Deprecate/warn usage of yaml.load(input)

The `load` and `load_all` methods will issue a warning when they are
called without the 'Loader=' parameter. The warning will point to a URL
that is always up to date with the latest information on the usage of
`load`.

There are several ways to stop the warning:

* Use `full_load(input)` - sugar for `yaml.load(input, FullLoader)`
  * FullLoader is the new safe but complete loader class
* Use `safe_load(input)` - sugar for `yaml.load(input, SafeLoader)`
  * Make sure your input YAML consists of the 'safe' subset
* Use `unsafe_load(input)` - sugar for `yaml.load(input, UnsafeLoader)`
  * Make sure your input YAML consists of the 'safe' subset
* Use `yaml.load(input, Loader=yaml.<loader>)`
  * Or shorter `yaml.load(input, yaml.<loader>)`
  * Where '<loader>' can be:
    * FullLoader - safe, complete Python YAML loading
    * SafeLoader - safe, partial Python YAML loading
    * UnsafeLoader - more explicit name for the old, unsafe 'Loader' class
* yaml.warnings({'YAMLLoadWarning': False})
  * Use this when you use third party modules that use `yaml.load(input)`
  * Only do this if input is trusted

The above `load()` expressions all have `load_all()` counterparts.

You can get the original unsafe behavior with:
* `yaml.unsafe_load(input)`
* `yaml.load(input, Loader=yaml.UnsafeLoader)`

In a future release, `yaml.load(input)` will raise an exception.

The new loader called FullLoader is almost entirely complete as
Loader/UnsafeLoader but it does it avoids all known code execution
paths. It is the preferred YAML loader, and the current default for
`yaml.load(input)` when you get the warning.

Here are some of the exploits that can be triggered with UnsafeLoader
but not with FullLoader:
```
python -c 'import os, yaml; yaml.full_load("!!python/object/new:os.system [echo EXPLOIT!]")'`
python -c 'import yaml; print yaml.full_load("!!python/object/new:abs [-5]")'
python -c 'import yaml; yaml.full_load("!!python/object/new:eval [exit(5)]")' ; echo $?
python -c 'import yaml; yaml.full_load("!!python/object/new:exit [5]")' ; echo $?

lib/yaml/__init__.py

@@ -8,14 +8,52 @@
 from loader import *
 from dumper import *
 
-__version__ = '4.1'
+__version__ = '3.13'
 
 try:
     from cyaml import *
     __with_libyaml__ = True
 except ImportError:
     __with_libyaml__ = False
 
+
+#------------------------------------------------------------------------------
+# Warnings control
+#------------------------------------------------------------------------------
+
+# 'Global' warnings state:
+_warnings_enabled = {
+    'YAMLLoadWarning': True,
+}
+
+# Get or set global warnings' state
+def warnings(settings=None):
+    if settings is None:
+        return _warnings_enabled
+
+    if type(settings) is dict:
+        for key in settings:
+            if key in _warnings_enabled:
+                _warnings_enabled[key] = settings[key]
+
+# Warn when load() is called without Loader=...
+class YAMLLoadWarning(RuntimeWarning):
+    pass
+
+def load_warning(method):
+    if _warnings_enabled['YAMLLoadWarning'] is False:
+        return
+
+    import warnings
+
+    message = "\n\
+  *** Calling yaml.%s() without Loader=... is deprecated.\n\
+  *** The default Loader is unsafe.\n\
+  *** Please read https://msg.pyyaml.org/load for full details." % method
+
+    warnings.warn(message, YAMLLoadWarning, stacklevel=3)
+
+#------------------------------------------------------------------------------
 def scan(stream, Loader=Loader):
     """
     Scan a YAML stream and produce scanning tokens.
@@ -61,45 +99,97 @@ def compose_all(stream, Loader=Loader):
     finally:
         loader.dispose()
 
-def load(stream, Loader=Loader):
+def load(stream, Loader=None):
     """
     Parse the first YAML document in a stream
     and produce the corresponding Python object.
     """
+    if Loader is None:
+        load_warning('load')
+        Loader = FullLoader
+
     loader = Loader(stream)
     try:
         return loader.get_single_data()
     finally:
         loader.dispose()
 
-def load_all(stream, Loader=Loader):
+def load_all(stream, Loader=None):
     """
     Parse all YAML documents in a stream
     and produce corresponding Python objects.
     """
+    if Loader is None:
+        load_warning('load_all')
+        Loader = FullLoader
+
     loader = Loader(stream)
     try:
         while loader.check_data():
             yield loader.get_data()
     finally:
         loader.dispose()
 
+def full_load(stream):
+    """
+    Parse the first YAML document in a stream
+    and produce the corresponding Python object.
+
+    Resolve all tags except those known to be
+    unsafe on untrusted input.
+    """
+    return load(stream, FullLoader)
+
+def full_load_all(stream):
+    """
+    Parse all YAML documents in a stream
+    and produce corresponding Python objects.
+
+    Resolve all tags except those known to be
+    unsafe on untrusted input.
+    """
+    return load_all(stream, FullLoader)
+
 def safe_load(stream):
     """
     Parse the first YAML document in a stream
     and produce the corresponding Python object.
-    Resolve only basic YAML tags.
+
+    Resolve only basic YAML tags. This is known
+    to be safe for untrusted input.
     """
     return load(stream, SafeLoader)
 
 def safe_load_all(stream):
     """
     Parse all YAML documents in a stream
     and produce corresponding Python objects.
-    Resolve only basic YAML tags.
+
+    Resolve only basic YAML tags. This is known
+    to be safe for untrusted input.
     """
     return load_all(stream, SafeLoader)
 
+def unsafe_load(stream):
+    """
+    Parse the first YAML document in a stream
+    and produce the corresponding Python object.
+
+    Resolve all tags, even those known to be
+    unsafe on untrusted input.
+    """
+    return load(stream, UnsafeLoader)
+
+def unsafe_load_all(stream):
+    """
+    Parse all YAML documents in a stream
+    and produce corresponding Python objects.
+
+    Resolve all tags, even those known to be
+    unsafe on untrusted input.
+    """
+    return load_all(stream, UnsafeLoader)
+
 def emit(events, stream=None, Dumper=Dumper,
         canonical=None, indent=None, width=None,
         allow_unicode=None, line_break=None):

lib/yaml/constructor.py

@@ -1,6 +1,12 @@
 
-__all__ = ['BaseConstructor', 'SafeConstructor', 'Constructor',
-    'ConstructorError']
+__all__ = [
+    'BaseConstructor',
+    'SafeConstructor',
+    'FullConstructor',
+    'UnsafeConstructor',
+    'Constructor',
+    'ConstructorError'
+]
 
 from error import *
 from nodes import *
@@ -464,7 +470,7 @@ def construct_undefined(self, node):
 SafeConstructor.add_constructor(None,
         SafeConstructor.construct_undefined)
 
-class Constructor(SafeConstructor):
+class FullConstructor(SafeConstructor):
 
     def construct_python_str(self, node):
         return self.construct_scalar(node).encode('utf-8')
@@ -481,18 +487,22 @@ def construct_python_complex(self, node):
     def construct_python_tuple(self, node):
         return tuple(self.construct_sequence(node))
 
-    def find_python_module(self, name, mark):
+    def find_python_module(self, name, mark, unsafe=False):
         if not name:
             raise ConstructorError("while constructing a Python module", mark,
                     "expected non-empty name appended to the tag", mark)
-        try:
-            __import__(name)
-        except ImportError, exc:
+        if unsafe:
+            try:
+                __import__(name)
+            except ImportError, exc:
+                raise ConstructorError("while constructing a Python module", mark,
+                        "cannot find module %r (%s)" % (name.encode('utf-8'), exc), mark)
+        if not name in sys.modules:
             raise ConstructorError("while constructing a Python module", mark,
-                    "cannot find module %r (%s)" % (name.encode('utf-8'), exc), mark)
+                    "module %r is not imported" % name.encode('utf-8'), mark)
         return sys.modules[name]
 
-    def find_python_name(self, name, mark):
+    def find_python_name(self, name, mark, unsafe=False):
         if not name:
             raise ConstructorError("while constructing a Python object", mark,
                     "expected non-empty name appended to the tag", mark)
@@ -501,11 +511,15 @@ def find_python_name(self, name, mark):
         else:
             module_name = '__builtin__'
             object_name = name
-        try:
-            __import__(module_name)
-        except ImportError, exc:
+        if unsafe:
+            try:
+                __import__(module_name)
+            except ImportError, exc:
+                raise ConstructorError("while constructing a Python object", mark,
+                        "cannot find module %r (%s)" % (module_name.encode('utf-8'), exc), mark)
+        if not module_name in sys.modules:
             raise ConstructorError("while constructing a Python object", mark,
-                    "cannot find module %r (%s)" % (module_name.encode('utf-8'), exc), mark)
+                    "module %r is not imported" % module_name.encode('utf-8'), mark)
         module = sys.modules[module_name]
         if not hasattr(module, object_name):
             raise ConstructorError("while constructing a Python object", mark,
@@ -532,12 +546,16 @@ def construct_python_module(self, suffix, node):
     class classobj: pass
 
     def make_python_instance(self, suffix, node,
-            args=None, kwds=None, newobj=False):
+            args=None, kwds=None, newobj=False, unsafe=False):
         if not args:
             args = []
         if not kwds:
             kwds = {}
         cls = self.find_python_name(suffix, node.start_mark)
+        if not (unsafe or isinstance(cls, type) or isinstance(cls, type(self.classobj))):
+            raise ConstructorError("while constructing a Python instance", node.start_mark,
+                    "expected a class, but found %r" % type(cls),
+                    node.start_mark)
         if newobj and isinstance(cls, type(self.classobj))  \
                 and not args and not kwds:
             instance = self.classobj()
@@ -571,6 +589,64 @@ def construct_python_object(self, suffix, node):
         state = self.construct_mapping(node, deep=deep)
         self.set_python_instance_state(instance, state)
 
+FullConstructor.add_constructor(
+    u'tag:yaml.org,2002:python/none',
+    FullConstructor.construct_yaml_null)
+
+FullConstructor.add_constructor(
+    u'tag:yaml.org,2002:python/bool',
+    FullConstructor.construct_yaml_bool)
+
+FullConstructor.add_constructor(
+    u'tag:yaml.org,2002:python/str',
+    FullConstructor.construct_python_str)
+
+FullConstructor.add_constructor(
+    u'tag:yaml.org,2002:python/unicode',
+    FullConstructor.construct_python_unicode)
+
+FullConstructor.add_constructor(
+    u'tag:yaml.org,2002:python/int',
+    FullConstructor.construct_yaml_int)
+
+FullConstructor.add_constructor(
+    u'tag:yaml.org,2002:python/long',
+    FullConstructor.construct_python_long)
+
+FullConstructor.add_constructor(
+    u'tag:yaml.org,2002:python/float',
+    FullConstructor.construct_yaml_float)
+
+FullConstructor.add_constructor(
+    u'tag:yaml.org,2002:python/complex',
+    FullConstructor.construct_python_complex)
+
+FullConstructor.add_constructor(
+    u'tag:yaml.org,2002:python/list',
+    FullConstructor.construct_yaml_seq)
+
+FullConstructor.add_constructor(
+    u'tag:yaml.org,2002:python/tuple',
+    FullConstructor.construct_python_tuple)
+
+FullConstructor.add_constructor(
+    u'tag:yaml.org,2002:python/dict',
+    FullConstructor.construct_yaml_map)
+
+FullConstructor.add_multi_constructor(
+    u'tag:yaml.org,2002:python/name:',
+    FullConstructor.construct_python_name)
+
+FullConstructor.add_multi_constructor(
+    u'tag:yaml.org,2002:python/module:',
+    FullConstructor.construct_python_module)
+
+FullConstructor.add_multi_constructor(
+    u'tag:yaml.org,2002:python/object:',
+    FullConstructor.construct_python_object)
+
+class UnsafeConstructor(FullConstructor):
+
     def construct_python_object_apply(self, suffix, node, newobj=False):
         # Format:
         #   !!python/object/apply       # (or !!python/object/new)
@@ -609,67 +685,25 @@ def construct_python_object_apply(self, suffix, node, newobj=False):
     def construct_python_object_new(self, suffix, node):
         return self.construct_python_object_apply(suffix, node, newobj=True)
 
-Constructor.add_constructor(
-    u'tag:yaml.org,2002:python/none',
-    Constructor.construct_yaml_null)
-
-Constructor.add_constructor(
-    u'tag:yaml.org,2002:python/bool',
-    Constructor.construct_yaml_bool)
-
-Constructor.add_constructor(
-    u'tag:yaml.org,2002:python/str',
-    Constructor.construct_python_str)
-
-Constructor.add_constructor(
-    u'tag:yaml.org,2002:python/unicode',
-    Constructor.construct_python_unicode)
-
-Constructor.add_constructor(
-    u'tag:yaml.org,2002:python/int',
-    Constructor.construct_yaml_int)
-
-Constructor.add_constructor(
-    u'tag:yaml.org,2002:python/long',
-    Constructor.construct_python_long)
-
-Constructor.add_constructor(
-    u'tag:yaml.org,2002:python/float',
-    Constructor.construct_yaml_float)
-
-Constructor.add_constructor(
-    u'tag:yaml.org,2002:python/complex',
-    Constructor.construct_python_complex)
-
-Constructor.add_constructor(
-    u'tag:yaml.org,2002:python/list',
-    Constructor.construct_yaml_seq)
-
-Constructor.add_constructor(
-    u'tag:yaml.org,2002:python/tuple',
-    Constructor.construct_python_tuple)
-
-Constructor.add_constructor(
-    u'tag:yaml.org,2002:python/dict',
-    Constructor.construct_yaml_map)
-
-Constructor.add_multi_constructor(
-    u'tag:yaml.org,2002:python/name:',
-    Constructor.construct_python_name)
+    def find_python_module(self, name, mark):
+        return super(UnsafeConstructor, self).find_python_module(name, mark, unsafe=True)
 
-Constructor.add_multi_constructor(
-    u'tag:yaml.org,2002:python/module:',
-    Constructor.construct_python_module)
+    def find_python_name(self, name, mark):
+        return super(UnsafeConstructor, self).find_python_name(name, mark, unsafe=True)
 
-Constructor.add_multi_constructor(
-    u'tag:yaml.org,2002:python/object:',
-    Constructor.construct_python_object)
+    def make_python_instance(self, suffix, node, args=None, kwds=None, newobj=False):
+        return super(UnsafeConstructor, self).make_python_instance(
+            suffix, node, args, kwds, newobj, unsafe=True)
 
-Constructor.add_multi_constructor(
+UnsafeConstructor.add_multi_constructor(
     u'tag:yaml.org,2002:python/object/apply:',
-    Constructor.construct_python_object_apply)
+    UnsafeConstructor.construct_python_object_apply)
 
-Constructor.add_multi_constructor(
+UnsafeConstructor.add_multi_constructor(
     u'tag:yaml.org,2002:python/object/new:',
-    Constructor.construct_python_object_new)
+    UnsafeConstructor.construct_python_object_new)
 
+# Constructor is same as UnsafeConstructor. Need to leave this in place in case
+# people have extended it directly.
+class Constructor(UnsafeConstructor):
+    pass