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

@@ -8,7 +8,7 @@ from nodes import *
 from loader import *
 from dumper import *
 
-__version__ = '4.1'
+__version__ = '3.13'
 
 try:
     from cyaml import *
@@ -16,6 +16,45 @@ try:
 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 = (
+        "calling yaml.%s() without Loader=... is deprecated, as the "
+        "default Loader is unsafe. 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.
@@ -61,22 +100,30 @@ 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():
@@ -84,11 +131,33 @@ def load_all(stream, Loader=Loader):
     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)
 
@@ -96,10 +165,32 @@ 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',
+__all__ = [
-    'ConstructorError']
+    'BaseConstructor',
+    'SafeConstructor',
+    'FullConstructor',
+    'UnsafeConstructor',
+    'Constructor',
+    'ConstructorError'
+]
 
 from error import *
 from nodes import *
@@ -464,7 +470,7 @@ SafeConstructor.add_constructor(
 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 @@ class Constructor(SafeConstructor):
     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)
+        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,
+                    "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 @@ class Constructor(SafeConstructor):
         else:
             module_name = '__builtin__'
             object_name = name
+        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,
+                    "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 @@ class Constructor(SafeConstructor):
     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()
@@ -609,67 +627,83 @@ class Constructor(SafeConstructor):
     def construct_python_object_new(self, suffix, node):
         return self.construct_python_object_apply(suffix, node, newobj=True)
 
-Constructor.add_constructor(
+FullConstructor.add_constructor(
     u'tag:yaml.org,2002:python/none',
-    Constructor.construct_yaml_null)
+    FullConstructor.construct_yaml_null)
 
-Constructor.add_constructor(
+FullConstructor.add_constructor(
     u'tag:yaml.org,2002:python/bool',
-    Constructor.construct_yaml_bool)
+    FullConstructor.construct_yaml_bool)
 
-Constructor.add_constructor(
+FullConstructor.add_constructor(
     u'tag:yaml.org,2002:python/str',
-    Constructor.construct_python_str)
+    FullConstructor.construct_python_str)
 
-Constructor.add_constructor(
+FullConstructor.add_constructor(
     u'tag:yaml.org,2002:python/unicode',
-    Constructor.construct_python_unicode)
+    FullConstructor.construct_python_unicode)
 
-Constructor.add_constructor(
+FullConstructor.add_constructor(
     u'tag:yaml.org,2002:python/int',
-    Constructor.construct_yaml_int)
+    FullConstructor.construct_yaml_int)
 
-Constructor.add_constructor(
+FullConstructor.add_constructor(
     u'tag:yaml.org,2002:python/long',
-    Constructor.construct_python_long)
+    FullConstructor.construct_python_long)
 
-Constructor.add_constructor(
+FullConstructor.add_constructor(
     u'tag:yaml.org,2002:python/float',
-    Constructor.construct_yaml_float)
+    FullConstructor.construct_yaml_float)
 
-Constructor.add_constructor(
+FullConstructor.add_constructor(
     u'tag:yaml.org,2002:python/complex',
-    Constructor.construct_python_complex)
+    FullConstructor.construct_python_complex)
 
-Constructor.add_constructor(
+FullConstructor.add_constructor(
     u'tag:yaml.org,2002:python/list',
-    Constructor.construct_yaml_seq)
+    FullConstructor.construct_yaml_seq)
 
-Constructor.add_constructor(
+FullConstructor.add_constructor(
     u'tag:yaml.org,2002:python/tuple',
-    Constructor.construct_python_tuple)
+    FullConstructor.construct_python_tuple)
 
-Constructor.add_constructor(
+FullConstructor.add_constructor(
     u'tag:yaml.org,2002:python/dict',
-    Constructor.construct_yaml_map)
+    FullConstructor.construct_yaml_map)
 
-Constructor.add_multi_constructor(
+FullConstructor.add_multi_constructor(
     u'tag:yaml.org,2002:python/name:',
-    Constructor.construct_python_name)
+    FullConstructor.construct_python_name)
 
-Constructor.add_multi_constructor(
+FullConstructor.add_multi_constructor(
     u'tag:yaml.org,2002:python/module:',
-    Constructor.construct_python_module)
+    FullConstructor.construct_python_module)
 
-Constructor.add_multi_constructor(
+FullConstructor.add_multi_constructor(
     u'tag:yaml.org,2002:python/object:',
-    Constructor.construct_python_object)
+    FullConstructor.construct_python_object)
 
-Constructor.add_multi_constructor(
+FullConstructor.add_multi_constructor(
     u'tag:yaml.org,2002:python/object/apply:',
-    Constructor.construct_python_object_apply)
+    FullConstructor.construct_python_object_apply)
 
-Constructor.add_multi_constructor(
+FullConstructor.add_multi_constructor(
     u'tag:yaml.org,2002:python/object/new:',
-    Constructor.construct_python_object_new)
+    FullConstructor.construct_python_object_new)
 
+class UnsafeConstructor(FullConstructor):
+
+    def find_python_module(self, name, mark):
+        return super(UnsafeConstructor, self).find_python_module(name, mark, unsafe=True)
+
+    def find_python_name(self, name, mark):
+        return super(UnsafeConstructor, self).find_python_name(name, mark, unsafe=True)
+
+    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 is same as UnsafeConstructor. Need to leave this in place in case
+# people have extended it directly.
+class Constructor(UnsafeConstructor):
+    pass

lib/yaml/cyaml.py

@@ -1,6 +1,8 @@
 
-__all__ = ['CBaseLoader', 'CSafeLoader', 'CLoader',
+__all__ = [
-        'CBaseDumper', 'CSafeDumper', 'CDumper']
+    'CBaseLoader', 'CSafeLoader', 'CFullLoader', 'CUnsafeLoader', 'CLoader',
+    'CBaseDumper', 'CSafeDumper', 'CDumper'
+]
 
 from _yaml import CParser, CEmitter
 
@@ -25,6 +27,20 @@ class CSafeLoader(CParser, SafeConstructor, Resolver):
         SafeConstructor.__init__(self)
         Resolver.__init__(self)
 
+class CFullLoader(CParser, FullConstructor, Resolver):
+
+    def __init__(self, stream):
+        CParser.__init__(self, stream)
+        FullConstructor.__init__(self)
+        Resolver.__init__(self)
+
+class CUnsafeLoader(CParser, UnsafeConstructor, Resolver):
+
+    def __init__(self, stream):
+        CParser.__init__(self, stream)
+        UnsafeConstructor.__init__(self)
+        Resolver.__init__(self)
+
 class CLoader(CParser, Constructor, Resolver):
 
     def __init__(self, stream):

lib/yaml/loader.py

@@ -1,5 +1,5 @@
 
-__all__ = ['BaseLoader', 'SafeLoader', 'Loader']
+__all__ = ['BaseLoader', 'FullLoader', 'SafeLoader', 'Loader', 'UnsafeLoader']
 
 from reader import *
 from scanner import *
@@ -18,6 +18,16 @@ class BaseLoader(Reader, Scanner, Parser, Composer, BaseConstructor, BaseResolve
         BaseConstructor.__init__(self)
         BaseResolver.__init__(self)
 
+class FullLoader(Reader, Scanner, Parser, Composer, FullConstructor, Resolver):
+
+    def __init__(self, stream):
+        Reader.__init__(self, stream)
+        Scanner.__init__(self)
+        Parser.__init__(self)
+        Composer.__init__(self)
+        FullConstructor.__init__(self)
+        Resolver.__init__(self)
+
 class SafeLoader(Reader, Scanner, Parser, Composer, SafeConstructor, Resolver):
 
     def __init__(self, stream):
@@ -38,3 +48,16 @@ class Loader(Reader, Scanner, Parser, Composer, Constructor, Resolver):
         Constructor.__init__(self)
         Resolver.__init__(self)
 
+# UnsafeLoader is the same as Loader (which is and was always unsafe on
+# untrusted input). Use of either Loader or UnsafeLoader should be rare, since
+# FullLoad should be able to load almost all YAML safely. Loader is left intact
+# to ensure backwards compatability.
+class UnsafeLoader(Reader, Scanner, Parser, Composer, Constructor, Resolver):
+
+    def __init__(self, stream):
+        Reader.__init__(self, stream)
+        Scanner.__init__(self)
+        Parser.__init__(self)
+        Composer.__init__(self)
+        Constructor.__init__(self)
+        Resolver.__init__(self)

setup.py

@@ -1,6 +1,6 @@
 
 NAME = 'PyYAML'
-VERSION = '4.1'
+VERSION = '3.13'
 DESCRIPTION = "YAML parser and emitter for Python"
 LONG_DESCRIPTION = """\
 YAML is a data serialization format designed for human readability

tests/lib/test_errors.py

@@ -3,7 +3,7 @@ import yaml, test_emitter
 
 def test_loader_error(error_filename, verbose=False):
     try:
-        list(yaml.load_all(open(error_filename, 'rb')))
+        list(yaml.load_all(open(error_filename, 'rb'), yaml.FullLoader))
     except yaml.YAMLError, exc:
         if verbose:
             print "%s:" % exc.__class__.__name__, exc
@@ -14,7 +14,7 @@ test_loader_error.unittest = ['.loader-error']
 
 def test_loader_error_string(error_filename, verbose=False):
     try:
-        list(yaml.load_all(open(error_filename, 'rb').read()))
+        list(yaml.load_all(open(error_filename, 'rb').read(), yaml.FullLoader))
     except yaml.YAMLError, exc:
         if verbose:
             print "%s:" % exc.__class__.__name__, exc
@@ -25,7 +25,7 @@ test_loader_error_string.unittest = ['.loader-error']
 
 def test_loader_error_single(error_filename, verbose=False):
     try:
-        yaml.load(open(error_filename, 'rb').read())
+        yaml.load(open(error_filename, 'rb').read(), yaml.FullLoader)
     except yaml.YAMLError, exc:
         if verbose:
             print "%s:" % exc.__class__.__name__, exc

tests/lib/test_input_output.py

@@ -17,7 +17,7 @@ def _unicode_open(file, encoding, errors='strict'):
 def test_unicode_input(unicode_filename, verbose=False):
     data = open(unicode_filename, 'rb').read().decode('utf-8')
     value = ' '.join(data.split())
-    output = yaml.load(_unicode_open(StringIO.StringIO(data.encode('utf-8')), 'utf-8'))
+    output = yaml.full_load(_unicode_open(StringIO.StringIO(data.encode('utf-8')), 'utf-8'))
     assert output == value, (output, value)
     for input in [data, data.encode('utf-8'),
                     codecs.BOM_UTF8+data.encode('utf-8'),
@@ -25,9 +25,9 @@ def test_unicode_input(unicode_filename, verbose=False):
                     codecs.BOM_UTF16_LE+data.encode('utf-16-le')]:
         if verbose:
             print "INPUT:", repr(input[:10]), "..."
-        output = yaml.load(input)
+        output = yaml.full_load(input)
         assert output == value, (output, value)
-        output = yaml.load(StringIO.StringIO(input))
+        output = yaml.full_load(StringIO.StringIO(input))
         assert output == value, (output, value)
 
 test_unicode_input.unittest = ['.unicode']
@@ -40,14 +40,14 @@ def test_unicode_input_errors(unicode_filename, verbose=False):
             codecs.BOM_UTF8+data.encode('utf-16-le')]:
 
         try:
-            yaml.load(input)
+            yaml.full_load(input)
         except yaml.YAMLError, exc:
             if verbose:
                 print exc
         else:
             raise AssertionError("expected an exception")
         try:
-            yaml.load(StringIO.StringIO(input))
+            yaml.full_load(StringIO.StringIO(input))
         except yaml.YAMLError, exc:
             if verbose:
                 print exc

tests/lib/test_recursive.py

@@ -30,7 +30,7 @@ def test_recursive(recursive_filename, verbose=False):
     output2 = None
     try:
         output1 = yaml.dump(value1)
-        value2 = yaml.load(output1)
+        value2 = yaml.load(output1, yaml.FullLoader)
         output2 = yaml.dump(value2)
         assert output1 == output2, (output1, output2)
     finally: