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Analyzing data flow in C#

You can use CodeQL to track the flow of data through a C# program to its use.

About this article

This article describes how data flow analysis is implemented in the CodeQL libraries for C# and includes examples to help you write your own data flow queries. The following sections describe how to use the libraries for local data flow, global data flow, and taint tracking. For a more general introduction to modeling data flow, see “About data flow analysis.”

Note

The new modular API for data flow described here is available alongside the previous library from CodeQL 2.13.0 onwards. For information about how the library has changed and how to migrate any existing queries to the modular API, see New dataflow API for CodeQL query writing.

Local data flow

Local data flow is data flow within a single method or callable. Local data flow is easier, faster, and more precise than global data flow, and is sufficient for many queries.

Using local data flow

The local data flow library is in the module DataFlow, which defines the class Node denoting any element that data can flow through. Nodes are divided into expression nodes (ExprNode) and parameter nodes (ParameterNode). You can map between data flow nodes and expressions/parameters using the member predicates asExpr and asParameter:

class Node {
  /** Gets the expression corresponding to this node, if any. */
  Expr asExpr() { ... }

  /** Gets the parameter corresponding to this node, if any. */
  Parameter asParameter() { ... }

 ...
}

or using the predicates exprNode and parameterNode:

/**
 * Gets the node corresponding to expression `e`.
 */
ExprNode exprNode(Expr e) { ... }

/**
 * Gets the node corresponding to the value of parameter `p` at function entry.
 */
ParameterNode parameterNode(Parameter p) { ... }

The predicate localFlowStep(Node nodeFrom, Node nodeTo) holds if there is an immediate data flow edge from the node nodeFrom to the node nodeTo. You can apply the predicate recursively, by using the + and * operators, or you can use the predefined recursive predicate localFlow.

For example, you can find flow from a parameter source to an expression sink in zero or more local steps:

DataFlow::localFlow(DataFlow::parameterNode(source), DataFlow::exprNode(sink))

Using local taint tracking

Local taint tracking extends local data flow by including non-value-preserving flow steps. For example:

var temp = x;
var y = temp + ", " + temp;

If x is a tainted string then y is also tainted.

The local taint tracking library is in the module TaintTracking. Like local data flow, a predicate localTaintStep(DataFlow::Node nodeFrom, DataFlow::Node nodeTo) holds if there is an immediate taint propagation edge from the node nodeFrom to the node nodeTo. You can apply the predicate recursively, by using the + and * operators, or you can use the predefined recursive predicate localTaint.

For example, you can find taint propagation from a parameter source to an expression sink in zero or more local steps:

TaintTracking::localTaint(DataFlow::parameterNode(source), DataFlow::exprNode(sink))

Examples

This query finds the filename passed to System.IO.File.Open:

import csharp

from Method fileOpen, MethodCall call
where fileOpen.hasQualifiedName("System.IO.File.Open")
  and call.getTarget() = fileOpen
select call.getArgument(0)

Unfortunately this will only give the expression in the argument, not the values which could be passed to it. So we use local data flow to find all expressions that flow into the argument:

import csharp

from Method fileOpen, MethodCall call, Expr src
where fileOpen.hasQualifiedName("System.IO.File.Open")
  and call.getTarget() = fileOpen
  and DataFlow::localFlow(DataFlow::exprNode(src), DataFlow::exprNode(call.getArgument(0)))
select src

Then we can make the source more specific, for example an access to a public parameter. This query finds instances where a public parameter is used to open a file:

import csharp

from Method fileOpen, MethodCall call, Parameter p
where fileOpen.hasQualifiedName("System.IO.File.Open")
  and call.getTarget() = fileOpen
  and DataFlow::localFlow(DataFlow::parameterNode(p), DataFlow::exprNode(call.getArgument(0)))
  and call.getEnclosingCallable().(Member).isPublic()
select p, "Opening a file from a public method."

This query finds calls to String.Format where the format string isn’t hard-coded:

import csharp

from Method format, MethodCall call, Expr formatString
where format.hasQualifiedName("System.String.Format")
  and call.getTarget() = format
  and formatString = call.getArgument(0)
  and formatString.getType() instanceof StringType
  and not exists(StringLiteral source | DataFlow::localFlow(DataFlow::exprNode(source), DataFlow::exprNode(formatString)))
select call, "Argument to 'string.Format' isn't hard-coded."

Exercises

Exercise 1: Write a query that finds all hard-coded strings used to create a System.Uri, using local data flow. (Answer)

Global data flow

Global data flow tracks data flow throughout the entire program, and is therefore more powerful than local data flow. However, global data flow is less precise than local data flow, and the analysis typically requires significantly more time and memory to perform.

Note

You can model data flow paths in CodeQL by creating path queries. To view data flow paths generated by a path query in CodeQL for VS Code, you need to make sure that it has the correct metadata and select clause. For more information, see Creating path queries.

Using global data flow

The global data flow library is used by implementing the signature DataFlow::ConfigSig and applying the module DataFlow::Global<ConfigSig>:

import csharp

module MyFlowConfiguration implements DataFlow::ConfigSig {
  predicate isSource(DataFlow::Node source) {
    ...
  }

  predicate isSink(DataFlow::Node sink) {
    ...
  }
}

module MyFlow = DataFlow::Global<MyFlowConfiguration>;

These predicates are defined in the configuration:

• isSource - defines where data may flow from.
• isSink - defines where data may flow to.
• isBarrier - optionally, restricts the data flow.
• isAdditionalFlowStep - optionally, adds additional flow steps.

The data flow analysis is performed using the predicate flow(DataFlow::Node source, DataFlow::Node sink):

from DataFlow::Node source, DataFlow::Node sink
where MyFlow::flow(source, sink)
select source, "Dataflow to $@.", sink, sink.toString()

Using global taint tracking

Global taint tracking is to global data flow what local taint tracking is to local data flow. That is, global taint tracking extends global data flow with additional non-value-preserving steps. The global taint tracking library is used by applying the module TaintTracking::Global<ConfigSig> to your configuration instead of DataFlow::Global<ConfigSig>:

import csharp

module MyFlowConfiguration implements DataFlow::ConfigSig {
  predicate isSource(DataFlow::Node source) {
    ...
  }

  predicate isSink(DataFlow::Node sink) {
    ...
  }
}

module MyFlow = TaintTracking::Global<MyFlowConfiguration>;

The resulting module has an identical signature to the one obtained from DataFlow::Global<ConfigSig>.

Flow sources

The data flow library contains some predefined flow sources. The class PublicCallableParameterFlowSource (defined in module semmle.code.csharp.dataflow.flowsources.PublicCallableParameter) represents data flow from public parameters, which is useful for finding security problems in a public API.

The class RemoteFlowSource (defined in module semmle.code.csharp.dataflow.flowsources.Remote) represents data flow from remote network inputs. This is useful for finding security problems in networked services.

Example

This query shows a data flow configuration that uses all public API parameters as data sources:

import csharp
import semmle.code.csharp.dataflow.flowsources.PublicCallableParameter

module MyFlowConfiguration implements DataFlow::ConfigSig {
  predicate isSource(DataFlow::Node source) {
    source instanceof PublicCallableParameterFlowSource
  }

  ...
}

Class hierarchy

• DataFlow::Node - an element behaving as a data flow node.
  • DataFlow::ExprNode - an expression behaving as a data flow node.
  • DataFlow::ParameterNode - a parameter data flow node representing the value of a parameter at function entry.
    • PublicCallableParameter - a parameter to a public method/callable in a public class.
  • RemoteFlowSource - data flow from network/remote input.
    • AspNetRemoteFlowSource - data flow from remote ASP.NET user input.
      • AspNetQueryStringRemoteFlowSource - data flow from System.Web.HttpRequest.
      • AspNetUserInputRemoveFlowSource - data flow from System.Web.IO.WebControls.TextBox.
    • WcfRemoteFlowSource - data flow from a WCF web service.
    • AspNetServiceRemoteFlowSource - data flow from an ASP.NET web service.

Examples

This data flow configuration tracks data flow from environment variables to opening files:

import csharp

module EnvironmentToFileConfiguration implements DataFlow::ConfigSig {
  predicate isSource(DataFlow::Node source) {
    exists(Method m |
      m = source.asExpr().(MethodCall).getTarget() and
      m.hasQualifiedName("System.Environment.GetEnvironmentVariable")
    )
  }

  predicate isSink(DataFlow::Node sink) {
    exists(MethodCall mc |
      mc.getTarget().hasQualifiedName("System.IO.File.Open") and
      sink.asExpr() = mc.getArgument(0)
    )
  }
}

module EnvironmentToFileFlow = DataFlow::Global<EnvironmentToFileConfiguration>;

from Expr environment, Expr fileOpen
where EnvironmentToFileFlow::flow(DataFlow::exprNode(environment), DataFlow::exprNode(fileOpen))
select fileOpen, "This 'File.Open' uses data from $@.",
  environment, "call to 'GetEnvironmentVariable'"

Exercises

Exercise 2: Find all hard-coded strings passed to System.Uri, using global data flow. (Answer)

Exercise 3: Define a class that represents flow sources from System.Environment.GetEnvironmentVariable. (Answer)

Exercise 4: Using the answers from 2 and 3, write a query to find all global data flow from System.Environment.GetEnvironmentVariable to System.Uri. (Answer)

Extending library data flow

Library data flow defines how data flows through libraries where the source code is not available, such as the .NET Framework, third-party libraries or proprietary libraries.

To define new library data flow, extend the class LibraryTypeDataFlow from the module semmle.code.csharp.dataflow.LibraryTypeDataFlow. Override the predicate callableFlow to define how data flows through the methods in the class. callableFlow has the signature

predicate callableFlow(CallableFlowSource source, CallableFlowSink sink, SourceDeclarationCallable callable, boolean preservesValue)

• callable - the Callable (such as a method, constructor, property getter or setter) performing the data flow.
• source - the data flow input.
• sink - the data flow output.
• preservesValue - whether the flow step preserves the value, for example if x is a string then x.ToString() preserves the value where as x.ToLower() does not.

Class hierarchy

• Callable - a callable (methods, accessors, constructors etc.)
  • SourceDeclarationCallable - an unconstructed callable.
• CallableFlowSource - the input of data flow into the callable.
  • CallableFlowSourceQualifier - the data flow comes from the object itself.
  • CallableFlowSourceArg - the data flow comes from an argument to the call.
• CallableFlowSink - the output of data flow from the callable.
  • CallableFlowSinkQualifier - the output is to the object itself.
  • CallableFlowSinkReturn - the output is returned from the call.
  • CallableFlowSinkArg - the output is an argument.
  • CallableFlowSinkDelegateArg - the output flows through a delegate argument (for example, LINQ).

Example

This example is adapted from LibraryTypeDataFlow.qll. It declares data flow through the class System.Uri, including the constructor, the ToString method, and the properties Query, OriginalString, and PathAndQuery.

import semmle.code.csharp.dataflow.LibraryTypeDataFlow
import semmle.code.csharp.frameworks.System

class SystemUriFlow extends LibraryTypeDataFlow, SystemUriClass {
  override predicate callableFlow(CallableFlowSource source, CallableFlowSink sink, SourceDeclarationCallable c, boolean preservesValue) {
    (
      constructorFlow(source, c) and
      sink instanceof CallableFlowSinkQualifier
      or
      methodFlow(c) and
      source instanceof CallableFlowSourceQualifier and
      sink instanceof CallableFlowSinkReturn
      or
      exists(Property p |
        propertyFlow(p) and
        source instanceof CallableFlowSourceQualifier and
        sink instanceof CallableFlowSinkReturn and
        c = p.getGetter()
      )
    )
    and
    preservesValue = false
  }

  private predicate constructorFlow(CallableFlowSourceArg source, Constructor c) {
    c = getAMember()
    and
    c.getParameter(0).getType() instanceof StringType
    and
    source.getArgumentIndex() = 0
  }

  private predicate methodFlow(Method m) {
    m.getDeclaringType() = getABaseType*()
    and
    m = getSystemObjectClass().getToStringMethod().getAnOverrider*()
  }

  private predicate propertyFlow(Property p) {
    p = getPathAndQueryProperty()
    or
    p = getQueryProperty()
    or
    p = getOriginalStringProperty()
  }
}

This defines a new class SystemUriFlow which extends LibraryTypeDataFlow to add another case. It extends SystemUriClass (the class representing System.Uri, defined in the module semmle.code.csharp.frameworks.System) to access methods such as getQueryProperty.

The predicate callableFlow declares data flow through System.Uri. The first case (constructorFlow) declares data flow from the first argument of the constructor to the object itself (CallableFlowSinkQualifier).

The second case declares data flow from the object (CallableFlowSourceQualifier) to the result of calling ToString on the object (CallableFlowSinkReturn).

The third case declares data flow from the object (CallableFlowSourceQualifier) to the return (CallableFlowSinkReturn) of the getters for the properties PathAndQuery, Query and OriginalString. Note that the properties (getPathAndQueryProperty, getQueryProperty and getOriginalStringProperty) are inherited from the class SystemUriClass.

In all three cases preservesValue = false, which means that these steps will only be included in taint tracking, not in (normal) data flow.

Exercises

Exercise 5: In System.Uri, what other properties could expose data? How could they be added to SystemUriFlow? (Answer)

Exercise 6: Implement the data flow for the class System.Exception. (Answer)

---

Answers

Exercise 1

import csharp

from Expr src, Call c
where DataFlow::localFlow(DataFlow::exprNode(src), DataFlow::exprNode(c.getArgument(0)))
  and c.getTarget().(Constructor).getDeclaringType().hasQualifiedName("System.Uri")
  and src.hasValue()
select src, "This string constructs 'System.Uri' $@.", c, "here"

Exercise 2

import csharp

module StringToUriConfig implements DataFlow::ConfigSig {
  predicate isSource(DataFlow::Node src) {
    src.asExpr().hasValue()
  }

  predicate isSink(DataFlow::Node sink) {
    exists(Call c | c.getTarget().(Constructor).getDeclaringType().hasQualifiedName("System.Uri")
    and sink.asExpr()=c.getArgument(0))
  }
}

module StringToUriFlow = DataFlow::Global<StringToUriConfig>;

from DataFlow::Node src, DataFlow::Node sink
where StringToUriFlow::flow(src, sink)
select src, "This string constructs a 'System.Uri' $@.", sink, "here"

Exercise 3

class EnvironmentVariableFlowSource extends DataFlow::ExprNode {
  EnvironmentVariableFlowSource() {
    this.getExpr().(MethodCall).getTarget().hasQualifiedName("System.Environment.GetEnvironmentVariable")
  }
}

Exercise 4

import csharp

class EnvironmentVariableFlowSource extends DataFlow::ExprNode {
  EnvironmentVariableFlowSource() {
    this.getExpr().(MethodCall).getTarget().hasQualifiedName("System.Environment.GetEnvironmentVariable")
  }
}

module EnvironmentToUriConfig implements DataFlow::ConfigSig {
  predicate isSource(DataFlow::Node src) {
    src instanceof EnvironmentVariableFlowSource
  }

  predicate isSink(DataFlow::Node sink) {
    exists(Call c | c.getTarget().(Constructor).getDeclaringType().hasQualifiedName("System.Uri")
    and sink.asExpr()=c.getArgument(0))
  }
}

module EnvironmentToUriFlow = DataFlow::Global<EnvironmentToUriConfig>;

from DataFlow::Node src, DataFlow::Node sink
where EnvironmentToUriFlow::flow(src, sink)
select src, "This environment variable constructs a 'System.Uri' $@.", sink, "here"

Exercise 5

All properties can flow data:

private predicate propertyFlow(Property p) {
  p = getAMember()
}

Exercise 6

This can be adapted from the SystemUriFlow class:

import semmle.code.csharp.dataflow.LibraryTypeDataFlow
import semmle.code.csharp.frameworks.System

class SystemExceptionFlow extends LibraryTypeDataFlow, SystemExceptionClass {
  override predicate callableFlow(CallableFlowSource source, CallableFlowSink sink, SourceDeclarationCallable c, boolean preservesValue) {
    (
      constructorFlow(source, c) and
      sink instanceof CallableFlowSinkQualifier
      or
      methodFlow(source, sink, c)
      or
      exists(Property p |
        propertyFlow(p) and
        source instanceof CallableFlowSourceQualifier and
        sink instanceof CallableFlowSinkReturn and
        c = p.getGetter()
      )
    )
    and
    preservesValue = false
  }

  private predicate constructorFlow(CallableFlowSourceArg source, Constructor c) {
    c = getAMember()
    and
    c.getParameter(0).getType() instanceof StringType
    and
    source.getArgumentIndex() = 0
  }

  private predicate methodFlow(CallableFlowSourceQualifier source, CallableFlowSinkReturn sink, SourceDeclarationMethod m) {
    m.getDeclaringType() = getABaseType*()
    and
    m = getSystemObjectClass().getToStringMethod().getAnOverrider*()
  }

  private predicate propertyFlow(Property p) {
    p = getAProperty() and p.hasName("Message")
  }
}

Further reading

• “Exploring data flow with path queries”

• CodeQL queries for C#
• Example queries for C#
• CodeQL library reference for C#

• “QL language reference”
• “CodeQL tools”

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