Add the ability to select which functions or processes you which to extract capabilities from (#2156)

CHANGELOG.md

@@ -3,7 +3,7 @@
 ## master (unreleased)
 
 ### New Features
-
+- cli: add the ability to select which specific functions or processes to analyze @yelhamer
 - webui: explore capa analysis results in a web-based UI online and offline #2224 @s-ff
 - support analyzing DRAKVUF traces #2143 @yelhamer
 - IDA extractor: extract names from dynamically resolved APIs stored in renamed global variables #2201 @Ana06

capa/exceptions.py

@@ -23,3 +23,15 @@ class UnsupportedOSError(ValueError):
 
 class EmptyReportError(ValueError):
     pass
+
+
+class InvalidArgument(ValueError):
+    pass
+
+
+class NonExistantFunctionError(ValueError):
+    pass
+
+
+class NonExistantProcessError(ValueError):
+    pass

capa/features/extractors/base_extractor.py

@@ -9,7 +9,9 @@
 import abc
 import hashlib
 import dataclasses
-from typing import Any, Dict, Tuple, Union, Iterator
+from copy import copy
+from types import MethodType
+from typing import Any, Set, Dict, Tuple, Union, Iterator
 from dataclasses import dataclass
 
 # TODO(williballenthin): use typing.TypeAlias directly when Python 3.9 is deprecated
@@ -296,6 +298,22 @@ class StaticFeatureExtractor:
         raise NotImplementedError()
 
 
+def FunctionFilter(extractor: StaticFeatureExtractor, functions: Set) -> StaticFeatureExtractor:
+    original_get_functions = extractor.get_functions
+
+    def filtered_get_functions(self):
+        yield from (f for f in original_get_functions() if f.address in functions)
+
+    # we make a copy of the original extractor object and then update its get_functions() method with the decorated filter one.
+    # this is in order to preserve the original extractor object's get_functions() method, in case it is used elsewhere in the code.
+    # an example where this is important is in our testfiles where we may use the same extractor object with different tests,
+    # with some of these tests needing to install a functions filter on the extractor object.
+    new_extractor = copy(extractor)
+    new_extractor.get_functions = MethodType(filtered_get_functions, extractor)  # type: ignore
+
+    return new_extractor
+
+
 @dataclass
 class ProcessHandle:
     """
@@ -467,4 +485,20 @@ class DynamicFeatureExtractor:
         raise NotImplementedError()
 
 
+def ProcessFilter(extractor: DynamicFeatureExtractor, processes: Set) -> DynamicFeatureExtractor:
+    original_get_processes = extractor.get_processes
+
+    def filtered_get_processes(self):
+        yield from (f for f in original_get_processes() if f.address.pid in processes)
+
+    # we make a copy of the original extractor object and then update its get_processes() method with the decorated filter one.
+    # this is in order to preserve the original extractor object's get_processes() method, in case it is used elsewhere in the code.
+    # an example where this is important is in our testfiles where we may use the same extractor object with different tests,
+    # with some of these tests needing to install a processes filter on the extractor object.
+    new_extractor = copy(extractor)
+    new_extractor.get_processes = MethodType(filtered_get_processes, extractor)  # type: ignore
+
+    return new_extractor
+
+
 FeatureExtractor: TypeAlias = Union[StaticFeatureExtractor, DynamicFeatureExtractor]

capa/main.py

@@ -17,7 +17,7 @@ import argparse
 import textwrap
 import contextlib
 from types import TracebackType
-from typing import Any, Dict, List, Optional
+from typing import Any, Set, Dict, List, Optional, TypedDict
 from pathlib import Path
 
 import colorama
@@ -62,6 +62,7 @@ from capa.helpers import (
     log_unsupported_drakvuf_report_error,
 )
 from capa.exceptions import (
+    InvalidArgument,
     EmptyReportError,
     UnsupportedOSError,
     UnsupportedArchError,
@@ -83,9 +84,17 @@ from capa.features.common import (
     FORMAT_FREEZE,
     FORMAT_RESULT,
     FORMAT_DRAKVUF,
+    STATIC_FORMATS,
+    DYNAMIC_FORMATS,
 )
 from capa.capabilities.common import find_capabilities, has_file_limitation, find_file_capabilities
-from capa.features.extractors.base_extractor import FeatureExtractor, StaticFeatureExtractor, DynamicFeatureExtractor
+from capa.features.extractors.base_extractor import (
+    ProcessFilter,
+    FunctionFilter,
+    FeatureExtractor,
+    StaticFeatureExtractor,
+    DynamicFeatureExtractor,
+)
 
 RULES_PATH_DEFAULT_STRING = "(embedded rules)"
 SIGNATURES_PATH_DEFAULT_STRING = "(embedded signatures)"
@@ -106,10 +115,17 @@ E_MISSING_CAPE_STATIC_ANALYSIS = 21
 E_MISSING_CAPE_DYNAMIC_ANALYSIS = 22
 E_EMPTY_REPORT = 23
 E_UNSUPPORTED_GHIDRA_EXECUTION_MODE = 24
+E_INVALID_INPUT_FORMAT = 25
+E_INVALID_FEATURE_EXTRACTOR = 26
 
 logger = logging.getLogger("capa")
 
 
+class FilterConfig(TypedDict, total=False):
+    processes: Set[int]
+    functions: Set[int]
+
+
 @contextlib.contextmanager
 def timing(msg: str):
     t0 = time.time()
@@ -276,6 +292,22 @@ def install_common_args(parser, wanted=None):
             help=f"select backend, {backend_help}",
         )
 
+    if "restrict-to-functions" in wanted:
+        parser.add_argument(
+            "--restrict-to-functions",
+            type=lambda s: s.replace(" ", "").split(","),
+            default=[],
+            help="provide a list of comma-separated function virtual addresses to analyze (static analysis).",
+        )
+
+    if "restrict-to-processes" in wanted:
+        parser.add_argument(
+            "--restrict-to-processes",
+            type=lambda s: s.replace(" ", "").split(","),
+            default=[],
+            help="provide a list of comma-separated process IDs to analyze (dynamic analysis).",
+        )
+
     if "os" in wanted:
         oses = [
             (OS_AUTO, "detect OS automatically - default"),
@@ -749,9 +781,10 @@ def get_extractor_from_cli(args, input_format: str, backend: str) -> FeatureExtr
 
     os_ = get_os_from_cli(args, backend)
     sample_path = get_sample_path_from_cli(args, backend)
+    extractor_filters = get_extractor_filters_from_cli(args, input_format)
 
     try:
-        return capa.loader.get_extractor(
+        extractor = capa.loader.get_extractor(
             args.input_file,
             input_format,
             os_,
@@ -761,6 +794,7 @@ def get_extractor_from_cli(args, input_format: str, backend: str) -> FeatureExtr
             disable_progress=args.quiet or args.debug,
             sample_path=sample_path,
         )
+        return apply_extractor_filters(extractor, extractor_filters)
     except UnsupportedFormatError as e:
         if input_format == FORMAT_CAPE:
             log_unsupported_cape_report_error(str(e))
@@ -780,6 +814,38 @@ def get_extractor_from_cli(args, input_format: str, backend: str) -> FeatureExtr
         raise ShouldExitError(E_CORRUPT_FILE) from e
 
 
+def get_extractor_filters_from_cli(args, input_format) -> FilterConfig:
+    if not hasattr(args, "restrict_to_processes") and not hasattr(args, "restrict_to_functions"):
+        # no processes or function filters were installed in the args
+        return {}
+
+    if input_format in STATIC_FORMATS:
+        if args.restrict_to_processes:
+            raise InvalidArgument("Cannot filter processes with static analysis.")
+        return {"functions": {int(addr, 0) for addr in args.restrict_to_functions}}
+    elif input_format in DYNAMIC_FORMATS:
+        if args.restrict_to_functions:
+            raise InvalidArgument("Cannot filter functions with dynamic analysis.")
+        return {"processes": {int(pid, 0) for pid in args.restrict_to_processes}}
+    else:
+        raise ShouldExitError(E_INVALID_INPUT_FORMAT)
+
+
+def apply_extractor_filters(extractor: FeatureExtractor, extractor_filters: FilterConfig):
+    if not any(extractor_filters.values()):
+        return extractor
+
+    # if the user specified extractor filters, then apply them here
+    if isinstance(extractor, StaticFeatureExtractor):
+        assert extractor_filters["functions"]
+        return FunctionFilter(extractor, extractor_filters["functions"])
+    elif isinstance(extractor, DynamicFeatureExtractor):
+        assert extractor_filters["processes"]
+        return ProcessFilter(extractor, extractor_filters["processes"])
+    else:
+        raise ShouldExitError(E_INVALID_FEATURE_EXTRACTOR)
+
+
 def main(argv: Optional[List[str]] = None):
     if sys.version_info < (3, 8):
         raise UnsupportedRuntimeError("This version of capa can only be used with Python 3.8+")
@@ -819,7 +885,20 @@ def main(argv: Optional[List[str]] = None):
     parser = argparse.ArgumentParser(
         description=desc, epilog=epilog, formatter_class=argparse.RawDescriptionHelpFormatter
     )
-    install_common_args(parser, {"input_file", "format", "backend", "os", "signatures", "rules", "tag"})
+    install_common_args(
+        parser,
+        {
+            "input_file",
+            "format",
+            "backend",
+            "os",
+            "signatures",
+            "rules",
+            "tag",
+            "restrict-to-functions",
+            "restrict-to-processes",
+        },
+    )
     parser.add_argument("-j", "--json", action="store_true", help="emit JSON instead of text")
     args = parser.parse_args(args=argv)
 

doc/usage.md

@@ -9,6 +9,22 @@ Use the `-t` option to run rules with the given metadata value (see the rule fie
 For example, `capa -t william.ballenthin@mandiant.com` runs rules that reference Willi's email address (probably as the author), or
 `capa -t communication` runs rules with the namespace `communication`.
 
+### only analyze selected functions
+Use the `--restrict-to-functions` option to extract capabilities from only a selected set of functions. This is useful for analyzing 
+large functions and figuring out their capabilities and their address of occurance; for example: PEB access, RC4 encryption, etc.
+
+To use this, you can copy the virtual addresses from your favorite disassembler and pass them to capa as follows:
+`capa sample.exe --restrict-to-functions 0x4019C0,0x401CD0`. If you add the `-v` option then capa will extract the interesting parts of a function for you.
+
+### only analyze selected processes
+Use the `--restrict-to-processes` option to extract capabilities from only a selected set of processes. This is useful for filtering the noise 
+generated from analyzing non-malicious processes that can be reported by some sandboxes, as well as reduce the execution time 
+by not analyzing such processes in the first place.
+
+To use this, you can pick the PIDs of the processes you are interested in from the sandbox-generated process tree (or from the sandbox-reported malware PID) 
+and pass that to capa as follows: `capa report.log --restrict-to-processes 3888,3214,4299`. If you add the `-v` option then capa will tell you 
+which threads perform what actions (encrypt/decrypt data, initiate a connection, etc.).
+
 ### IDA Pro plugin: capa explorer
 Please check out the [capa explorer documentation](/capa/ida/plugin/README.md).
 
@@ -16,4 +32,4 @@ Please check out the [capa explorer documentation](/capa/ida/plugin/README.md).
 Set the environment variable `CAPA_SAVE_WORKSPACE` to instruct the underlying analysis engine to 
 cache its intermediate results to the file system. For example, vivisect will create `.viv` files.
 Subsequently, capa may run faster when reprocessing the same input file.
-This is particularly useful during rule development as you repeatedly test a rule against a known sample.
+This is particularly useful during rule development as you repeatedly test a rule against a known sample.

tests/test_capabilities.py

@@ -9,6 +9,7 @@
 import textwrap
 
 import capa.capabilities.common
+from capa.features.extractors.base_extractor import FunctionFilter
 
 
 def test_match_across_scopes_file_function(z9324d_extractor):
@@ -174,6 +175,37 @@ def test_subscope_bb_rules(z9324d_extractor):
     assert "test rule" in capabilities
 
 
+def test_match_specific_functions(z9324d_extractor):
+    rules = capa.rules.RuleSet(
+        [
+            capa.rules.Rule.from_yaml(
+                textwrap.dedent(
+                    """
+                    rule:
+                        meta:
+                            name: receive data
+                            scopes:
+                                static: function
+                                dynamic: call
+                            examples:
+                            - 9324d1a8ae37a36ae560c37448c9705a:0x401CD0
+                        features:
+                            - or:
+                                - api: recv
+                    """
+                )
+            )
+        ]
+    )
+    extractor = FunctionFilter(z9324d_extractor, {0x4019C0})
+    capabilities, meta = capa.capabilities.common.find_capabilities(rules, extractor)
+    matches = capabilities["receive data"]
+    # test that we received only one match
+    assert len(matches) == 1
+    # and that this match is from the specified function
+    assert matches[0][0] == 0x4019C0
+
+
 def test_byte_matching(z9324d_extractor):
     rules = capa.rules.RuleSet(
         [