Xcheck 对 Java 工具类代码的分析尝试

0x00 背景

log4j2 和 spring cloud function 爆出严重的安全漏洞，作为静态代码扫描工具，Xcheck 经常会被问到——是否能够发现此类漏洞。

从设计初衷出发，Xcheck 不会对框架代码和工具类代码（第三方 Jar 包）进行有效扫描。

主要有以下原因：

此类代码与上层应用代码相比，通常来说漏洞相对少，且使用到的 Java 语言特性比较复杂，Xcheck 使用的污点传播模型来进行分析，既不够高效，效果也不够理想
工具类代码的入口缺乏统一的特征，如果要通过自定义规则来逐一标注，不利于实现自动化批量分析

尝试自己使用CodeQL，以及参考网上的文章，在不添加专用规则的前提下并没有实现精准的分析出 log4j2 的漏洞。

那么换一个思路，如果只需要得出近似的结果，Xcheck 是否可以做？

近期，基于 Java 引擎进行扩展，实现了对框架代码和工具类代码的分析，能够得出一些近似结果，可以辅助做安全研究的同学用于分析。

0x01 log4j2

分析后，根据lookup函数得出可能的入口函数有 426 个，其中虽然包含了所需要的函数，但是也存在不少干扰数据：

AbstractLogger.debug(Marker, String, Object, Object, Object, Object, Object, ) AbstractLogger.debug(Marker, String, Object, Object, Object, Object, Object, Object, Object, ) AbstractLogger.debug(Marker, String, Object, Object, Object, Object, Object, Object, Object, Object, ) AbstractLogger.debug(Marker, String, Object, Object, Object, Object, Object, Object, Object, Object, Object, ) AbstractLogger.debug(Marker, String, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, ) AbstractLogger.debug(String, Object, Object, Object, Object, Object, Object, ) AbstractLogger.debug(String, Object, Object, Object, Object, Object, Object, Object, Object, ) AbstractLogger.debug(String, Object, Object, Object, Object, Object, Object, Object, Object, Object, ) AbstractLogger.debug(String, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, ) AbstractLogger.enter(String, MessageSupplier, ) AbstractLogger.error(Marker, String, Object, Object, Object, Object, ) AbstractLogger.error(Marker, String, Object, Object, Object, Object, Object, ) AbstractLogger.error(Marker, String, Object, Object, Object, Object, Object, Object, ) AbstractLogger.error(Marker, String, Object, Object, Object, Object, Object, Object, Object, ) AbstractLogger.error(Marker, String, Object, Object, Object, Object, Object, Object, Object, Object, ) AbstractLogger.error(Marker, String, Object, Object, Object, Object, Object, Object, Object, Object, Object, ) AbstractLogger.error(Marker, String, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, ) AbstractLogger.error(String, Object, Object, Object, Object, Object, ) AbstractLogger.error(String, Object, Object, Object, Object, Object, Object, ) AbstractLogger.error(String, Object, Object, Object, Object, Object, Object, Object, ) AbstractLogger.error(String, Object, Object, Object, Object, Object, Object, Object, Object, ) AbstractLogger.error(String, Object, Object, Object, Object, Object, Object, Object, Object, Object, ) AbstractLogger.error(String, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, ) AbstractLogger.fatal(Marker, String, Object, Object, Object, ) AbstractLogger.fatal(Marker, String, Object, Object, Object, Object, ) AbstractLogger.fatal(Marker, String, Object, Object, Object, Object, Object, ) AbstractLogger.fatal(Marker, String, Object, Object, Object, Object, Object, Object, ) AbstractLogger.fatal(Marker, String, Object, Object, Object, Object, Object, Object, Object, ) AbstractLogger.fatal(Marker, String, Object, Object, Object, Object, Object, Object, Object, Object, ) AbstractLogger.fatal(Marker, String, Object, Object, Object, Object, Object, Object, Object, Object, Object, ) AbstractLogger.fatal(Marker, String, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, ) AbstractLogger.fatal(String, Object, Object, Object, Object, ) AbstractLogger.fatal(String, Object, Object, Object, Object, Object, ) AbstractLogger.fatal(String, Object, Object, Object, Object, Object, Object, ) AbstractLogger.fatal(String, Object, Object, Object, Object, Object, Object, Object, ) AbstractLogger.fatal(String, Object, Object, Object, Object, Object, Object, Object, Object, ) AbstractLogger.fatal(String, Object, Object, Object, Object, Object, Object, Object, Object, Object, ) AbstractLogger.fatal(String, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, ) AbstractLogger.log(Level, Marker, String, Object, Object, Object, ) AbstractLogger.log(Level, Marker, String, Object, Object, Object, Object, ) AbstractLogger.log(Level, Marker, String, Object, Object, Object, Object, Object, ) AbstractLogger.log(Level, Marker, String, Object, Object, Object, Object, Object, Object, ) AbstractLogger.log(Level, Marker, String, Object, Object, Object, Object, Object, Object, Object, ) AbstractLogger.log(Level, Marker, String, Object, Object, Object, Object, Object, Object, Object, Object, ) AbstractLogger.log(Level, Marker, String, Object, Object, Object, Object, Object, Object, Object, Object, Object, ) AbstractLogger.log(Level, Marker, String, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, ) AbstractLogger.log(Level, String, Object, Object, Object, Object, ) AbstractLogger.log(Level, String, Object, Object, Object, Object, Object, ) AbstractLogger.log(Level, String, Object, Object, Object, Object, Object, Object, ) AbstractLogger.log(Level, String, Object, Object, Object, Object, Object, Object, Object, ) AbstractLogger.log(Level, String, Object, Object, Object, Object, Object, Object, Object, Object, ) AbstractLogger.log(Level, String, Object, Object, Object, Object, Object, Object, Object, Object, Object, ) AbstractLogger.log(Level, String, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, ) AbstractLogger.log(Message, ) AbstractLogger.readObject(ObjectInputStream, ) AbstractLogger.throwing(Level, T, ) AbstractLogger.trace(Marker, String, Object, Object, Object, Object, Object, ) AbstractLogger.trace(Marker, String, Object, Object, Object, Object, Object, Object, ) AbstractLogger.trace(Marker, String, Object, Object, Object, Object, Object, Object, Object, ) AbstractLogger.trace(Marker, String, Object, Object, Object, Object, Object, Object, Object, Object, ) AbstractLogger.trace(Marker, String, Object, Object, Object, Object, Object, Object, Object, Object, Object, ) AbstractLogger.trace(Marker, String, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, ) AbstractLogger.trace(String, Object, Object, Object, Object, Object, Object, ) AbstractLogger.trace(String, Object, Object, Object, Object, Object, Object, Object, ) AbstractLogger.trace(String, Object, Object, Object, Object, Object, Object, Object, Object, ) AbstractLogger.trace(String, Object, Object, Object, Object, Object, Object, Object, Object, Object, ) AbstractLogger.trace(String, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, ) AbstractLogger.warn(Marker, String, Object, Object, Object, Object, Object, ) AbstractLogger.warn(Marker, String, Object, Object, Object, Object, Object, Object, ) AbstractLogger.warn(Marker, String, Object, Object, Object, Object, Object, Object, Object, ) AbstractLogger.warn(Marker, String, Object, Object, Object, Object, Object, Object, Object, Object, ) AbstractLogger.warn(Marker, String, Object, Object, Object, Object, Object, Object, Object, Object, Object, ) AbstractLogger.warn(Marker, String, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, ) AbstractLogger.warn(String, Object, Object, Object, Object, Object, Object, ) AbstractLogger.warn(String, Object, Object, Object, Object, Object, Object, Object, ) AbstractLogger.warn(String, Object, Object, Object, Object, Object, Object, Object, Object, ) AbstractLogger.warn(String, Object, Object, Object, Object, Object, Object, Object, Object, Object, ) AbstractLogger.warn(String, Object, Object, Object, Object, Object, Object, Object, Object, Object, Object, ) ... ...

Xcheck 能够找到从风险函数到入口函数的调用链路，但是，在不做专用规则的前提下，无法生成与真实路径一致的结果。

在此类较为复杂的场景下，Xckeck可以做的是，根据Debug过程中的实际数据，实时生成可能的调用链路，从而辅助漏洞挖掘：

0x02 Spring Cloud Function

分析后，根据parseExpression函数得出可能的入口函数有 4 个：

FunctionDeployerConfiguration.functionArchiveUnDeployer(FunctionDeployerProperties, FunctionRegistry, ApplicationArguments, MavenProperties, ApplicationContext, ) HeaderEnricher.apply(Object, ) RoutingFunction.apply(Object, ) SimpleFunctionRegistry.FunctionInvocationWrapper.accept(Object, )

其中调试确认 RoutingFunction.apply(Object)是真实触发路径上的函数：

Xcheck 分析出的调用路径与实际结果吻合：

RoutingFunction.apply(Object) RoutingFunction.route(Object, String) RoutingFunction.functionFromExpression(String, Object, String) org.springframework.expression.spel.standard.SpelExpressionParser.parseExpression()

还缺少 doApply 到 post 的调用链条，是由于工具在处理上存在一点小的瑕疵，后续会补齐。

0x03 其它

尝试分析常见的 Java 框架和工具类代码，发现很多之前没有关注到的有意思代码，以下是一些归纳总结。

很多软件都会对配置文件中的内容调用表达式语言进行解析，例如：

Mybatis3 在处理某些标签时，会调用 OGNL 对标签属性进行解析

spring-cloud-stream-binder-rabbit 的部分 bindings 配置信息会被 SPEL 解析

如果你发现有写配置文件或者XML文件的漏洞，可以考虑上述特性的利用
反序列化还是广泛存在于各种软件中，如果有新的反序列化链条被发现，会有很大影响，例如：

spring-cloud-integration的session数据的持久化逻辑中
代码中的各种注解，很多都用到表达式语言模块进行解析，例如：

jeecg-boot、resilience4j、spring-batch

0x04 总结

目前主流 SAST 工具对于框架类代码和工具类代码，在不做专用规则的前提下，只能输出近似调用链条的结果
对于调用关系复杂场景（如 log4j2），输出的调用链条更多的意义在于辅助安全研究人员进行调试分析
对于调用关系简单的场景（如 spring cloud function），输出的调用链条比较接近于准确结果