fix: prevent extracting archived files outside of target path

This PR is meant to fix an arbitrary file write vulnerability, that can be
achieved using a specially crafted zip archive, that holds path traversal
filenames. When the filename gets concatenated to the target extraction
directory, the final path ends up outside of the target folder.

A sample malicious zip file named Zip.Evil.zip was used,
and when running the code below, resulted in the creation of C:/Temp/evil.txt
outside of the intended target directory.

There are various possible ways to avoid this issue, some include checking
for .. (dot dot) characters in the filename, but the best solution in our
opinion is to check if the final target filename, starts with the target
folder (after both are resolved to their absolute path).

Stay secure,
Snyk Team

Author: odinn1984

src/SharpCompress/Archives/IArchiveEntryExtensions.cs

@@ -48,6 +48,7 @@ public static void WriteToDirectory(this IArchiveEntry entry, string destination
         {
             string destinationFileName;
             string file = Path.GetFileName(entry.Key);
+            string fullDestinationDirectoryPath = Path.GetFullPath(destinationDirectory);
 
             options = options ?? new ExtractionOptions()
                                  {
@@ -58,19 +59,35 @@ public static void WriteToDirectory(this IArchiveEntry entry, string destination
             if (options.ExtractFullPath)
             {
                 string folder = Path.GetDirectoryName(entry.Key);
-                string destdir = Path.Combine(destinationDirectory, folder);
+                string destdir = Path.GetFullPath(
+                                    Path.Combine(fullDestinationDirectoryPath, folder)
+                                 );
+
                 if (!Directory.Exists(destdir))
                 {
+                    if (!destdir.StartsWith(fullDestinationDirectoryPath))
+                    {
+                        throw new ExtractionException("Entry is trying to create a directory outside of the destination directory.");
+                    }
+
                     Directory.CreateDirectory(destdir);
                 }
                 destinationFileName = Path.Combine(destdir, file);
             }
             else
             {
-                destinationFileName = Path.Combine(destinationDirectory, file);
+                destinationFileName = Path.Combine(fullDestinationDirectoryPath, file);
             }
+
             if (!entry.IsDirectory)
             {
+                destinationFileName = Path.GetFullPath(destinationFileName);
+
+                if (!destinationFileName.StartsWith(fullDestinationDirectoryPath))
+                {
+                    throw new ExtractionException("Entry is trying to write a file outside of the destination directory.");
+                }
+
                 entry.WriteToFile(destinationFileName, options);
             }
         }

tests/SharpCompress.Test/Zip/ZipArchiveTests.cs

@@ -433,7 +433,34 @@ public void Zip_Deflate_PKWear_Multipy_Entry_Access()
                     }
                 }
             }
+        }
+
+        [Fact]
+        public void Zip_Evil_Throws_Exception()
+        {
+            Exception expectedExcetpion = null;
+            string zipFile = Path.Combine(TEST_ARCHIVES_PATH, "Zip.Evil.zip");
+
+            try
+            { 
+                using (var archive = ZipArchive.Open(zipFile))
+                {
+                    foreach (var entry in archive.Entries.Where(entry => !entry.IsDirectory))
+                    {
+                        entry.WriteToDirectory(SCRATCH_FILES_PATH, new ExtractionOptions()
+                        {
+                            ExtractFullPath = true,
+                            Overwrite = true
+                        });
+                    }
+                }
+            }
+            catch (Exception ex)
+            {
+                expectedExcetpion = ex;
+            }
 
+            Assert.NotEqual(expectedExcetpion, null);
         }
 
         class NonSeekableMemoryStream : MemoryStream