TranslateProject/sources/tech/20150415 Strong SSL Security on nginx.md

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增强 nginx 的 SSL 安全性

本文向你介绍如何在 nginx 服务器上设置高安全性的 SSL。我们通过取消 SSL 压缩降低 CRIME 攻击威胁;禁用协议上存在安全缺陷的 SSLv3 及更低版本并设置更健壮的加密套件来尽可能启用正向保密Forward Secrecy我们也启用了 HSTS 和 HPKP。这样我们就拥有了一个健壮而可经受考验的 SSL 配置,并可以在 Qually Labs 的 SSL 测试中得到 A 级评分。

不求甚解的话,可以从 https://cipherli.st 上找到 nginx 、Apache 和 Lighttpd 的安全设置,复制粘帖即可。

本教程在 Digital Ocean 的 VPS 上测试通过。如果你喜欢这篇教程,想要支持作者的站点的话,购买 Digital Ocean 的 VPS 时请使用如下链接:https://www.digitalocean.com/?refcode=7435ae6b8212

本教程可以通过发布于 2014/1/21 的 SSL 实验室测试的严格要求(之前就通过了测试,如果你按照本文进行的话,可以得到一个 A+ 评分)。

你可以从下列链接中找到这方面的进一步内容:

我们需要编辑 nginx 的配置,在 Ubuntu/Debian 上是 /etc/nginx/sited-enabled/yoursite.com,在 RHEL/CentOS 上是 /etc/nginx/conf.d/nginx.conf

本文中我们需要编辑443端口SSLserver 配置中的部分。在文末我们会给出完整的配置例子。

在编辑之前切记备份一下配置文件!

The BEAST attack and RC4

In short, by tampering with an encryption algorithm's CBC - cipher block chaining - mode's, portions of the encrypted traffic can be secretly decrypted. More info on the above link.

Recent browser versions have enabled client side mitigation for the beast attack. The recommendation was to disable all TLS 1.0 ciphers and only offer RC4. However, [RC4 has a growing list of attacks against it],(http://www.isg.rhul.ac.uk/tls/) many of which have crossed the line from theoretical to practical. Moreover, there is reason to believe that the NSA has broken RC4, their so-called "big breakthrough."

Disabling RC4 has several ramifications. One, users with shitty browsers such as Internet Explorer on Windows XP will use 3DES in lieu. Triple-DES is more secure than RC4, but it is significantly more expensive. Your server will pay the cost for these users. Two, RC4 mitigates BEAST. Thus, disabling RC4 makes TLS 1.0 users susceptible to that attack, by moving them to AES-CBC (the usual server-side BEAST "fix" is to prioritize RC4 above all else). I am confident that the flaws in RC4 significantly outweigh the risks from BEAST. Indeed, with client-side mitigation (which Chrome and Firefox both provide), BEAST is a nonissue. But the risk from RC4 only grows: More cryptanalysis will surface over time.

Factoring RSA-EXPORT Keys (FREAK)

FREAK is a man-in-the-middle (MITM) vulnerability discovered by a group of cryptographers at INRIA, Microsoft Research and IMDEA. FREAK stands for "Factoring RSA-EXPORT Keys."

The vulnerability dates back to the 1990s, when the US government banned selling crypto software overseas, unless it used export cipher suites which involved encryption keys no longer than 512-bits.

It turns out that some modern TLS clients - including Apple's SecureTransport and OpenSSL - have a bug in them. This bug causes them to accept RSA export-grade keys even when the client didn't ask for export-grade RSA. The impact of this bug can be quite nasty: it admits a 'man in the middle' attack whereby an active attacker can force down the quality of a connection, provided that the client is vulnerable and the server supports export RSA.

There are two parts of the attack as the server must also accept "export grade RSA."

The MITM attack works as follows:

  • In the client's Hello message, it asks for a standard 'RSA' ciphersuite.
  • The MITM attacker changes this message to ask for 'export RSA'.
  • The server responds with a 512-bit export RSA key, signed with its long-term key.
  • The client accepts this weak key due to the OpenSSL/SecureTransport bug.
  • The attacker factors the RSA modulus to recover the corresponding RSA decryption key.
  • When the client encrypts the 'pre-master secret' to the server, the attacker can now decrypt it to recover the TLS 'master secret'.
  • From here on out, the attacker sees plaintext and can inject anything it wants.

The ciphersuite offered here on this page does not enable EXPORT grade ciphers. Make sure your OpenSSL is updated to the latest available version and urge your clients to also use upgraded software.

Heartbleed

Heartbleed is a security bug disclosed in April 2014 in the OpenSSL cryptography library, which is a widely used implementation of the Transport Layer Security (TLS) protocol. Heartbleed may be exploited regardless of whether the party using a vulnerable OpenSSL instance for TLS is a server or a client. It results from improper input validation (due to a missing bounds check) in the implementation of the DTLS heartbeat extension (RFC6520), thus the bug's name derives from "heartbeat". The vulnerability is classified as a buffer over-read, a situation where more data can be read than should be allowed.

What versions of the OpenSSL are affected by Heartbleed?

Status of different versions:

  • OpenSSL 1.0.1 through 1.0.1f (inclusive) are vulnerable
  • OpenSSL 1.0.1g is NOT vulnerable
  • OpenSSL 1.0.0 branch is NOT vulnerable
  • OpenSSL 0.9.8 branch is NOT vulnerable

The bug was introduced to OpenSSL in December 2011 and has been out in the wild since OpenSSL release 1.0.1 on 14th of March 2012. OpenSSL 1.0.1g released on 7th of April 2014 fixes the bug.

By updating OpenSSL you are not vulnerable to this bug.

SSL Compression (CRIME attack)

The CRIME attack uses SSL Compression to do its magic. SSL compression is turned off by default in nginx 1.1.6+/1.0.9+ (if OpenSSL 1.0.0+ used) and nginx 1.3.2+/1.2.2+ (if older versions of OpenSSL are used).

If you are using al earlier version of nginx or OpenSSL and your distro has not backported this option then you need to recompile OpenSSL without ZLIB support. This will disable the use of OpenSSL using the DEFLATE compression method. If you do this then you can still use regular HTML DEFLATE compression.

SSLv2 and SSLv3

SSL v2 is insecure, so we need to disable it. We also disable SSLv3, as TLS 1.0 suffers a downgrade attack, allowing an attacker to force a connection to use SSLv3 and therefore disable forward secrecy.

Again edit the config file:

ssl_protocols TLSv1 TLSv1.1 TLSv1.2;

Poodle and TLS-FALLBACK-SCSV

SSLv3 allows exploiting of the POODLE bug. This is one more major reason to disable this.

Google have proposed an extension to SSL/TLS named TLSFALLBACKSCSV that seeks to prevent forced SSL downgrades. This is automatically enabled if you upgrade OpenSSL to the following versions:

  • OpenSSL 1.0.1 has TLSFALLBACKSCSV in 1.0.1j and higher.
  • OpenSSL 1.0.0 has TLSFALLBACKSCSV in 1.0.0o and higher.
  • OpenSSL 0.9.8 has TLSFALLBACKSCSV in 0.9.8zc and higher.

More info on the NGINX documentation

The Cipher Suite

Forward Secrecy ensures the integrity of a session key in the event that a long-term key is compromised. PFS accomplishes this by enforcing the derivation of a new key for each and every session.

This means that when the private key gets compromised it cannot be used to decrypt recorded SSL traffic.

The cipher suites that provide Perfect Forward Secrecy are those that use an ephemeral form of the Diffie-Hellman key exchange. Their disadvantage is their overhead, which can be improved by using the elliptic curve variants.

The following two ciphersuites are recommended by me, and the latter by the Mozilla Foundation.

The recommended cipher suite:

ssl_ciphers 'AES128+EECDH:AES128+EDH';

The recommended cipher suite for backwards compatibility (IE6/WinXP):

ssl_ciphers "ECDHE-RSA-AES256-GCM-SHA384:ECDHE-RSA-AES128-GCM-SHA256:DHE-RSA-AES256-GCM-SHA384:DHE-RSA-AES128-GCM-SHA256:ECDHE-RSA-AES256-SHA384:ECDHE-RSA-AES128-SHA256:ECDHE-RSA-AES256-SHA:ECDHE-RSA-AES128-SHA:DHE-RSA-AES256-SHA256:DHE-RSA-AES128-SHA256:DHE-RSA-AES256-SHA:DHE-RSA-AES128-SHA:ECDHE-RSA-DES-CBC3-SHA:EDH-RSA-DES-CBC3-SHA:AES256-GCM-SHA384:AES128-GCM-SHA256:AES256-SHA256:AES128-SHA256:AES256-SHA:AES128-SHA:DES-CBC3-SHA:HIGH:!aNULL:!eNULL:!EXPORT:!DES:!MD5:!PSK:!RC4";

If your version of OpenSSL is old, unavailable ciphers will be discarded automatically. Always use the full ciphersuite above and let OpenSSL pick the ones it supports.

The ordering of a ciphersuite is very important because it decides which algorithms are going to be selected in priority. The recommendation above prioritizes algorithms that provide perfect forward secrecy.

Older versions of OpenSSL may not return the full list of algorithms. AES-GCM and some ECDHE are fairly recent, and not present on most versions of OpenSSL shipped with Ubuntu or RHEL.

Prioritization logic

  • ECDHE+AESGCM ciphers are selected first. These are TLS 1.2 ciphers and not widely supported at the moment. No known attack currently target these ciphers.
  • PFS ciphersuites are preferred, with ECDHE first, then DHE.
  • AES 128 is preferred to AES 256. There has been discussions on whether AES256 extra security was worth the cost, and the result is far from obvious. At the moment, AES128 is preferred, because it provides good security, is really fast, and seems to be more resistant to timing attacks.
  • In the backward compatible ciphersuite, AES is preferred to 3DES. BEAST attacks on AES are mitigated in TLS 1.1 and above, and difficult to achieve in TLS 1.0. In the non-backward compatible ciphersuite, 3DES is not present.
  • RC4 is removed entirely. 3DES is used for backward compatibility. See discussion in #RC4_weaknesses

Mandatory discards

  • aNULL contains non-authenticated Diffie-Hellman key exchanges, that are subject to Man-In-The-Middle (MITM) attacks
  • eNULL contains null-encryption ciphers (cleartext)
  • EXPORT are legacy weak ciphers that were marked as exportable by US law
  • RC4 contains ciphers that use the deprecated ARCFOUR algorithm
  • DES contains ciphers that use the deprecated Data Encryption Standard
  • SSLv2 contains all ciphers that were defined in the old version of the SSL standard, now deprecated
  • MD5 contains all the ciphers that use the deprecated message digest 5 as the hashing algorithm

Extra settings

Make sure you also add these lines:

ssl_prefer_server_ciphers on;
ssl_session_cache shared:SSL:10m;

When choosing a cipher during an SSLv3 or TLSv1 handshake, normally the client's preference is used. If this directive is enabled, the server's preference will be used instead.

Forward Secrecy & Diffie Hellman Ephemeral Parameters

The concept of forward secrecy is simple: client and server negotiate a key that never hits the wire, and is destroyed at the end of the session. The RSA private from the server is used to sign a Diffie-Hellman key exchange between the client and the server. The pre-master key obtained from the Diffie-Hellman handshake is then used for encryption. Since the pre-master key is specific to a connection between a client and a server, and used only for a limited amount of time, it is called Ephemeral.

With Forward Secrecy, if an attacker gets a hold of the server's private key, it will not be able to decrypt past communications. The private key is only used to sign the DH handshake, which does not reveal the pre-master key. Diffie-Hellman ensures that the pre-master keys never leave the client and the server, and cannot be intercepted by a MITM.

All versions of nginx as of 1.4.4 rely on OpenSSL for input parameters to Diffie-Hellman (DH). Unfortunately, this means that Ephemeral Diffie-Hellman (DHE) will use OpenSSL's defaults, which include a 1024-bit key for the key-exchange. Since we're using a 2048-bit certificate, DHE clients will use a weaker key-exchange than non-ephemeral DH clients.

We need generate a stronger DHE parameter:

cd /etc/ssl/certs
openssl dhparam -out dhparam.pem 4096

And then tell nginx to use it for DHE key-exchange:

ssl_dhparam /etc/ssl/certs/dhparam.pem;

OCSP Stapling

When connecting to a server, clients should verify the validity of the server certificate using either a Certificate Revocation List (CRL), or an Online Certificate Status Protocol (OCSP) record. The problem with CRL is that the lists have grown huge and takes forever to download.

OCSP is much more lightweight, as only one record is retrieved at a time. But the side effect is that OCSP requests must be made to a 3rd party OCSP responder when connecting to a server, which adds latency and potential failures. In fact, the OCSP responders operated by CAs are often so unreliable that browser will fail silently if no response is received in a timely manner. This reduces security, by allowing an attacker to DoS an OCSP responder to disable the validation.

The solution is to allow the server to send its cached OCSP record during the TLS handshake, therefore bypassing the OCSP responder. This mechanism saves a roundtrip between the client and the OCSP responder, and is called OCSP Stapling.

The server will send a cached OCSP response only if the client requests it, by announcing support for the status_request TLS extension in its CLIENT HELLO.

Most servers will cache OCSP response for up to 48 hours. At regular intervals, the server will connect to the OCSP responder of the CA to retrieve a fresh OCSP record. The location of the OCSP responder is taken from the Authority Information Access field of the signed certificate.

HTTP Strict Transport Security

When possible, you should enable HTTP Strict Transport Security (HSTS), which instructs browsers to communicate with your site only over HTTPS.

HTTP Public Key Pinning Extension

You should also enable the HTTP Public Key Pinning Extension.

Public Key Pinning means that a certificate chain must include a whitelisted public key. It ensures only whitelisted Certificate Authorities (CA) can sign certificates for *.example.com, and not any CA in your browser store.

I've written an article about it that has background theory and configuration examples for Apache, Lighttpd and NGINX: https://raymii.org/s/articles/HTTPPublicKeyPinningExtension_HPKP.html

Config Example

server {

  listen [::]:443 default_server;

  ssl on;
  ssl_certificate_key /etc/ssl/cert/raymii_org.pem;
  ssl_certificate /etc/ssl/cert/ca-bundle.pem;

  ssl_ciphers 'AES128+EECDH:AES128+EDH:!aNULL';

  ssl_protocols TLSv1 TLSv1.1 TLSv1.2;
  ssl_session_cache shared:SSL:10m;

  ssl_stapling on;
  ssl_stapling_verify on;
  resolver 8.8.4.4 8.8.8.8 valid=300s;
  resolver_timeout 10s;

  ssl_prefer_server_ciphers on;
  ssl_dhparam /etc/ssl/certs/dhparam.pem;

  add_header Strict-Transport-Security max-age=63072000;
  add_header X-Frame-Options DENY;
  add_header X-Content-Type-Options nosniff;

  root /var/www/;
  index index.html index.htm;
  server_name raymii.org;

}

Conclusion

If you have applied the above config lines you need to restart nginx:

# Check the config first:
/etc/init.d/nginx configtest
# Then restart:
/etc/init.d/nginx restart

Now use the SSL Labs test to see if you get a nice A. And, of course, have a safe, strong and future proof SSL configuration!


via: https://raymii.org/s/tutorials/Strong_SSL_Security_On_nginx.html

作者:Remy van Elst 译者:译者ID 校对:校对者ID

本文由 LCTT 原创翻译,Linux中国 荣誉推出