CVE-2019-13574 (published 2019-07-12T03:15Z)

In lib/mini_magick/image.rb in MiniMagick before 4.9.4, a fetched remote image filename could cause remote command execution because Image.open input is directly passed to Kernel#open, which accepts a '|' character followed by a command.

Severity: MEDIUM
CVSS v2 base score: 6.8 (AV:N/AC:M/Au:N/C:P/I:P/A:P)
CVSS v3 base score: 7.8, HIGH (CVSS:3.0/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H)

Weakness information:
cwe_id: CWE-78
cwe_name: Improper Neutralization of Special Elements used in an OS Command ('OS Command Injection')
description: The product constructs all or part of an OS command using externally-influenced input from an upstream component, but it does not neutralize or incorrectly neutralizes special elements that could modify the intended OS command when it is sent to a downstream component.
extended_description: 
		   This weakness can lead to a vulnerability in environments in which the attacker does not have direct access to the operating system, such as in web applications. Alternately, if the weakness occurs in a privileged program, it could allow the attacker to specify commands that normally would not be accessible, or to call alternate commands with privileges that the attacker does not have. The problem is exacerbated if the compromised process does not follow the principle of least privilege, because the attacker-controlled commands may run with special system privileges that increases the amount of damage.
            There are at least two subtypes of OS command injection:
               
				 The application intends to execute a single, fixed program that is under its own control. It intends to use externally-supplied inputs as arguments to that program. For example, the program might use system("nslookup [HOSTNAME]") to run nslookup and allow the user to supply a HOSTNAME, which is used as an argument. Attackers cannot prevent nslookup from executing. However, if the program does not remove command separators from the HOSTNAME argument, attackers could place the separators into the arguments, which allows them to execute their own program after nslookup has finished executing.
				 The application accepts an input that it uses to fully select which program to run, as well as which commands to use. The application simply redirects this entire command to the operating system. For example, the program might use "exec([COMMAND])" to execute the [COMMAND] that was supplied by the user. If the COMMAND is under attacker control, then the attacker can execute arbitrary commands or programs. If the command is being executed using functions like exec() and CreateProcess(), the attacker might not be able to combine multiple commands together in the same line.
               
            From a weakness standpoint, these variants represent distinct programmer errors. In the first variant, the programmer clearly intends that input from untrusted parties will be part of the arguments in the command to be executed. In the second variant, the programmer does not intend for the command to be accessible to any untrusted party, but the programmer probably has not accounted for alternate ways in which malicious attackers can provide input.
         
         

Fix information:
repo_url: https://github.com/minimagick/minimagick
commit_details: repo_url: https://github.com/minimagick/minimagick
author: Janko Marohnić
author_date: 2019-05-26 17:30:14+02:00
msg: Don't allow remote shell execution

Kernel#open accepts a string of format "| <shell command>" which
executes the specified shell command and otherwise presumably acts as
IO.popen. The open-uri standard library overrides Kernel#open to also
accept URLs.

However, the overridden Kernel#open just delegates to URI#open, so we
switch to using that directly and avoid the remote shell execution
vulnerability. For files we just use File.open, which should have the
same behaviour as Kernel#open.
repository: repo_url: https://github.com/minimagick/minimagick
repo_name: minimagick/minimagick
description: mini replacement for RMagick
repo_language: Ruby
owner: minimagick
file_changes: filename: image.rb
change_type: ModificationType.MODIFY
diff: @@ -82,17 +82,15 @@ module MiniMagick
     def self.open(path_or_url, ext = nil, options = {})
       options, ext = ext, nil if ext.is_a?(Hash)
 
-      ext ||=
-        if File.exist?(path_or_url)
-          File.extname(path_or_url)
-        else
-          File.extname(URI(path_or_url).path)
-        end
+      uri = URI(path_or_url.to_s)
 
+      ext ||= File.extname(uri.path)
       ext.sub!(/:.*/, '') # hack for filenames or URLs that include a colon
 
-      Kernel.open(path_or_url, "rb", options) do |file|
-        read(file, ext)
+      if uri.is_a?(URI::HTTP) || uri.is_a?(URI::FTP)
+        uri.open(options) { |file| read(file, ext) }
+      else
+        File.open(uri.to_s, "rb", options) { |file| read(file, ext) }
       end
     end
 

programming_language: Ruby
filename: image_spec.rb
change_type: ModificationType.MODIFY
diff: @@ -76,6 +76,14 @@ require "webmock/rspec"
           expect(File.extname(image.path)).to eq ".jpg"
         end
 
+        it "doesn't allow remote shell execution" do
+          expect {
+            described_class.open("| touch file.txt") # Kernel#open accepts this
+          }.to raise_error(URI::InvalidURIError)
+
+          expect(File.exist?("file.txt")).to eq(false)
+        end
+
         it "accepts open-uri options" do
           stub_request(:get, "http://example.com/image.jpg")
             .with(headers: {"Foo" => "Bar"})

programming_language: Ruby