while chunkheader = file.read(8)
-        return false if chunkheader.to_s.length != 8
-
-        chunk_len, chunk_name = chunkheader.unpack("Na4")
-        current_pos = file.tell
-        yield chunk_name, chunk_len, file
-        if chunk_name == "IEND"
-          iend_reached = true
-          break
-        end
-        file.seek(current_pos+chunk_len+4, IO::SEEK_SET)
-      end
-    end
-    return iend_reached
-  end
-  
-  def inspect!
-    actl_corrupted = false
-    
-    read_success = each_chunk do |name, len, file|
-      if name == 'acTL'
-        if len != 8 then
-          actl_corrupted = true
-        else
-          framedata = file.read(4)
-          if framedata.to_s.length != 4
-            actl_corrupted = true
-          else
-            framecount = framedata.unpack("N")[0]
-            if framecount < 1 
-              actl_corrupted = true
-            else
-              @animated = true
-              @frames = framecount
-            end
-          end
-        end
-      end
-    end
-    @corrupted = !read_success || actl_corrupted 
-    return !@corrupted
-  end
-  
-  def corrupted?
-    @corrupted
-  end
-  
-  def animated?
-    !@corrupted && @animated && @frames > 1
-  end
+class APNGInspector
+  attr_reader :frames
+
+  PNG_MAGIC_NUMBER = ["89504E470D0A1A0A"].pack('H*')
+
+  def initialize(file_path)
+    @file_path = file_path
+    @corrupted = false
+    @animated = false
+  end
+
+  #PNG file consists of 8-byte magic number, followed by arbitrary number of chunks
+  #Each chunk has the following structure:
+  #4-byte length (unsigned int, can be zero)
+  #4-byte name (ASCII string consisting of letters A-z)
+  #(length)-byte data
+  #4-byte CRC
+  #
+  #Any data after chunk named IEND is irrelevant
+  #APNG frame count is inside a chunk named acTL, in first 4 bytes of data.
+
+
+  #This function calls associated block for each PNG chunk
+  #parameters passed are |chunk_name, chunk_length, file_descriptor|
+  #returns true if file is read succesfully from start to IEND,
+  #or if 100 000 chunks are read; returns false otherwise.
+  def each_chunk
+    iend_reached = false
+    File.open(@file_path, 'rb') do |file|
+      #check if file is not PNG at all
+      return false if file.read(8) != PNG_MAGIC_NUMBER
+
+      chunks = 0
+
+      #We could be dealing with large number of chunks,
+      #so the code should be optimized to create as few objects as possible.
+      #All literal strings are frozen and read() function uses string buffer.
+      chunkheader = ''
+      while file.read(8, chunkheader)
+        #ensure that first 8 bytes from chunk were read properly
+        if chunkheader == nil || chunkheader.length < 8
+          return false
+        end
+
+        current_pos = file.tell
+
+        chunk_len, chunk_name = chunkheader.unpack("Na4".freeze)
+        return false if chunk_name =~ /[^A-Za-z]/
+        yield chunk_name, chunk_len, file
+
+        #no need to read further if IEND is reached
+        if chunk_name == "IEND".freeze
+          iend_reached = true
+          break
+        end
+
+        #check if we processed too many chunks already
+        #if we did, file is probably maliciously formed
+        #fail gracefully without marking the file as corrupt
+        chunks += 1
+        if chunks > 100000
+          iend_reached = true
+          break
+        end
+
+        #jump to the next chunk - go forward by chunk length + 4 bytes CRC
+        file.seek(current_pos+chunk_len+4, IO::SEEK_SET)
+      end
+    end
+    return iend_reached
+  end
+
+  def inspect!
+    actl_corrupted = false
+
+    read_success = each_chunk do |name, len, file|
+      if name == 'acTL'.freeze
+        framecount = parse_actl(len, file)
+        if framecount < 1
+          actl_corrupted = true
+        else
+          @animated = true
+          @frames = framecount
+        end
+      end
+    end
+
+    @corrupted = !read_success || actl_corrupted
+    return !@corrupted
+  end
+
+  def corrupted?
+    @corrupted
+  end
+
+  def animated?
+    !@corrupted && @animated
+  end
+
+  private
+
+    #return number of frames in acTL or -1 on failure
+    def parse_actl(len, file)
+      return -1 if len != 8
+      framedata = file.read(4)
+      if framedata == nil || framedata.length != 4
+        return -1
+      end
+      return framedata.unpack("N".freeze)[0]
+    end
+
 end
\ No newline at end of file
diff --git a/test/files/apng/actl_wronglen.png b/test/files/apng/actl_wronglen.png
new file mode 100644
index 000000000..b91ae886e
Binary files /dev/null and b/test/files/apng/actl_wronglen.png differ
diff --git a/test/files/apng/actl_zero_frames.png b/test/files/apng/actl_zero_frames.png
new file mode 100644
index 000000000..4ba18e9e4
Binary files /dev/null and b/test/files/apng/actl_zero_frames.png differ
diff --git a/test/files/apng/broken.png b/test/files/apng/broken.png
new file mode 100644
index 000000000..d310545df
Binary files /dev/null and b/test/files/apng/broken.png differ
diff --git a/test/files/apng/empty.png b/test/files/apng/empty.png
new file mode 100644
index 000000000..e69de29bb
diff --git a/test/files/apng/iend_missing.png b/test/files/apng/iend_missing.png
new file mode 100644
index 000000000..4c008b425
Binary files /dev/null and b/test/files/apng/iend_missing.png differ
diff --git a/test/files/apng/jpg.png b/test/files/apng/jpg.png
new file mode 100644
index 000000000..dc221d891
Binary files /dev/null and b/test/files/apng/jpg.png differ
diff --git a/test/files/apng/misaligned_chunks.png b/test/files/apng/misaligned_chunks.png
new file mode 100644
index 000000000..7d4da55fd
Binary files /dev/null and b/test/files/apng/misaligned_chunks.png differ
diff --git a/test/files/apng/normal_apng.png b/test/files/apng/normal_apng.png
new file mode 100644
index 000000000..f79642592
Binary files /dev/null and b/test/files/apng/normal_apng.png differ
diff --git a/test/files/apng/not_apng.png b/test/files/apng/not_apng.png
new file mode 100644
index 000000000..70075493f
Binary files /dev/null and b/test/files/apng/not_apng.png differ
diff --git a/test/files/apng/single_frame.png b/test/files/apng/single_frame.png
new file mode 100644
index 000000000..79f4dc9ab
Binary files /dev/null and b/test/files/apng/single_frame.png differ
diff --git a/test/unit/apng_inspector_test.rb b/test/unit/apng_inspector_test.rb
new file mode 100644
index 000000000..1f734b532
--- /dev/null
+++ b/test/unit/apng_inspector_test.rb
@@ -0,0 +1,64 @@
+require "test_helper"
+
+class DTextTest < ActiveSupport::TestCase
+  def inspect(filename)
+    apng = APNGInspector.new("#{Rails.root}/test/files/apng/#{filename}")
+    apng.inspect!
+    apng
+  end
+  context "APNG inspector" do
+    should "correctly parse normal APNG file" do
+      apng = inspect('normal_apng.png')
+      assert_equal(3, apng.frames)
+      assert_equal(true, apng.animated?)
+      assert_equal(false, apng.corrupted?)
+    end
+
+    should "recognize 1-frame APNG as animated" do
+      apng = inspect('single_frame.png')
+      assert_equal(1, apng.frames)
+      assert_equal(true, apng.animated?)
+      assert_equal(false, apng.corrupted?)
+    end
+
+    should "correctly parse normal PNG file" do
+      apng = inspect('not_apng.png')
+      assert_equal(false, apng.animated?)
+      assert_equal(false, apng.corrupted?)
+    end
+
+    should "handle empty file" do
+      apng = inspect('empty.png')
+      assert_equal(false, apng.animated?)
+      assert_equal(true, apng.corrupted?)
+    end
+
+    should "handle corrupted files" do
+      apng = inspect('iend_missing.png')
+      assert_equal(false, apng.animated?)
+      assert_equal(true, apng.corrupted?)
+      apng = inspect('misaligned_chunks.png')
+      assert_equal(false, apng.animated?)
+      assert_equal(true, apng.corrupted?)
+      apng = inspect('broken.png')
+      assert_equal(false, apng.animated?)
+      assert_equal(true, apng.corrupted?)
+    end
+
+    should "handle incorrect acTL chunk" do
+      apng = inspect('actl_wronglen.png')
+      assert_equal(false, apng.animated?)
+      assert_equal(true, apng.corrupted?)
+      apng = inspect('actl_zero_frames.png')
+      assert_equal(false, apng.animated?)
+      assert_equal(true, apng.corrupted?)
+    end
+
+    should "handle non-png files" do
+      apng = inspect('jpg.png')
+      assert_equal(false, apng.animated?)
+      assert_equal(true, apng.corrupted?)
+    end
+
+  end
+end