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Java bytecode analyzer customizable via JSON rules. It is a command-line tool that receives a path containing one or moreJar orWar files, analyzes them using the provided rules and generates HTML reports with the results.

usage: java -jar cba-cli.jar [OPTIONS] -a DIRECTORY_TO_ANALYZE -a,--analyze <pathToAnalyze>    Path of the directory to run the                                 analysis.-c,--checks <checks...>          Space separated list of custom checks that are going to be run in the analysis. -f,--custom-file <customFile>   Specify a file in JSON format to run                                 custom rules. Read more in                                 https://github.com/fergarrui/custom-bytecode-analyzer. -h,--help                       Print this message. -i,--items-report <maxItems>    Max number of items per report. If the                                 number of issues found exceeds this                                 value, the report will be split into                                 different files. Useful if expecting too                                 many issues in the report. Default: 50. -o,--output <outputDir>         Directory to save the report. Warning -                                 if there are already saved reports in                                 this directory they will be overwritten.                                 Default is "report". -v,--verbose-debug              Increase verbosity to debug mode. -vv,--verbose-trace             Increase verbosity to trace mode - makes it slower, use it only when you need.

Rules file can be specified using-f,--custom-file argument . The file is in JSON format and has the following structure:

• rules : array(rule)
  • name : string
  • fields : array(field)
    • visibility : (public|protected|private)
    • type : string
    • valueRegex : string (java regular expression) - only supported if the field isfinal
    • nameRegex : string (java regular expression)
    • report : boolean (default: true)
  • interfaces : array(string)
  • superClass : string
  • annotations : array(annotation)
    • type : string
    • report : boolean (default: true)
  • methods : array(method)
    • name : string
    • visibility : (public|protected|private)
    • parameters : array(parameter)
      • type : string
      • report : boolean (default: true)
      • annotations : array(annotation)
        • type : string
        • report : boolean (default: true)
    • variables : array(variable)
      • type : string
      • nameRegex : string (java regular expression)
      • annotations : array(annotation)
        • type : string
        • report : boolean (default: true)
      • report (default: true)
    • annotations : array(annotation)
      • type : string
      • report : boolean (default: true)
    • report : boolean (default: true)
  • invocations : array(invocation)
    • owner : string
    • method : method
      • name : string
      • visibility : (public|protected|private)
    • from : method
      • name : string
      • visibility : (public|protected|private)
    • notFrom : method
      • name : string
      • visibility : (public|protected|private)
    • report : boolean (default:true)

You can also checknet.nandgr.cba.custom.model.Rules.java to see the structure in Java code.

There are already several rules under the directoryexamples .Anyway, below are listed examples for every rule.

If we need to find classes with custom deserialization, we can do it quite easily. A class defines custom deserialization by implementingprivate void readObject(ObjectInputStream in). So we only need to find all classes where that method is defined. It would be enough just to define a rule as:

{"rules": [{"name":"Custom deserialization","methods": [{"name":"readObject","visibility":"private","parameters" : [{"type" :"java.io.ObjectInputStream"      }]}]}]}

It will report methods withprivate visibility,readObject as name and a parameter of typejava.io.ObjectOutputStream. Parameters are an array, if more than one is specified, all of them have to match to be reported. Since we only have one rule, a report named: custom-deserialization-0.html will be created.

In this case, one rule with two methods have to be defined. The same one than in the previous example for deserialization, and a new one to matchprivate void writeObject(ObjectOutputStream out). As shown in the JSON structure above, the property rules.rule.methods is an array of methods, so a rule like this can be written:

{"rules": [{"name":"Custom serialization and deserialization","methods": [{"name":"readObject","visibility":"private","parameters" : [{"type" :"java.io.ObjectInputStream"      }]    },{"name":"writeObject","report":"false","visibility":"private","parameters" : [{"type" :"java.io.ObjectOutputStream"      }]    }]  }]}

The propertyreport was set to false to avoid reporting twice for the same rule. We are using the second method just as a condition, but reporting onlyreadObject methods should be enough for the purpose of this rule.

If a property is not defined, it will always match as true. For example, this rule would return all methods definitions:

{"rules": [{"name":"Method definitions","methods": [{}]}]}

Method invocations can also be found. The JSON in this case would be:

{"rules": [{"name":"String equals","invocations": [{"owner":"java.lang.String","method": {"name":"equals"}}]}]}

The propertyowner specifies the class containing the method.

Another method invocation example a bit more useful than the previous one:

{"rules": [{"name":"Method invocation by reflection","invocations": [{"owner":"java.lang.reflect.Method","method": {"name":"invoke"}}]}]}

It is the same than any method invocation, but the name of the method in this case, should be<init>.

{"rules": [{"name" :"String instantiation","invocations" : [{"owner" :"java.lang.String","method" : {"name" :"<init>"        }    }]  }]}

This rule will find occurrences of:

[...]Strings =newString("foo");[...]

In this example, we want to find deserialization usages (not classes defining serialization behaviors like in the previous examples). Deserialization happens whenObjectInputStream.readObject() is invoked. for example in this code snippet:

ObjectInputStreamin =newObjectInputStream(fileInputStream);Objecto =in.readObject();

So we need to find method invocations fromObjectInputStream namedreadObject. But it will find a lot of false positives in a researching context, because when a class defines custom deserialization, they make an invocation to this method inside aprivate void readObject(ObjectInputStream in) method, and that would pollute the report too much. If we want to exclude those cases and keep only genuine deserialization,notFrom property can be used:

{"rules": [{"name":"Deserialization usage","invocations": [{"owner":"java.io.ObjectInputStream","method": {"name":"readObject"},"notFrom": {"name":"readObject","visibility":"private"}}]}]}

This file will findjava.io.ObjectInputStream.readObject() invocations if the invocation is not done insideprivate void readObject(ObjectInputStream in) method.

A class compiled with this code will not be reported:

privatevoidreadObject(ObjectInputStreamin)throwsIOException,ClassNotFoundException {Objecto =in.readObject();}

But this one will be reported:

publicObjectdeserializeObject(ObjectInputStreamin)throwsIOException,ClassNotFoundException {Objecto =in.readObject();returno;}

The propertyfrom can be set in invocations in exactly the same way thannotFrom, but the result will be the opposite: it will only match if the invocation is made from the defined method.

The propertysuperClass can be used in this case. If we want to find all classes extendingjavax.servlet.http.HttpServlet, a rule can be:

{"rules": [{"name":"Java servlets","superClass" :"javax.servlet.http.HttpServlet"  }]}

A rule can be written to find classes implementing an array of interfaces. if more than one interface is defined in the rule, the class has to implement all of them to be reported. If we want to find classes implementingjavax.net.ssl.X509TrustManager, the rule would be:

{"rules": [{"name":"X509TrustManager implementations","interfaces" : ["javax.net.ssl.X509TrustManager"]  }]}

Please note thatinterfaces is anarray, so make sure you add the strings between square brackets, e.g:["interface1", "interface2", ...].

Annotations are also supported. Multiple annotations properties can be defined in a rule (finding class annotations), in methods o variables (parameters or local variables). If all of them are found in the analyzed class, it will be reported.For example, if we want to find Spring endpoints, we would search for classes or methods annotated withorg.springframework.web.bind.annotation.RequestMapping. So, the rule can be:

{"rules": [{"name":"Spring endpoint - class annotation","annotations" : [{"type" :"org.springframework.web.bind.annotation.RequestMapping"    }]  },  {"name":"Spring endpoint - method annotation","methods" : [{"annotations" : [{"type" :"org.springframework.web.bind.annotation.RequestMapping"        }]      }]  }]}

The propertyrule.fields can be used to find class fields. If we want to find private String fields with password names, a rule like this one could be used:

{"rules": [{"name" :"Password fields","fields" : [      {"visibility" :"private","type" :"java.lang.String","nameRegex" :"(password|pass|psswd|passwd)"      }    ]  }]}

To find variables,rule.variables can be used. This property will report local variables and method arguments variables.If we want to find all variables of typejavax.servlet.http.Part, a rule could be:

{"rules": [{"name" :"Servlet upload file","methods" : [{"variables" : [{"type" :"javax.servlet.http.Part"      }]    }]  }]}

Multiple rules can be defined in the same JSON file. They will be processed and reported separately and they will not affect each other. We can combine some of the previous examples rules:

{"rules": [{"name":"Custom deserialization","methods": [{"name":"readObject","visibility":"private","parameters": [{"type" :"java.io.ObjectInputStream"      }]}]},{"name":"Method invocation by reflection","invocations": [{"owner":"java.lang.reflect.Method","method": {"name":"invoke"}}]}]}

Here, we have two rules ("Custom deserialization" and "Method invocation by reflection"). They will be processed as if you do it in two separated executions. And a report per rule will be generated. If the rules have the same name, they will be reported in the same file.

The project can be downloaded and built to add more complex custom rules in Java code that are not covered by the JSON format. There are already three examples under the packagenet.nandgr.cba.visitor.checks. Those areCustomDeserializationCheck, DeserializationCheck and InvokeMethodCheck. You can create your own rules by extendingnet.nandgr.cba.custom.visitor.base.CustomAbstractClassVisitor.

As mentioned above, the reports are created by default underreport folder. Every rule will have a separate file unless they have the same name.If the report is too big, you can split it using the-i,--items-report <maxItems> parameter, each of them will hold the argument specified or less (if it is the last one).Every reported item, specifies the jar where it is found, the class name and the method name (if it is relevant). It also shows the decompiled version of the class to ease a quick visual check.Example of how the items are shown for a rule to findjava.io.File instantiations:

When searching for security bugs it is very useful to have a call graph. At the moment, a simpleDOT compatible file is created under thereport directory.The graph contains all the possible flows where the found issues can be invoked from. For example, if a rule to find deserialization is used,a graph containing all possible paths leading to the method that calls the deserialization will be generated.

The file iscall-graph.dot and it would look like this (this is an extremely simple example):

graph callGraph {"demo.callgraph.Class1:method1" -- "demo.callgraph.Class2:method2""demo.callgraph.Class3:method3" -- "demo.callgraph.Class2:method2"}

To display it in a visual way,DOT can be used (or any compatible software). For example, to convert the file tosvg:

dot -Tsvg call-graph.dot -o call-graph.svg

This is done automatically by default if DOT is found in the system PATH. If not,DOT can be installed in Debian based systems usingsudo apt-get install graphviz.

It will create a SVG file namedcall-graph.svg that can be converted into PNG or visualized using programs likeinkscape or justfirefox.

A very simple example of the above file call-graph.dot, would be:

There are some limitations, like for example, if the searched item is in ajava.lang.Runnable.run() or similar method, it will not find where the thread is executed from.Also, the graph is cleaning cycles to avoidStackOverflowErrors, it is made in a bit of conservative way so the memory of the system is not drained during an analysis of a large directory.

More options will be added in future versions.

java -jar cba-cli-<version>.jar -a /path/with/jars -f /path/with/json/file/rules.json

To use custom java rules, class names have to be specified as arguments of-c.

java -jar cba-cli-<version>.jar -a /path/with/jars -c DeserializationCheck

Accepts a space separated list, so multiple custom rules can be defined (each of the rules will create a separate report):

java -jar cba-cli-<version>.jar -a /path/with/jars -c DeserializationCheck InvokeMethodCheck CustomDeserializationCheck YourCustomRule

java -jar cba-cli-<version>.jar -a /path/with/jars -f /path/with/json/file/rules.json -c YourCustomRule1 YourCustomRule2

To find errors, verbosity can be increased.Debug level:

java -jar cba-cli-<version>.jar -a /path/with/jars -c YourCustomRule1 -v

Trace level:

java -jar cba-cli-<version>.jar -a /path/with/jars -c YourCustomRule1 -vv

At the moment, the APK has to be converted to JAR first to be analyzed.

• Download dex2jar :https://github.com/pxb1988/dex2jar
• Convert DEX to JAR
  • d2j-dex2jar.sh -f -o app_to_analyze.jar app_to_analyze.apk
• Run cba-cli.jar as usual passing as-a parameter the directory containing the converted jar file.

There is already an executable jar file underbin directory at:https://github.com/fergarrui/custom-bytecode-analyzer/blob/master/bin/cba-cli-0.1-SNAPSHOT.jar . If you want to do modifications or add custom rules, the project can be built doing:

git clone https://github.com/fergarrui/custom-bytecode-analyzer.gitcd custom-bytecode-analyzermvn clean package

Two jars will be generated undertarget folder.cba-cli-<version>.jar contains all dependencies and is executable. Can be run usingjava -jar cba-cli-<version>.jar