Ruby: Add Module#const_get as a code execution

Module#const_get takes a single string argument and interprets it as the
name of a constant. It then looks up the constant and returns its value.

    Object.const_get("Math::PI")
    # => 3.141592653589793

By itself, this method is not as dangerous as e.g. eval, but if the
value returned is a class that is then instantiated, this can allow an
attacker to instantiate arbitrary Ruby classes.

As a result, I think it's safe to say that any remote input flowing into
this call is a potential vulnerability. A real-world example of this is
https://github.com/advisories/GHSA-52p9-v744-mwjj.

ruby/ql/lib/codeql/ruby/frameworks/StandardLibrary.qll

@@ -335,6 +335,18 @@ class ModuleEvalCallCodeExecution extends CodeExecution::Range, DataFlow::CallNo
   override DataFlow::Node getCode() { result = this.getArgument(0) }
 }
 
+/**
+ * A call to `Module#const_get`, which interprets its argument as a Ruby constant.
+ * Passing user input to this method may result in instantiation of arbitrary Ruby classes.
+ */
+class ModuleConstGetCallCodeExecution extends CodeExecution::Range, DataFlow::CallNode {
+  ModuleConstGetCallCodeExecution() {
+    this.asExpr().getExpr().(UnknownMethodCall).getMethodName() = "const_get"
+  }
+
+  override DataFlow::Node getCode() { result = this.getArgument(0) }
+}
+
 /** Flow summary for `Regexp.escape` and its alias, `Regexp.quote`. */
 class RegexpEscapeSummary extends SummarizedCallable {
   RegexpEscapeSummary() { this = "Regexp.escape" }

ruby/ql/test/library-tests/frameworks/StandardLibrary.expected

@@ -93,3 +93,10 @@ loggerLoggingCallInputs
 | Logging.rb:73:5:73:63 | call to log | Logging.rb:73:36:73:45 | "message1" |
 | Logging.rb:74:5:74:76 | call to log | Logging.rb:74:36:74:45 | "message2" |
 | Logging.rb:74:5:74:76 | call to log | Logging.rb:74:48:74:58 | "progname2" |
+moduleConstGetCallCodeExecutions
+| const_get.rb:1:1:1:24 | call to const_get | const_get.rb:1:18:1:23 | "Math" |
+| const_get.rb:2:1:2:28 | call to const_get | const_get.rb:2:22:2:27 | "Math" |
+| const_get.rb:3:1:3:20 | call to const_get | const_get.rb:3:16:3:19 | "PI" |
+| const_get.rb:4:1:4:19 | call to const_get | const_get.rb:4:16:4:18 | :PI |
+| const_get.rb:22:1:22:33 | call to const_get | const_get.rb:22:18:22:32 | "Foo::Baz::VAL" |
+| const_get.rb:23:1:23:25 | call to const_get | const_get.rb:23:15:23:24 | "Bar::VAL" |

ruby/ql/test/library-tests/frameworks/StandardLibrary.ql

@@ -32,3 +32,7 @@ query DataFlow::Node moduleEvalCallCodeExecutions(ModuleEvalCallCodeExecution e)
 }
 
 query DataFlow::Node loggerLoggingCallInputs(LoggerLoggingCall c) { result = c.getAnInput() }
+
+query DataFlow::Node moduleConstGetCallCodeExecutions(ModuleConstGetCallCodeExecution e) {
+  result = e.getCode()
+}

ruby/ql/test/library-tests/frameworks/const_get.rb

@@ -0,0 +1,27 @@
+Object.const_get("Math")
+self.class.const_get("Math")
+Math.const_get("PI")
+Math.const_get(:PI)
+
+module Foo
+  class Bar
+    VAL = 10
+
+    def const_get(x)
+      "my custom const_get method"
+    end
+  end
+
+  class Baz < Bar
+    def self.const_get(x)
+        "another custom const_get method"
+    end
+  end
+end
+
+Object.const_get("Foo::Baz::VAL")
+Foo.const_get("Bar::VAL")
+
+# Should not be identified as a use of Module#const_get
+Foo::Bar.new.const_get 5
+Foo::Baz.const_get 5

ruby/ql/test/query-tests/security/cwe-094/CodeInjection.expected

@@ -3,6 +3,8 @@ edges
 | CodeInjection.rb:3:12:3:24 | ...[...] :  | CodeInjection.rb:6:10:6:13 | code |
 | CodeInjection.rb:3:12:3:24 | ...[...] :  | CodeInjection.rb:18:20:18:23 | code |
 | CodeInjection.rb:3:12:3:24 | ...[...] :  | CodeInjection.rb:21:21:21:24 | code |
+| CodeInjection.rb:3:12:3:24 | ...[...] :  | CodeInjection.rb:27:15:27:18 | code |
+| CodeInjection.rb:3:12:3:24 | ...[...] :  | CodeInjection.rb:30:19:30:22 | code |
 nodes
 | CodeInjection.rb:3:12:3:17 | call to params :  | semmle.label | call to params :  |
 | CodeInjection.rb:3:12:3:24 | ...[...] :  | semmle.label | ...[...] :  |
@@ -10,9 +12,13 @@ nodes
 | CodeInjection.rb:9:10:9:15 | call to params | semmle.label | call to params |
 | CodeInjection.rb:18:20:18:23 | code | semmle.label | code |
 | CodeInjection.rb:21:21:21:24 | code | semmle.label | code |
+| CodeInjection.rb:27:15:27:18 | code | semmle.label | code |
+| CodeInjection.rb:30:19:30:22 | code | semmle.label | code |
 subpaths
 #select
 | CodeInjection.rb:6:10:6:13 | code | CodeInjection.rb:3:12:3:17 | call to params :  | CodeInjection.rb:6:10:6:13 | code | This code execution depends on $@. | CodeInjection.rb:3:12:3:17 | call to params | a user-provided value |
 | CodeInjection.rb:9:10:9:15 | call to params | CodeInjection.rb:9:10:9:15 | call to params | CodeInjection.rb:9:10:9:15 | call to params | This code execution depends on $@. | CodeInjection.rb:9:10:9:15 | call to params | a user-provided value |
 | CodeInjection.rb:18:20:18:23 | code | CodeInjection.rb:3:12:3:17 | call to params :  | CodeInjection.rb:18:20:18:23 | code | This code execution depends on $@. | CodeInjection.rb:3:12:3:17 | call to params | a user-provided value |
 | CodeInjection.rb:21:21:21:24 | code | CodeInjection.rb:3:12:3:17 | call to params :  | CodeInjection.rb:21:21:21:24 | code | This code execution depends on $@. | CodeInjection.rb:3:12:3:17 | call to params | a user-provided value |
+| CodeInjection.rb:27:15:27:18 | code | CodeInjection.rb:3:12:3:17 | call to params :  | CodeInjection.rb:27:15:27:18 | code | This code execution depends on $@. | CodeInjection.rb:3:12:3:17 | call to params | a user-provided value |
+| CodeInjection.rb:30:19:30:22 | code | CodeInjection.rb:3:12:3:17 | call to params :  | CodeInjection.rb:30:19:30:22 | code | This code execution depends on $@. | CodeInjection.rb:3:12:3:17 | call to params | a user-provided value |

ruby/ql/test/query-tests/security/cwe-094/CodeInjection.rb

@@ -22,6 +22,15 @@ class UsersController < ActionController::Base
 
     # GOOD
     Bar.class_eval(code)
+    
+    # BAD
+    const_get(code)
+    
+    # BAD
+    Foo.const_get(code)
+    
+    # GOOD
+    Bar.const_get(code)
   end
 
   def update
@@ -50,4 +59,8 @@ class Bar
   def self.class_eval(x)
     true
   end
+  
+  def self.const_get(x)
+    true
+  end
 end