Advanced Topics

This page documents less straightforward bits of Pysa.

Tainting Specific `kwargs`#

Sometimes, a function can have potential sinks mixed together with benign parameters in the keyword arguments (kwargs) that it accepts. In these cases, tainting the whole kwargs variable will result in false positives when tainted data flows into a benign kwarg. Instead, for a function like this:

def eval_and_log(**kwargs):
    eval(kwargs["eval"])
    logging.debug(kwargs["log"])

We can lie a bit in our .pysa file, and break out the dangerous argument for tainting:

def eval_and_log(*, eval: TaintSink[RemoteCodeExecution], **kwargs): ...

This allows us to catch flows only into the eval keyword argument.

Instance attributes versus class attributes#

Models can specify sources and sinks on attributes, following the type annotation syntax:

django.http.request.HttpRequest.GET: TaintSource[UserControlled]

Any access to request.GET will be tainted when request is an instance of HttpRequest or any of its children. However, note that the access to the class attribute (i.e, HttpRequest.GET) won't be considered tainted.

To specify sources and sinks on class attributes, use the __class__ prefix:

django.http.request.HttpRequest.__class__.GET: TaintSource[UserControlled]

To specify a source on both the class attribute and instance attribute, simply use both lines.

Literal String Sources And Sinks#

Some security vulnerabilities are best captured by modelling strings of a given form flowing to dangerous functions, or format strings that match a pattern getting tainted data passed in.

To mark all literal strings matching a pattern as sources, you first need to add a regular expression corresponding to the pattern to your taint.config:

{
  "sources": [
    {
      "name": "IPAddress"
    }
  ],
  "implicit_sources": {
     "literal_strings": [
       {
         "regexp": "\\d{1,3}(\\.\\d{1,3})+",
         "kind": "IPAddress",
         "description": "String that looks like an IP address."
       }
     ]
  }
}

With this regex in place, whenever Pysa sees a string such as 123.456.789.123, it will flag it as a taint source with the kind IPAddress.

def test() -> None:
    ip_address = "123.456.789.123"
    dont_pass_an_ip_address(ip_address) # Pysa will now flag this.

The converse of supporting literal strings as sinks is also supported, with a narrower use case. The syntax allows you to model data being used to format f-strings. To add a literal sink, first add the literal_sink to your configuration

{
  "sinks": [
    {
      "name": "MayBeRendered"
    }
  ],
  "implicit_sinks": {
     "literal_strings": [
       {
         "regexp": "^<.*>$",
         "kind": "MayBeRendered",
         "description": "Indicates a string whose contents may be rendered."
       }
     ]
  }

Now, Pysa will treat any values flowing into a f-string as a regular sink:

def may_render(parameter: str) -> None:
    result = f"<content={parameter}>"

Combined Source Rules#

Some security vulnerabilities are better modeled as multiple sources reaching a sink. For example, leaking credentials via requests.get could be modeled as user controlled data flowing into the url parameter and credentials flowing into the params parameter. These flows can be modeled by combined source rules.

Sources for combined source rules are declared as normal in taint.config. Sinks, however, need to be unique to the combined source rule and are declared inside the rule definition. The rule itself is declared in the combined_source_rules top level entry. The rule lists all the same things as a reglular rule, but also ties labels to its sources:

{
  "sources": [
    { "name": "UserControlled" },
    { "name": "Credentials" }
  ],
  "combined_source_rules": [
    {
       "name": "Credentials leaked through requests",
       "sources": { "url": "UserControlled", "creds": "Credentials" },
       "partial_sink": "UserControlledRequestWithCreds",
       "code": 1,
       "message_format": "Credentials leaked through requests"
    }
  ]
}

Sources are declared as normal in .pysa files. Instead of specifying sinks with a TaintSink annotation, however, PartialSink annotations are used to specify where each source needs to flow for the combined source rule. These PartialSink must reference the labels that were declared in multi_sink_labels:

def requests.api.get(
  url: PartialSink[UserControlledRequestWithCreds[url]],
  params: PartialSink[UserControlledRequestWithCreds[creds]],
  **kwargs
): ...

With the above configuration, Pysa can detect cases where UserControlled flows into url and Credentials flow into params at the same time.

Prevent Inferring Models with `SkipAnalysis`#

In addition to the models defined in .pysa files, Pysa will infer models for functions based what sources, sinks, etc. they call in their body. The SkipAnalysis annotation can be used to prevent Pysa from inferring models, and instead force it to use only the user defined models for determining taint flow:

@SkipAnalysis
def qualifier.dont_generate_models(argument): ...

SkipAnalysis can be applied at the class level as a shorthand to prevent pysa from infering models for all functions in a class:

class skip_analysis.SkipMe(SkipAnalysis): ...

Ignoring overrides#

When a method is called on a base class, Pysa has to assume that that call could actually invoke any subclass methods that override the base class's method. For heavily overriden methods, this can lead to both performance impacts and false positives. When running Pysa, you may see messages such as this in the output:

2020-09-02 09:25:50,677 WARNING `object.__init__` has 106 overrides, this might slow down the analysis considerably.

The above message indicates that 106 subclasses of object have overridden __init__. If Pysa sees taint flowing into object.__init__, then it will treat all 106 overrides of object.__init__ as also receiving that taint.

The @SkipOverrides decorator can be applied to deal with false positives or performance issues from having too many overrides on a given function:

@SkipOverrides
def object.__init__(self): ...

This annotation will cause Pysa not to propagate taint into to and from overridden methods on subclasses, when analyzing functions that call the overriden method on the base class.

maximum_overrides_to_analyze can be added the the options block of taint.config to limit the number of overrides that Pysa will analyze:

{
  "sources": [],
  "sinks": [],
  "features": [],
  "rules": [],
  "options": {
    "maximum_overrides_to_analyze": 60
  }
}

This can speed up the analysis, but it will lead to false negatives, because Pysa will only propagate taint to or from 60 (in the case of the above example) overriden methods on subclasses. The remaining overriding methods will be ignored and treated as if they weren't actually overriding the base class method.

By default, Pysa skips overrides on some functions that are typically problematic. You can find the full list of default-skipped functions in stubs/taint/skipped_overrides.pysa

Limit the trace length for better signal and performance#

By default, Pysa will find all flows from sources to sinks matching a rule. This can lead to very long traces which are hard to understand and tend to be false positives. This also brings down the performance a lot.

Pysa provides a --maximum-trace-length <integer> command line argument which limits the length of traces that it finds. In general, this will also make Pysa faster.

This option can also be added in the taint.config as follows:

{
  "sources": [],
  "sinks": [],
  "features": [],
  "rules": [],
  "options": {
    "maximum_trace_length": 20
  }
}

Note that this is not a silver bullet and that this might hide security vulnerabilities. Use it with caution.

Limit the tito depth for better signal and performance#

Pysa infers automatically when a function propagate the taint from one argument to its return value. This is called tito, for Taint In Taint Out. In practice, infering it can be very expensive since the taint can go through an arbitrary number of hops (i.e, depth).

For instance:

def foo(x):
  return x
def bar(x):
  return foo(x)
def baz(x):
  return bar(x)

In this example, baz propagates the taint on its argument to the return value using 3 hops.

Pysa provides a --maximum-tito-depth <integer> command line argument which limints the depth of inferred propagations. In combination with the trace length limit, this usually makes Pysa faster.