diff --git a/lib/rubygems/package/tar_header.rb b/lib/rubygems/package/tar_header.rb
index c54bd14d5780..d5573571142e 100644
--- a/lib/rubygems/package/tar_header.rb
+++ b/lib/rubygems/package/tar_header.rb
@@ -104,25 +104,30 @@ def self.from(stream)
     fields = header.unpack UNPACK_FORMAT
 
     new :name     => fields.shift,
-        :mode     => fields.shift.oct,
-        :uid      => fields.shift.oct,
-        :gid      => fields.shift.oct,
-        :size     => fields.shift.oct,
-        :mtime    => fields.shift.oct,
-        :checksum => fields.shift.oct,
+        :mode     => strict_oct(fields.shift),
+        :uid      => strict_oct(fields.shift),
+        :gid      => strict_oct(fields.shift),
+        :size     => strict_oct(fields.shift),
+        :mtime    => strict_oct(fields.shift),
+        :checksum => strict_oct(fields.shift),
         :typeflag => fields.shift,
         :linkname => fields.shift,
         :magic    => fields.shift,
-        :version  => fields.shift.oct,
+        :version  => strict_oct(fields.shift),
         :uname    => fields.shift,
         :gname    => fields.shift,
-        :devmajor => fields.shift.oct,
-        :devminor => fields.shift.oct,
+        :devmajor => strict_oct(fields.shift),
+        :devminor => strict_oct(fields.shift),
         :prefix   => fields.shift,
 
         :empty => empty
   end
 
+  def self.strict_oct(str)
+    return str.oct if str =~ /\A[0-7]*\z/
+    raise ArgumentError, "#{str.inspect} is not an octal string"
+  end
+
   ##
   # Creates a new TarHeader using +vals+
 
diff --git a/test/rubygems/test_gem_package_tar_header.rb b/test/rubygems/test_gem_package_tar_header.rb
index d33877057d0b..a0719a753163 100644
--- a/test/rubygems/test_gem_package_tar_header.rb
+++ b/test/rubygems/test_gem_package_tar_header.rb
@@ -143,5 +143,25 @@ def test_update_checksum
     assert_equal '012467', @tar_header.checksum
   end
 
+  def test_from_bad_octal
+    test_cases = [
+      "00000006,44\000", # bogus character
+      "00000006789\000", # non-octal digit
+      "+0000001234\000", # positive sign
+      "-0000001000\000", # negative sign
+      "0x000123abc\000", # radix prefix
+    ]
+
+    test_cases.each do |val|
+      header_s = @tar_header.to_s
+      # overwrite the size field
+      header_s[124, 12] = val
+      io = TempIO.new header_s
+      assert_raises ArgumentError do
+        new_header = Gem::Package::TarHeader.from io
+      end
+    end
+  end
+
 end