Debugging False Negatives

False Negatives occur when there is a legitimate flow of tainted data from a source to a sink, but Pysa fails to catch it.

Common Causes of False Negatives#

Missing Type Information#

Pysa relies on type information from Pyre to identify sources and sinks, and to build the call graph needed to follow the propagation of taint between the two. Just becasue type information is available somewhere in the code, does not mean Pyre will know the type of an object in the exact place where Pysa needs it. See the documentation on Coverage Increasing Strategies for tips on how to increase type coverage. The following examples demonstrate how lost type information leads to lost flows.

Missing Sources/Sinks#

HttpRequest.GET is a common source of UserControled data in Django. If the request objects are not explicitly typed as HttpRequst, however, Pysa will fail to detect obvious issues:

from django.http import HttpRequest
def this_is_missed(request):
# This flow WILL NOT be found, because Pysa does not know the type of
# 'request' at this point and thus does not know 'request.GET' is a source
# (even though the type is known in 'run')
eval(request.GET["command"])
def this_is_caught(request: HttpRequest):
# This flow WILL be found.
eval(request.GET["command"])
def run(request: HttpRequest):
this_is_missed(request)
this_is_caught(request)

Incomplete Call Graph#

Pysa relies on type information in order to build a call graph that accurately tracks a method call of foo.bar(x) to the def bar(self, x) implementation. Without type information on bar, Pysa will be unable to figure out how to dispatch the call and the flow will be lost:

from django.http import HttpRequest
class Runner:
def run(self, command: str) -> None:
eval(command)
def this_is_missed(request: HttpRequest, runner):
# This flow WILL NOT be found, because Pysa does not know the type of
# 'runner', and thus does not know where the 'run' call dispatches to
runner.run(request.GET["command"])
def this_is_caught(request: HttpRequest, runner: Runner):
# This flow WILL be found.
runner.run(request.GET["command"])

Globals#

To allow for parallel processing, Pysa is limited in it's ability to track taint flows through global variables. For example, Pysa will not detect an issue in the following code:

user_controlled_data = ""
def load_data(request: HttpRequest) -> None:
user_controlled_data = request.GET["data"]
def run_command(request: HttpRequest) -> None:
load_data(request)
eval(user_controlled_data)

The best workaround is to avoid using globals in your code. If a refactor isn't possible, but you do know what globals should be considered tainted, you can explicitly declare the global tainted in your .pysa files.

Methodology for Debugging False Negatives#

  1. Identify the flow you expect to see
    1. Source
    2. Sink
    3. Every function call/return that propagates the tainted data from the source to the sink
    4. Every variable that the tainted data passes through, within the identified functions. This usually includes the parameter which initially received the taint, and then 0 or more local variables that hold the tainted data as it is transformed in some way.
  2. Add a reveal_taint and reveal_type statement to each of the variables identified in the previous step
  3. Run Pysa using the same command you used when the false negative manifested, but also include the --noninteractive flag (eg. pyre --noninteractive analyze)
  4. Start following the flow from source to sink in your code, and find the corresponding output for each reveal_taint statement.
    • Note that each time Pysa analyzes a function (could be many times), it will dump the latest taint information, so the last instance of reveal_taint output for a given line will be the most accurate and is the one you should look at.
    • reveal_taint output exposes some of the implementation details of Pysa, by giving you Revealed forward taint and Revealed backward taint messages. Without going into those details, you should expect to see either the source name (eg. UserControlled) you care about appearing in the Revealed forward taint output, or the sink name (eg. RemoteCodeExecution) you care about in the Revealed backward taint output.
    1. For each reveal_taint, following the flow of tainted data from source to sink, locate the output in the logs that reveals the taint (eg. integration_test.reveal_taint:20:4-20:16: Revealed forward taint for ``command``:).
    2. If you see your source or sink name in the output, then go back to 1) and carry on with the next reveal_taint statement. If you do not see the source or sink name, then that means the cause of the false negative is likely between your previous reveal_taint and the one you're currently looking at. Refer to the "Commom Causes of False Negatives" section above for ideas on the cause, and how to fix it.

Example#

Pysa will not be able to detect a vulnerability in the following code:

from django.http import HttpRequest, HttpResponse
class Runner:
def run(self, command: str) -> None:
eval(command)
def get_command(request: HttpRequest) -> str:
command = request.GET["command"]
return command
def execute_command(runner: Runner, command):
runner.run(command)
def start(request: HttpRequest):
command = get_command(request)
runner = Runner()
execute_command(runner, command)

Folling the above debugging steps we identify the flow of data from beginning to end, and add debugging statements:

from django.http import HttpRequest, HttpResponse
class Runner:
def run(self, command: str) -> None:
reveal_type(command)
reveal_taint(command)
eval(command) # 5. User controlled data reaches the sink in here
def get_command(request: HttpRequest) -> str:
command = request.GET["command"] # 1. User controlled data originates here
reveal_type(command)
reveal_taint(command)
return command
def execute_command(runner: Runner, command):
reveal_type(command)
reveal_taint(command)
reveal_type(runner)
reveal_taint(runner)
runner.run(command) # 4. User controlled data is passed in here
def start(request: HttpRequest):
command = get_command(request) # 2. User controlled data is returned here
reveal_type(command)
reveal_taint(command)
runner = Runner()
execute_command(runner, command) # 3. User controlled data is passed in here

See the appendix for the full output of running pyre --noninteractive analyze on this example.

Starting at 1), we see this in the output:

2020-12-28 13:02:36,486 [PID 3382063] WARNING integration_test.reveal_taint:13:4-13:16: Revealed forward taint for `command`: @integration_test.reveal_taint:11:14-11:25 -> UserControlled -> SimpleFeature: [Features.Simple.LeafName {leaf = "Obj{django.http.request.HttpRequest.GET}";
2020-12-28 13:02:36,486 [PID 3382063] WARNING port = None}], ComplexFeature: [], TraceLength: 0, FirstIndex: ["command"], FirstField: []
2020-12-28 13:02:36,486 [PID 3382063] WARNING UserControlled_Payload -> SimpleFeature: [Features.Simple.LeafName {leaf = "Obj{django.http.request.HttpRequest.GET}";
2020-12-28 13:02:36,486 [PID 3382063] WARNING port = None}], ComplexFeature: [], TraceLength: 0, FirstIndex: ["command"], FirstField: []
2020-12-28 13:02:36,486 [PID 3382063] WARNING
2020-12-28 13:02:36,486 [PID 3382063] WARNING integration_test.reveal_taint:13:4-13:25: Revealed backward taint for `command`: declaration -> LocalReturn -> SimpleFeature: [], ComplexFeature: [(Features.Complex.ReturnAccessPath [])], TraceLength: 4611686018427387903, FirstIndex: [], FirstField: []

Removing the timestamps and other noise gives us:

integration_test.reveal_taint:13:4-13:16:
Revealed forward taint for `command`:
@integration_test.reveal_taint:11:14-11:25 -> UserControlled ->
SimpleFeature: [
Features.Simple.LeafName {
leaf = "Obj{django.http.request.HttpRequest.GET}"; port = None
}
],
ComplexFeature: [],
TraceLength: 0,
FirstIndex: ["command"],
FirstField: []
UserControlled_Payload -> SimpleFeature: [
Features.Simple.LeafName {
leaf = "Obj{django.http.request.HttpRequest.GET}"; port = None
}
],
ComplexFeature: [],
TraceLength: 0,
FirstIndex: ["command"],
FirstField: []
integration_test.reveal_taint:13:4-13:25:
Revealed backward taint for `command`:
declaration -> LocalReturn ->
SimpleFeature: [],
ComplexFeature: [(Features.Complex.ReturnAccessPath [])],
TraceLength: 4611686018427387903,
FirstIndex: [],
FirstField: []

For debugging false negatives, the only portion we care about is:

Revealed forward taint for `command`:
@integration_test.reveal_taint:11:14-11:25 -> UserControlled

This confirms that on line 11 (characters 14-25), we did indeed detect that command was tainted as UserControlled.

Moving on to 2, the forward taint output again tells us that we have UserControlled taint on command at line 26 (characters 4-16).

Starting with 4, we notice that we no longer see UserControlled or RemoteCodeExecution in our revealed forward or backwards taint:

2020-12-28 13:02:35,472 [PID 3382063] WARNING integration_test.reveal_taint:18:4-18:16: Revealed forward taint for `command`:
2020-12-28 13:02:35,472 [PID 3382063] WARNING
2020-12-28 13:02:35,472 [PID 3382063] WARNING integration_test.reveal_taint:18:4-18:25: Revealed backward taint for `command`:
2020-12-28 13:02:35,472 [PID 3382063] WARNING

This has helped us narrow down the problem to the execute_command function. In the end, the problem was that we did not have type information on runner, so Pysa did not know where the definition of runner.run was. Without knowing where the definition was, Pysa couldn't know that run containted a sink and thus couldn't know that command eventually reached that sink.

Appendix#

Subset of the output from running pyre --noninteractive analyze on the example:

2020-12-28 13:02:31,719 [PID 3382063] PERFORMANCE Overrides recorded: 2.408138s
2020-12-28 13:02:31,719 [PID 3382063] INFO Building call graph...
2020-12-28 13:02:34,166 [PID 3382063] PERFORMANCE Call graph built: 2.447174s
2020-12-28 13:02:34,166 [PID 3382063] INFO Call graph edges: 100
2020-12-28 13:02:34,166 [PID 3382063] INFO Computing overrides...
2020-12-28 13:02:34,311 [PID 3382063] PERFORMANCE Computed overrides: 0.144886s
2020-12-28 13:02:34,311 [PID 3382063] PERFORMANCE Pre-fixpoint computation for static analysis: 7.664068s
2020-12-28 13:02:34,311 [PID 3382063] INFO Analysis fixpoint started for 3075 overrides 68 functions...
2020-12-28 13:02:34,311 [PID 3382063] INFO Iteration #0. 3143 Callables [...]
2020-12-28 13:02:35,471 [PID 3382063] WARNING integration_test.reveal_taint:6:8-6:19: Revealed type for command: str
2020-12-28 13:02:35,471 [PID 3382063] WARNING integration_test.reveal_taint:7:8-7:20: Revealed forward taint for `command`:
2020-12-28 13:02:35,471 [PID 3382063] WARNING
2020-12-28 13:02:35,471 [PID 3382063] WARNING integration_test.reveal_taint:7:8-7:29: Revealed backward taint for `command`: @integration_test.reveal_taint:8:13-8:20 -> RemoteCodeExecution -> SimpleFeature: [Features.Simple.LeafName {leaf = "eval"; port = None}], ComplexFeature: [], TraceLength: 0, FirstIndex: [], FirstField: []
2020-12-28 13:02:35,472 [PID 3382063] WARNING
2020-12-28 13:02:35,472 [PID 3382063] WARNING integration_test.reveal_taint:17:4-17:15: Revealed type for command: typing.Any
2020-12-28 13:02:35,472 [PID 3382063] WARNING integration_test.reveal_taint:18:4-18:16: Revealed forward taint for `command`:
2020-12-28 13:02:35,472 [PID 3382063] WARNING
2020-12-28 13:02:35,472 [PID 3382063] WARNING integration_test.reveal_taint:19:4-19:15: Revealed type for command: typing.Any
2020-12-28 13:02:35,472 [PID 3382063] WARNING integration_test.reveal_taint:20:4-20:16: Revealed forward taint for `command`:
2020-12-28 13:02:35,472 [PID 3382063] WARNING
2020-12-28 13:02:35,472 [PID 3382063] WARNING integration_test.reveal_taint:20:4-20:25: Revealed backward taint for `command`:
2020-12-28 13:02:35,472 [PID 3382063] WARNING
2020-12-28 13:02:35,472 [PID 3382063] WARNING integration_test.reveal_taint:18:4-18:25: Revealed backward taint for `command`:
2020-12-28 13:02:35,472 [PID 3382063] WARNING
2020-12-28 13:02:35,473 [PID 3382063] WARNING integration_test.reveal_taint:12:4-12:15: Revealed type for command: str
2020-12-28 13:02:35,473 [PID 3382063] WARNING integration_test.reveal_taint:13:4-13:16: Revealed forward taint for `command`: @integration_test.reveal_taint:11:14-11:25 -> UserControlled -> SimpleFeature: [Features.Simple.LeafName {leaf = "Obj{django.http.request.HttpRequest.GET}";
2020-12-28 13:02:35,473 [PID 3382063] WARNING port = None}], ComplexFeature: [], TraceLength: 0, FirstIndex: ["command"], FirstField: []
2020-12-28 13:02:35,473 [PID 3382063] WARNING UserControlled_Payload -> SimpleFeature: [Features.Simple.LeafName {leaf = "Obj{django.http.request.HttpRequest.GET}";
2020-12-28 13:02:35,473 [PID 3382063] WARNING port = None}], ComplexFeature: [], TraceLength: 0, FirstIndex: ["command"], FirstField: []
2020-12-28 13:02:35,473 [PID 3382063] WARNING
2020-12-28 13:02:35,473 [PID 3382063] WARNING integration_test.reveal_taint:13:4-13:25: Revealed backward taint for `command`: declaration -> LocalReturn -> SimpleFeature: [], ComplexFeature: [(Features.Complex.ReturnAccessPath [])], TraceLength: 4611686018427387903, FirstIndex: [], FirstField: []
2020-12-28 13:02:35,473 [PID 3382063] WARNING
2020-12-28 13:02:35,480 [PID 3382063] WARNING integration_test.reveal_taint:25:4-25:15: Revealed type for command: str
2020-12-28 13:02:35,480 [PID 3382063] WARNING integration_test.reveal_taint:26:4-26:16: Revealed forward taint for `command`: via [email protected]_test.reveal_taint:24:14-24:34[integration_test.reveal_taint.get_command][{ root = LocalResult; path = [] }] -> UserControlled -> SimpleFeature: [Features.Simple.LeafName {leaf = "Obj{django.http.request.HttpRequest.GET}";
2020-12-28 13:02:35,480 [PID 3382063] WARNING port = None}], ComplexFeature: [], TraceLength: 1, FirstIndex: ["command"], FirstField: []
2020-12-28 13:02:35,480 [PID 3382063] WARNING UserControlled_Payload -> SimpleFeature: [Features.Simple.LeafName {leaf = "Obj{django.http.request.HttpRequest.GET}";
2020-12-28 13:02:35,480 [PID 3382063] WARNING port = None}], ComplexFeature: [], TraceLength: 1, FirstIndex: ["command"], FirstField: []
2020-12-28 13:02:35,480 [PID 3382063] WARNING
2020-12-28 13:02:35,481 [PID 3382063] WARNING integration_test.reveal_taint:26:4-26:25: Revealed backward taint for `command`:
2020-12-28 13:02:35,481 [PID 3382063] WARNING
2020-12-28 13:02:35,546 [PID 3382063] PERFORMANCE Expensive callables for iteration 0:
2020-12-28 13:02:35,577 [PID 3382063] INFO Iteration #0, 3143 callables, heap size 46105024 took 1.266790s
2020-12-28 13:02:35,578 [PID 3382063] INFO Iteration #1. 3038 Callables [...]
2020-12-28 13:02:36,482 [PID 3382063] WARNING integration_test.reveal_taint:6:8-6:19: Revealed type for command: str
2020-12-28 13:02:36,482 [PID 3382063] WARNING integration_test.reveal_taint:7:8-7:20: Revealed forward taint for `command`:
2020-12-28 13:02:36,483 [PID 3382063] WARNING
2020-12-28 13:02:36,483 [PID 3382063] WARNING integration_test.reveal_taint:7:8-7:29: Revealed backward taint for `command`: @integration_test.reveal_taint:8:13-8:20 -> RemoteCodeExecution -> SimpleFeature: [Features.Simple.LeafName {leaf = "eval"; port = None}], ComplexFeature: [], TraceLength: 0, FirstIndex: [], FirstField: []
2020-12-28 13:02:36,483 [PID 3382063] WARNING
2020-12-28 13:02:36,486 [PID 3382063] WARNING integration_test.reveal_taint:12:4-12:15: Revealed type for command: str
2020-12-28 13:02:36,486 [PID 3382063] WARNING integration_test.reveal_taint:13:4-13:16: Revealed forward taint for `command`: @integration_test.reveal_taint:11:14-11:25 -> UserControlled -> SimpleFeature: [Features.Simple.LeafName {leaf = "Obj{django.http.request.HttpRequest.GET}";
2020-12-28 13:02:36,486 [PID 3382063] WARNING port = None}], ComplexFeature: [], TraceLength: 0, FirstIndex: ["command"], FirstField: []
2020-12-28 13:02:36,486 [PID 3382063] WARNING UserControlled_Payload -> SimpleFeature: [Features.Simple.LeafName {leaf = "Obj{django.http.request.HttpRequest.GET}";
2020-12-28 13:02:36,486 [PID 3382063] WARNING port = None}], ComplexFeature: [], TraceLength: 0, FirstIndex: ["command"], FirstField: []
2020-12-28 13:02:36,486 [PID 3382063] WARNING
2020-12-28 13:02:36,486 [PID 3382063] WARNING integration_test.reveal_taint:13:4-13:25: Revealed backward taint for `command`: declaration -> LocalReturn -> SimpleFeature: [], ComplexFeature: [(Features.Complex.ReturnAccessPath [])], TraceLength: 4611686018427387903, FirstIndex: [], FirstField: []
2020-12-28 13:02:36,486 [PID 3382063] WARNING
2020-12-28 13:02:36,486 [PID 3382063] WARNING integration_test.reveal_taint:25:4-25:15: Revealed type for command: str
2020-12-28 13:02:36,486 [PID 3382063] WARNING integration_test.reveal_taint:26:4-26:16: Revealed forward taint for `command`: via [email protected]_test.reveal_taint:24:14-24:34[integration_test.reveal_taint.get_command][{ root = LocalResult; path = [] }] -> UserControlled -> SimpleFeature: [Features.Simple.LeafName {leaf = "Obj{django.http.request.HttpRequest.GET}";
2020-12-28 13:02:36,486 [PID 3382063] WARNING port = None}], ComplexFeature: [], TraceLength: 1, FirstIndex: ["command"], FirstField: []
2020-12-28 13:02:36,486 [PID 3382063] WARNING UserControlled_Payload -> SimpleFeature: [Features.Simple.LeafName {leaf = "Obj{django.http.request.HttpRequest.GET}";
2020-12-28 13:02:36,487 [PID 3382063] WARNING port = None}], ComplexFeature: [], TraceLength: 1, FirstIndex: ["command"], FirstField: []
2020-12-28 13:02:36,487 [PID 3382063] WARNING
2020-12-28 13:02:36,492 [PID 3382063] WARNING integration_test.reveal_taint:26:4-26:25: Revealed backward taint for `command`:
2020-12-28 13:02:36,492 [PID 3382063] WARNING
2020-12-28 13:02:36,552 [PID 3382063] PERFORMANCE Expensive callables for iteration 1:
2020-12-28 13:02:36,585 [PID 3382063] INFO Iteration #1, 3038 callables, heap size 46521728 took 1.007461s
2020-12-28 13:02:36,585 [PID 3382063] INFO Iteration #2. 23 Callables [...]
2020-12-28 13:02:37,018 [PID 3382063] PERFORMANCE Expensive callables for iteration 2:
2020-12-28 13:02:37,018 [PID 3382063] INFO Iteration #2, 23 callables, heap size 46530432 took 0.432597s
2020-12-28 13:02:37,018 [PID 3382063] INFO Iteration #3. 2 Callables [integration_test.string_concatenation.bad_1 (fun), integration_test.string_concatenation.bad_2 (fun)]
2020-12-28 13:02:37,130 [PID 3382063] PERFORMANCE Expensive callables for iteration 3:
2020-12-28 13:02:37,131 [PID 3382063] INFO Iteration #3, 2 callables, heap size 46532352 took 0.113038s
2020-12-28 13:02:37,131 [PID 3382063] INFO Iteration #4. 0 Callables []
2020-12-28 13:02:37,131 [PID 3382063] INFO Fixpoint iterations: 4
2020-12-28 13:02:37,348 [PID 3382063] PERFORMANCE Analysis fixpoint complete: 3.037628s
2020-12-28 13:02:37,369 [PID 3382063] PERFORMANCE Analyze: 18.289002s
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