Date: Mon, 2 Jun 2014 13:13:56 -0700
From: David Fifield <david@...software.com>
To: fulldisclosure@...lists.org
Subject: [FD] GoAgent vulnerabilities: CA cert with known private key,
	TLS MITM

There is an HTML version of this document with screenshots at
https://www.bamsoftware.com/sec/goagent-advisory.html.

  * GoAgent installs a root CA certificate with a known private key
      * Test page
      * Mitigation
      * How to remove the GoAgent certificate
  * Improper TLS validation makes GoAgent susceptible to
    man-in-the-middle attacks
      * Mitigation

  * GoAgent 导入公开私钥根证书的问题
      * 测试页面
      * 如何防止风险
      * 如何删除 GoAgent CA 证书
  * GoAgent 没有进行正确的 TLS 验证，存在中间人攻击的风险
      * 如何防止风险

GoAgent (https://github.com/goagent/goagent) is a popular censorship
circumvention tool that uses Google App Engine as a proxy to bypass
China's Great Firewall. GoAgent has serious security flaws that put its
users at risk of man-in-the-middle attacks.

  * At startup, GoAgent installs trusted root "GoAgent CA" certificate
    whose private key is publicly known. With this private key, anyone
    may impersonate a certificate authority to attack GoAgent users. The
    risk of this flaw exists even when not using GoAgent, if GoAgent has
    been used any time in the past.
  * Improper TLS validation, and a lack of validation by default, mean
    that the HTTPS tunnel used by GoAgent can be subverted by an active
    attacker.

These flaws have been previously identified on GoAgent's issue tracker,
but are not yet widely disclosed, especially among the non-Chinese
community.

    https://code.google.com/p/goagent/issues/detail?id=11091
    http://translate.google.com/translate?hl=en&ie=UTF8&sl=zh-CN&tl=en&u=http://code.google.com/p/goagent/issues/detail%3Fid%3D11091

    https://code.google.com/p/goagent/issues/detail?id=8031
    http://translate.google.com/translate?hl=en&ie=UTF8&sl=zh-CN&tl=en&u=http://code.google.com/p/goagent/issues/detail%3Fid%3D8031


== GoAgent installs a root CA certificate with a known private key ==

At startup, GoAgent installs a system-wide root CA certificate with a
fixed and publicly known private key. Because the private key is known,
anyone can impersonate the "GoAgent CA" and sign certificates for almost
any web site. The trusted root CA certificate remains installed even
after GoAgent is turned off or removed. Depending on the circumstances
of GoAgent's installation, the certificate may also affect browsers
other than the one used with GoAgent, and other users of the same
computer.

The root CA certificate that GoAgent installs has the fingerprints:

    SHA1 Fingerprint=AB:70:2C:DF:18:EB:E8:B4:38:C5:28:69:CD:4A:5D:EF:48:B4:0E:33
    MD5 Fingerprint=56:B1:20:86:1B:0A:B0:61:38:00:1B:C3:67:CF:0C:CC

The file with "-----BEGIN RSA PRIVATE KEY-----" may be downloaded at
this URL, along with the certificate for "GoAgent CA":

    https://github.com/goagent/goagent/blob/c4386808ea943e2ebed25f1e5264943354e3f9cb/local/CA.crt

According to the version history, the key has been the same since June
2011 or earlier:

    https://github.com/goagent/goagent/blob/fa9959e577395e48a477fd5495afbc2363a51baa/local/CA.key

GoAgent has two parts: a local proxy program, proxy.py, that runs on a
user's computer, and a remote proxy program, gae.py, that runs on App
Engine.

    https://github.com/goagent/goagent/blob/c4386808ea943e2ebed25f1e5264943354e3f9cb/local/proxy.py
    https://github.com/goagent/goagent/blob/c4386808ea943e2ebed25f1e5264943354e3f9cb/server/gae/gae.py

Users upload a personal copy of gae.py to App Engine. The user's browser
talks to proxy.py on localhost, and proxy.py talks to gae.py on App
Engine.

When run, GoAgent attempts to install the GoAgent CA certificate. The
responsible function is CertUtil.import_ca:

    https://github.com/goagent/goagent/blob/c4386808ea943e2ebed25f1e5264943354e3f9cb/local/proxy.py#L337

The installation code is different for different operating systems, and
varies as to what browsers and users it affects. In some cases it
requires administrator privileges. On Windows, it calls the
CertAddEncodedCertificateToStore function. On OS X, it runs the command

    security find-certificate -a -c "GoAgent" | grep "GoAgent" >/dev/null || security add-trusted-cert -d -r trustRoot -k "/Library/Keychains/System.keychain" "<pwd>/CA.crt"

On Ubuntu, it copies the certificate file to
/usr/local/share/ca-certificates and calls update-ca-certificates. On
other varieties of GNU/Linux, it attempts to modify the NSS database
with the command

    certutil -L -d sql:$HOME/.pki/nssdb | grep "GoAgent" || certutil -d sql:$HOME/.pki/nssdb -A -t "C,," -n "GoAgent" -i "<pwd>/CA.crt"

The automatic installation is ineffective for Firefox, which has its own
certificate store. GoAgent's installation guide and FAQ show how to
import the certificate into Firefox manually.

    https://code.google.com/p/goagent/wiki/InstallGuide
    http://translate.google.com/translate?hl=en&ie=UTF8&sl=zh-CN&tl=en&u=http://code.google.com/p/goagent/wiki/InstallGuide

    https://code.google.com/p/goagent/wiki/FAQ
    http://translate.google.com/translate?hl=en&ie=UTF8&sl=zh-CN&tl=en&u=http://code.google.com/p/goagent/wiki/FAQ

The GoAgent CA certificate is used to do a local (intentional)
man-in-the-middle of HTTPS connections between the browser and proxy.py.
GoAgent works by encoding HTTP requests received by proxy.py and sending
them to gae.py, where gae.py makes the encoded request. gae.py then
encodes the HTTP response and sends it back to proxy.py, where it is
decoded and returned to the browser. In order for GoAgent to work with
HTTPS sites, it needs to undo the encryption so that gae.py will know
what URL to request. When proxy.py receives a CONNECT request (meaning
an HTTPS site is requested), it generates and serves a fake certificate
signed by the GoAgent CA. From the user's point of view, all HTTPS sites
are verified by "GoAgent". In some browsers, certificate pinning
prevents the GoAgent technique from working for a small number of sites.
(A consequence of GoAgent's model is that HTTPS is not end-to-end. It is
HTTPS between the user and App Engine, and HTTPS between App Engine and
the web site, but App Engine gets to see the plaintext.)


=== Test page ===

To test whether you are affected by the GoAgent CA vulnerability, access
this web page:

    https://goagent-cert-test.bamsoftware.com/ 

The web page uses a certificate signed by the GoAgent CA. If you are not
vulnerable, your browser will display a certificate warning. If you are
vulnerable, the web page will be shown with no warning.


=== Mitigation ===

GoAgent has the ability to regenerate CA.crt with a new, truly private
key. Delete the file local/CA.crt and start GoAgent again. The new
certificate and private key will be unique to your installation, and
only those with access to your computer will be able to attack your
HTTPS sessions.

This patch file removes the CA.crt file from a GoAgent Git repository.
Apply it with the command "git am 0001-Remove-static-CA.crt.patch". If
you have a copy of the code that is not a Git repository, delete the
file local/CA.crt manually.

    https://www.bamsoftware.com/sec/0001-Remove-static-CA.crt.patch

If you have ever run GoAgent in the past, it is important to also remove
the "GoAgent CA" certificate with SHA-1 fingerprint
AB:70:2C:DF:18:EB:E8:B4:38:C5:28:69:CD:4A:5D:EF:48:B4:0E:33 from
anywhere it might be installed. See the next section for how to do that
on selected operating systems.


=== How to remove the GoAgent certificate ===

In Firefox, open the preferences, then go to Advanced, Certificates,
View Certificates, Authorities, and select "GoAgent CA" in the list.
Click the "Delete or Distrust..." button, then click OK.

On Ubuntu, delete the file /usr/local/share/ca-certificates/GoAgent.crt
and run the command

    update-ca-certificates --fresh

On Windows, refer to these instructions:

    http://technet.microsoft.com/en-us/library/cc754841.aspx#BKMK_addlocal 

Follow the instructions under "Adding certificates to the Trusted Root
Certification Authorities store for a local computer", but at step 8,
right-click on "GoAgent CA" and select "Delete".

A screenshot of the Windows Trusted Root Certification Authorities store
with "GoAgent CA" selected

On Mac OS X, open the application called Keychain Access. Click the lock
to unlock it and enter your password. In the side panel, select "System"
and "Certificates". Highlight "GoAgent CA" and press the delete key on
the keyboard. Click the "Delete" button and enter your password again.


== Improper TLS validation makes GoAgent susceptible to man-in-the-middle attacks ==

By default, GoAgent establishes an HTTPS tunnel between the local
proxy.py and gae.py running on App Engine. (In the default proxy.ini
configuration file, the default setting is gae.mode=https.) However by
default, it does not validate the server's certificate (gae.validate=0),
making possible an HTTPS man-in-the-middle attack between GoAgent and
App Engine. Further, the gae.validate option also controls whether
validation is done between App Engine and the remote web server, so that
is disabled by default as well, enabling HTTPS man-in-the-middle between
App Engine and the destination web site.

Even when validation is enabled (gae.validate=1), the validation works
differently than usual: It does not match against the remote hostname,
but rather checks that the certificate's organizationName begins with
"Google ".

    https://github.com/goagent/goagent/blob/c4386808ea943e2ebed25f1e5264943354e3f9cb/local/proxy.py#L1623

Setting gae.validate=1 causes gae.py to validate its HTTP transactions.
Except that validation is not enabled by default, there doesn't appear
to be a problem with the validation in gae.py.

    https://github.com/goagent/goagent/blob/c4386808ea943e2ebed25f1e5264943354e3f9cb/server/gae/gae.py#L184

GoAgent optionally obfuscates its encoded HTTP requests and responses by
encrypting them with RC4 and a password-derived key. The obfuscation
option is enabled by setting gae.password and gae.options=rc4 in the
proxy.ini configuration file, as well as the __password__ variable in
gae.py.

    https://github.com/goagent/goagent/blob/c4386808ea943e2ebed25f1e5264943354e3f9cb/server/gae/gae.py#L5

However, the RC4 encryption is intended only for obfuscation, and does
not mitigate a man-in-the-middle attack. It offers no confidentiality,
because the password is sent along with each ciphertext in a G-password
header. Also, the same keystream is reused for all messages in both
directions, making it possible to XOR ciphertexts and get the XOR of
plaintexts. The RC4 feature prevents other users from trivially using up
your App Engine bandwidth quota, but doesn't provide additional
protection against a network adversary.


=== Mitigation ===

In the proxy.ini configuration file, make sure to set gae.mode=https
(the default) and gae.validate=1 (not the default). These settings
enable validation both between GoAgent and App Engine, and between App
Engine and the remote web site. There remains a risk of
man-in-the-middle by someone able to get a valid certificate with
"Google " in the organizationName.


------------------------------------------------------------------------


GoAgent (https://github.com/goagent/goagent) 利用 Google App Engine
(GAE) 来绕过 GFW 的封锁，以免费、快速、稳定的特点深受网民的欢迎。但
是，GoAgent 的安装和配置中存在两点严重安全风险的问题却鲜为人知。这两点安
全风险都可能被攻击者利用进行 “中间人攻击（man-in-the-middle attack）” 来
窃取 GoAgent 用户的网络帐号密码等敏感信息，其概括描述如下：

  * GoAgent 在启动时会尝试自动往系统的可信根证书中导入一个名为 “GoAgent
    CA” 的证书。由于这个证书的私钥是公开的，导致任何人都可以利用这个私钥
    来伪造任意网站的证书进行 HTTPS 中间人攻击。即使在不开启 GoAgent 时，
    这种攻击的风险仍然存在。换而言之，一旦这个证书被导入，攻击者可以用此
    绕过几乎所有网站的 HTTPS 保护。
  * GoAgent 本身对 TLS 证书的认证存在问题，而且默认时不对证书进行检查，
    这导致在使用 GoAgent 时存在 HTTPS 中间人攻击的风险。

事实上曾经有用户在 GoAgent 主页上的问题跟踪列表中指出了这两个安全问题
（见以下链接），但既没有修复也没有广泛公开，多数用户，尤其是非中文用户可
能并不知情。下面是这两个问题的详细解释。

    https://code.google.com/p/goagent/issues/detail?id=11091
    https://code.google.com/p/goagent/issues/detail?id=8031 

== GoAgent 导入公开私钥根证书的问题 ==

GoAgent 在启动时会尝试在系统中导入一个根证书来避免访问 HTTPS 网站时的证
书报警，但在默认情况下所导入证书的私钥是公开的。因为私钥公开，任何人可以
作为 “GoAgent CA” 来签发任何网站的证书。即使在 GoAgent 没有启动甚至卸载
的情况下，这个公钥仍会遗留在系统中。在有些系统中，GoAgent 所导入的根证书
不仅被 GoAgent 默认使用的浏览器信任，其他的浏览器也可能会信任这一根证
书，从而受到这一问题的影响。

GoAgent 所导入的这一公开私钥根证书的指纹是：

    SHA1 Fingerprint=AB:70:2C:DF:18:EB:E8:B4:38:C5:28:69:CD:4A:5D:EF:48:B4:0E:33
    MD5 Fingerprint=56:B1:20:86:1B:0A:B0:61:38:00:1B:C3:67:CF:0C:CC

包含这一 “GoAgent CA” 证书以其私钥（文件中 “-----BEGIN RSA PRIVATE
KEY-----” 位置）的文件 URL 为：

    https://github.com/goagent/goagent/blob/c4386808ea943e2ebed25f1e5264943354e3f9cb/local/CA.crt

根据版本信息，这一证书和私钥从 2011 年 6 月甚至更早的时间以来一直保持不变。

    https://github.com/goagent/goagent/blob/fa9959e577395e48a477fd5495afbc2363a51baa/local/CA.key

GoAgent 主要包含两个部分：一个在用户计算机上运行的本地代理程序
proxy.py，以及一个在 GAE 上运行的远程代理程序 gae.py。

    https://github.com/goagent/goagent/blob/c4386808ea943e2ebed25f1e5264943354e3f9cb/local/proxy.py
    https://github.com/goagent/goagent/blob/c4386808ea943e2ebed25f1e5264943354e3f9cb/server/gae/gae.py

安装时，用户需要上传 gae.py 到 GAE。用户浏览器通过设置一个本地代理将
HTTP/HTTPS 请求转发到 proxy.py，再由 proxy.py 和 gae.py 进行通信。

默认情况下，GoAgent 在启动时试图导入上述 GoAgent CA 证书。具体的代码为
proxy.py 中的 CertUtil.import_ca：

    https://github.com/goagent/goagent/blob/c4386808ea943e2ebed25f1e5264943354e3f9cb/local/proxy.py#L337

这个函数会根据用户操作系统通过不同的方式尝试导入证书，在某些情况下会需要
管理员 （root/administrator）权限。在 Windows 下，这个函数会调用
CertAddEncodedCertificateToStore 这一 API。在 OS X 下，会尝试执行系统命令

    security find-certificate -a -c "GoAgent" | grep "GoAgent" >/dev/null || security add-trusted-cert -d -r trustRoot -k "/Library/Keychains/System.keychain" "<pwd>/CA.crt"

在 Ubuntu 下，会拷贝证书文件到 /usr/local/share/ca-certificates 然后执
行 update-ca-certificates。在其他 GNU/Linux 发行版中，会尝试执行以下命
令更改 NSS 数据库：

    certutil -L -d sql:$HOME/.pki/nssdb | grep "GoAgent" || certutil -d sql:$HOME/.pki/nssdb -A -t "C,," -n "GoAgent" -i "<pwd>/CA.crt"

由于 Firefox 采用了不同的方式存储证书，这一自动安装过程不会导入 GoAgent
CA 证书到 Firefox 中。但是 GoAgent 的安装指南和 FAQ 中说明了如何手动导入
这一证书：

    https://code.google.com/p/goagent/wiki/InstallGuide
    https://code.google.com/p/goagent/wiki/FAQ 

这一证书随后被 proxy.py 用来作为 HTTPS 中间人来避免浏览器在访问 HTTPS
网站时出现报警。GoAgent 的工作原理如下：首先 proxy.py 将浏览器的 HTTP
请求进行编码并转发给 gae.py，gae.py 完成收到的请求然后将结果进行编码
后返回给 proxy.py，最后 proxy.py 将结果转发给浏览器来完成 “翻墙” 过
程。由于 GAE 的限制 （免费 app 无法使用 socket 接口），对于 HTTPS 请
求，proxy.py 无法进行透明转发，只能作为中间人先和浏览器完成连接，然后
获得其中的明文请求以后在转发给 gae.py。当收到 CONNECT 请求（这意味着
浏览器正在浏览一个 HTTPS 网站）， proxy.py 首先利用 GoAgent CA 签发一
个假的证书来和浏览器完成握手，从用户的角度，所有的 HTTPS 网站的证书都是
由事先导入的 “GoAgent CA” 认证的，所以不会报警。有些浏览器会对少数网站的
证书进行特别的检查（Certificate Pinning），这种情况下 "GoAgent CA” 所签
发的证书可能会触发证书不安全的报警。GoAgent 的这种工作方式导致 HTTPS 不
再是浏览器到网站的端到端安全通信，而变成了 proxy.py 到 GAE，以及 GAE
到网站两段独立的 HTTPS 连接，GAE 能够看到请求和应答的明文。


=== 测试页面===

请访问

    https://goagent-cert-test.bamsoftware.com/ 

来进行测试。这个页面使用了一个由 GoAgent CA 签发的证书。如果你的浏览器没
有受到影响，会显示报警信息；如果没有看到报警，则表明你的浏览器导入了公开
的 GoAgent CA 证书，存在严重安全风险。


=== 如何防止风险 ===

GoAgent 本身带有生成证书文件 CA.crt 的功能。只需要删除 local/CA.crt 文件
就能保证 GoAgent 所导入的证书是唯一的，不会被网络上的攻击者利用来进行攻击。

下面的 “补丁（patch）” 文件会帮助你从 GoAgent 的 git 仓库中删除 CA.crt
文件，请下载补丁文件并执行以下命令：
git am 0001-Remove-static-CA.crt.patch. 如果你不是通过 git 获得
GoAgent（例如直接从 http://code.google.com/p/goagent/ 上的链接下载得到）
，请手动删除 local/CA.crt 文件。

    https://www.bamsoftware.com/sec/0001-Remove-static-CA.crt.patch

如果你以前曾经使用过 GoAgent，务必要检查系统中任何可能的地方，删除 SHA-1
指纹为 AB:70:2C:DF:18:EB:E8:B4:38:C5:28:69:CD:4A:5D:EF:48:B4:0E:33 的
“GoAgent CA” 证书（建议使用浏览器访问上面的测试页面进行检查）。下面说明
在常见系统中检查和删除 GoAgent CA 证书的方法。


=== 如何删除 GoAgent CA 证书 ===

Firefox 中，打开 “preferences”，”Advanced”, “Certificates”, “View
Certificates”, “Authorities”，然后在证书列表中找到 “GoAgent CA”，选中并
点击 “Delete or Distrust...” 按钮，然后确认。

Ubuntu 下，删除 /usr/local/share/ca-certificates/GoAgent.crt 然后执行

    update-ca-certificates --fresh

Windows 下，请参考以下链接：

    http://technet.microsoft.com/en-us/library/cc754841.aspx#BKMK_addlocal 

中 "Adding certificates to the Trusted Root Certification Authorities
store for a local computer" 的步骤，但是在 step 8 时右键选中 "GoAgent
CA" 然后选择 "Delete"。

在 Mac OS X下，打开 “Keychain Access” 应用，点击锁图标并输入密码解锁。在
边上的控制面板中，选择 "System" 以及 "Certificates"，选中 "GoAgent CA"
然后按 “Delete” 键，点击 "Delete" 按钮并输入你的密码确认。


== GoAgent 没有进行正确的 TLS 验证，存在中间人攻击的风险 ==

默认情况下，GoAgent 会通过 HTTPS 来保护本地 proxy.py 和 GAE 服务器上
的 gae.py 之间的通信 （在配置文件 proxy.ini 中相关的设置默认为
gae.mode=https）。但是同样在默认情况下，GoAgent 不会要求对 GAE 服务器
的证书进行验证（gae.validate=0），这导致本地 proxy.py 和 App Engine
服务器之间的通信存在 HTTPS 中间人攻击的风险。此外， gae.validate 配置
项同样控制 App Engine 上的 gae.py 是否对网站服务器的证书进行验证，默认
配置下这一配置为 0 导致 gae.py 也不会对网站证书进行验证，使得 gae.py
和网站之间的通信同样存在 HTTPS 中间人攻击的风险

即使修改配置启用证书验证（gae.validate=1），GoAgent 对 App Engine 服务
器证书的验证也并不严格：在 proxy.py 中只是对证书的 organizationName 进
行了粗略的检查（是否为 “Google ” 开头），而没有对主机名（hostname）进行
匹配。

    https://github.com/goagent/goagent/blob/c4386808ea943e2ebed25f1e5264943354e3f9cb/local/proxy.py#L1623

将配置改为 gae.validate=1 同时会启用 gae.py 段对网站服务器证书的验
证，这部分的事先没有明显的问题。

    https://github.com/goagent/goagent/blob/c4386808ea943e2ebed25f1e5264943354e3f9cb/server/gae/gae.py#L184

GoAgent 还提供了一个可选功能，通过 RC4 和一个共享密钥来对 proxy.py 和
gae.py 之间的数据进行进一步的混淆。启用这一功能需要在 proxy.ini 中设置
gae.password，以及 gae.options=rc4，并在 gae.py 中设置
__password__ 变量。

    https://github.com/goagent/goagent/blob/c4386808ea943e2ebed25f1e5264943354e3f9cb/server/gae/gae.py#L5

但是，这里的 RC4 加密只能起到一个简单的混淆作用，无法在不启用 HTTPS 的情
况下利用这一功能来防止中间人攻击。GoAgent 中的 RC4 无法实现数据的机密
性，因为密码本身会通过一个 G-password 头在 proxy.py 和 gae.py 中传
送，而且在两段通信中会使用同样的密码流（keystream），导致攻击者很容易通
过密文的 XOR 操作来获得 XOR 过的明文（见流密码的重用问题，Stream Cipher
Key Reuse），进而得到明文。在这里 RC4 只能起到防止其他 GoAgent 用户共享
服务端流量的目的，无法提供更多的保护来防止网络攻击。

=== 如何防止风险 ===

确认在 proxy.ini 中设置了 gae.mode=https （默认），并且启用了证书验
证 gae.validate=1（非默认）。这样的设定基本上能够防止 proxy.py 和
GAE 服务器，以及 GAE 服务器和网站服务器之间的 HTTPS 中间人攻击。由于
proxy.py 中没有对证书的主机名进行严格匹配，proxy.py 和 GAE 服务器 的
通信仍存在（相对较小的）风险，如果有人能够申请到 organizationName 字段以
“Google ” 开头的证书，仍然能够成功进行HTTPS 中间人攻击。

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