Analyzing data flow in Java¶
You can use CodeQL to track the flow of data through a Java program to its use.
About this article¶
This article describes how data flow analysis is implemented in the CodeQL libraries for Java 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.”
Local data flow¶
Local data flow is data flow within a single method or callable. Local data flow is usually 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. Node
s 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 by using the predefined recursive predicate localFlow
, which is equivalent to localFlowStep*
.
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:
String temp = x;
String 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 by using the predefined recursive predicate localTaint
, which is equivalent to localTaintStep*
.
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 new FileReader(..)
.
import java
from Constructor fileReader, Call call
where
fileReader.getDeclaringType().hasQualifiedName("java.io", "FileReader") and
call.getCallee() = fileReader
select call.getArgument(0)
Unfortunately, this only gives 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 java
import semmle.code.java.dataflow.DataFlow
from Constructor fileReader, Call call, Expr src
where
fileReader.getDeclaringType().hasQualifiedName("java.io", "FileReader") and
call.getCallee() = fileReader 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 where a public parameter is passed to new FileReader(..)
:
import java
import semmle.code.java.dataflow.DataFlow
from Constructor fileReader, Call call, Parameter p
where
fileReader.getDeclaringType().hasQualifiedName("java.io", "FileReader") and
call.getCallee() = fileReader and
DataFlow::localFlow(DataFlow::parameterNode(p), DataFlow::exprNode(call.getArgument(0)))
select p
This query finds calls to formatting functions where the format string is not hard-coded.
import java
import semmle.code.java.dataflow.DataFlow
import semmle.code.java.StringFormat
from StringFormatMethod format, MethodAccess call, Expr formatString
where
call.getMethod() = format and
call.getArgument(format.getFormatStringIndex()) = formatString and
not exists(DataFlow::Node source, DataFlow::Node sink |
DataFlow::localFlow(source, sink) and
source.asExpr() instanceof StringLiteral and
sink.asExpr() = formatString
)
select call, "Argument to String format method isn't hard-coded."
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¶
You use the global data flow library by extending the class DataFlow::Configuration
:
import semmle.code.java.dataflow.DataFlow
class MyDataFlowConfiguration extends DataFlow::Configuration {
MyDataFlowConfiguration() { this = "MyDataFlowConfiguration" }
override predicate isSource(DataFlow::Node source) {
...
}
override predicate isSink(DataFlow::Node sink) {
...
}
}
These predicates are defined in the configuration:
isSource
—defines where data may flow fromisSink
—defines where data may flow toisBarrier
—optional, restricts the data flowisAdditionalFlowStep
—optional, adds additional flow steps
The characteristic predicate MyDataFlowConfiguration()
defines the name of the configuration, so "MyDataFlowConfiguration"
should be a unique name, for example, the name of your class.
The data flow analysis is performed using the predicate hasFlow(DataFlow::Node source, DataFlow::Node sink)
:
from MyDataFlowConfiguration dataflow, DataFlow::Node source, DataFlow::Node sink
where dataflow.hasFlow(source, sink)
select source, "Data flow to $@.", sink, sink.toString()
Using global taint tracking¶
Global taint tracking is to global data flow as local taint tracking is to local data flow. That is, global taint tracking extends global data flow with additional non-value-preserving steps. You use the global taint tracking library by extending the class TaintTracking::Configuration
:
import semmle.code.java.dataflow.TaintTracking
class MyTaintTrackingConfiguration extends TaintTracking::Configuration {
MyTaintTrackingConfiguration() { this = "MyTaintTrackingConfiguration" }
override predicate isSource(DataFlow::Node source) {
...
}
override predicate isSink(DataFlow::Node sink) {
...
}
}
These predicates are defined in the configuration:
isSource
—defines where taint may flow fromisSink
—defines where taint may flow toisSanitizer
—optional, restricts the taint flowisAdditionalTaintStep
—optional, adds additional taint steps
Similar to global data flow, the characteristic predicate MyTaintTrackingConfiguration()
defines the unique name of the configuration.
The taint tracking analysis is performed using the predicate hasFlow(DataFlow::Node source, DataFlow::Node sink)
.
Flow sources¶
The data flow library contains some predefined flow sources. The class RemoteFlowSource
(defined in semmle.code.java.dataflow.FlowSources
) represents data flow sources that may be controlled by a remote user, which is useful for finding security problems.
Examples¶
This query shows a taint-tracking configuration that uses remote user input as data sources.
import java
import semmle.code.java.dataflow.FlowSources
class MyTaintTrackingConfiguration extends TaintTracking::Configuration {
MyTaintTrackingConfiguration() {
this = "..."
}
override predicate isSource(DataFlow::Node source) {
source instanceof RemoteFlowSource
}
...
}
Exercises¶
Exercise 2: Write a query that finds all hard-coded strings used to create a java.net.URL
, using global data flow. (Answer)
Exercise 3: Write a class that represents flow sources from java.lang.System.getenv(..)
. (Answer)
Exercise 4: Using the answers from 2 and 3, write a query which finds all global data flows from getenv
to java.net.URL
. (Answer)
Answers¶
Exercise 1¶
import semmle.code.java.dataflow.DataFlow
from Constructor url, Call call, StringLiteral src
where
url.getDeclaringType().hasQualifiedName("java.net", "URL") and
call.getCallee() = url and
DataFlow::localFlow(DataFlow::exprNode(src), DataFlow::exprNode(call.getArgument(0)))
select src
Exercise 2¶
import semmle.code.java.dataflow.DataFlow
class Configuration extends DataFlow::Configuration {
Configuration() {
this = "LiteralToURL Configuration"
}
override predicate isSource(DataFlow::Node source) {
source.asExpr() instanceof StringLiteral
}
override predicate isSink(DataFlow::Node sink) {
exists(Call call |
sink.asExpr() = call.getArgument(0) and
call.getCallee().(Constructor).getDeclaringType().hasQualifiedName("java.net", "URL")
)
}
}
from DataFlow::Node src, DataFlow::Node sink, Configuration config
where config.hasFlow(src, sink)
select src, "This string constructs a URL $@.", sink, "here"
Exercise 3¶
import java
class GetenvSource extends MethodAccess {
GetenvSource() {
exists(Method m | m = this.getMethod() |
m.hasName("getenv") and
m.getDeclaringType() instanceof TypeSystem
)
}
}
Exercise 4¶
import semmle.code.java.dataflow.DataFlow
class GetenvSource extends DataFlow::ExprNode {
GetenvSource() {
exists(Method m | m = this.asExpr().(MethodAccess).getMethod() |
m.hasName("getenv") and
m.getDeclaringType() instanceof TypeSystem
)
}
}
class GetenvToURLConfiguration extends DataFlow::Configuration {
GetenvToURLConfiguration() {
this = "GetenvToURLConfiguration"
}
override predicate isSource(DataFlow::Node source) {
source instanceof GetenvSource
}
override predicate isSink(DataFlow::Node sink) {
exists(Call call |
sink.asExpr() = call.getArgument(0) and
call.getCallee().(Constructor).getDeclaringType().hasQualifiedName("java.net", "URL")
)
}
}
from DataFlow::Node src, DataFlow::Node sink, GetenvToURLConfiguration config
where config.hasFlow(src, sink)
select src, "This environment variable constructs a URL $@.", sink, "here"