Transport Layer Protection Cheat Sheet
Last revision (mm/dd/yy): 02/7/2017
This cheat sheet provides a simple model to follow when implementing transport layer protection for an application. Although the concept of SSL is known to many, the actual details and security specific decisions of implementation are often poorly understood and frequently result in insecure deployments. This article establishes clear rules which provide guidance on securely designing and configuring transport layer security for an application. This article is focused on the use of SSL/TLS between a web application and a web browser, but we also encourage the use of SSL/TLS or other network encryption technologies, such as VPN, on back end and other non-browser based connections.
An architectural decision must be made to determine the appropriate method to protect data when it is being transmitted. The most common options available to corporations are Virtual Private Networks (VPN) or a SSL/TLS model commonly used by web applications. The selected model is determined by the business needs of the particular organization. For example, a VPN connection may be the best design for a partnership between two companies that includes mutual access to a shared server over a variety of protocols. Conversely, an Internet facing enterprise web application would likely be best served by a SSL/TLS model.
TLS is mainly a defence against man-in-the-middle attacks. An TLS Threat Model is one that starts with the question "What is the business impact of an attacker's ability to observe, intercept and manipulate the traffic between the client and the server".
This cheat sheet will focus on security considerations when the SSL/TLS model is selected. This is a frequently used model for publicly accessible web applications.
Providing Transport Layer Protection with SSL/TLS
The primary benefit of transport layer security is the protection of web application data from unauthorized disclosure and modification when it is transmitted between clients (web browsers) and the web application server, and between the web application server and back end and other non-browser based enterprise components.
The server validation component of TLS provides authentication of the server to the client. If configured to require client side certificates, TLS can also play a role in client authentication to the server. However, in practice client side certificates are not often used in lieu of username and password based authentication models for clients.
TLS also provides two additional benefits that are commonly overlooked; integrity guarantees and replay prevention. A TLS stream of communication contains built-in controls to prevent tampering with any portion of the encrypted data. In addition, controls are also built-in to prevent a captured stream of TLS data from being replayed at a later time.
It should be noted that TLS provides the above guarantees to data during transmission. TLS does not offer any of these security benefits to data that is at rest. Therefore appropriate security controls must be added to protect data while at rest within the application or within data stores.
The basic requirements for using TLS are: access to a Public Key Infrastructure (PKI) in order to obtain certificates, access to a directory or an Online Certificate Status Protocol (OCSP) responder in order to check certificate revocation status, and agreement/ability to support a minimum configuration of protocol versions and protocol options for each version.
SSL vs. TLS
The terms, Secure Socket Layer (SSL) and Transport Layer Security (TLS) are often used interchangeably. In fact, SSL v3.1 is equivalent to TLS v1.0. However, different versions of SSL and TLS are supported by modern web browsers and by most modern web frameworks and platforms. For the purposes of this cheat sheet we will refer to the technology generically as TLS. Recommendations regarding the use of SSL and TLS protocols, as well as browser support for TLS, can be found in the rule below titled "Only Support Strong Protocols".
When to Use a FIPS 140-2 Validated Cryptomodule
If the web application may be the target of determined attackers (a common threat model for Internet accessible applications handling sensitive data), it is strongly advised to use TLS services that are provided by FIPS 140-2 validated cryptomodules.
A cryptomodule, whether it is a software library or a hardware device, basically consists of three parts:
The security of a cryptomodule and its services (and the web applications that call the cryptomodule) depend on the correct implementation and integration of each of these three parts. In addition, the cryptomodule must be used and accessed securely. The includes consideration for:
In order to leverage the benefits of TLS it is important to use a TLS service (e.g. library, web framework, web application server) which has been FIPS 140-2 validated. In addition, the cryptomodule must be installed, configured and operated in either an approved or an allowed mode to provide a high degree of certainty that the FIPS 140-2 validated cryptomodule is providing the expected security services in the expected manner.
If the system is legally required to use FIPS 140-2 encryption (e.g., owned or operated by or on behalf of the U.S. Government) then TLS must be used and SSL disabled. Details on why SSL is unacceptable are described in Section 7.1 of Implementation Guidance for FIPS PUB 140-2 and the Cryptographic Module Validation Program.
Further reading on the use of TLS to protect highly sensitive data against determined attackers can be viewed in SP800-52 Guidelines for the Selection and Use of Transport Layer Security (TLS) Implementations
Secure Server Design
Rule - Use TLS or Other Strong Transport Everywhere
All networks, both external and internal, must utilize TLS or an equivalent transport layer security mechanism for all communication. It is not sufficient to claim that access to the internal network is "restricted to employees". Numerous recent data compromises have shown that the internal network can be breached by attackers. In these attacks, sniffers have been installed to access unencrypted sensitive data sent on the internal network.
The login page and all subsequent authenticated pages must be exclusively accessed over TLS. The initial login page, referred to as the "login landing page", must be served over TLS. Failure to utilize TLS for the login landing page allows an attacker to modify the login form action, causing the user's credentials to be posted to an arbitrary location. Failure to utilize TLS for authenticated pages after the login enables an attacker to view the unencrypted session ID and compromise the user's authenticated session.
Even marketing or other low-security websites still require TLS. Lack of TLS leads to a lack of integrity which allows attackers to modify content in transit. Also, sites that do not provide TLS are marked lower in pagerank for SEO.
Rule - Do Not Provide Non-TLS Pages for Secure Content
All pages which are available over TLS must not be available over a non-TLS connection. A user may inadvertently bookmark or manually type a URL to a HTTP page (e.g. http://example.com/myaccount) within the authenticated portion of the application. If this request is processed by the application then the response, and any sensitive data, would be returned to the user over the clear text HTTP.
Rule - Do Not Mix TLS and Non-TLS Content
A page that is available over TLS must be comprised completely of content which is transmitted over TLS. The page must not contain any content that is transmitted over unencrypted HTTP. This includes content from unrelated third party sites.
An attacker could intercept any of the data transmitted over the unencrypted HTTP and inject malicious content into the user's page. This malicious content would be included in the page even if the overall page is served over TLS. In addition, an attacker could steal the user's session cookie that is transmitted with any non-TLS requests. This is possible if the cookie's 'secure' flag is not set. See the rule 'Use "Secure" Cookie Flag'
Rule - Use "Secure" Cookie Flag
The "Secure" flag must be set for all user cookies. Failure to use the "secure" flag enables an attacker to access the session cookie by tricking the user's browser into submitting a request to an unencrypted page on the site. This attack is possible even if the server is not configured to offer HTTP content since the attacker is monitoring the requests and does not care if the server responds with a 404 or doesn't respond at all.
Rule - Keep Sensitive Data Out of the URL
Sensitive data must not be transmitted via URL arguments. A more appropriate place is to store sensitive data in a server side repository or within the user's session. When using TLS the URL arguments and values are encrypted during transit. However, there are two methods that the URL arguments and values could be exposed.
1. The entire URL is cached within the local user's browser history. This may expose sensitive data to any other user of the workstation.
2. The entire URL is exposed if the user clicks on a link to another HTTPS site. This may expose sensitive data within the referral field to the third party site. This exposure occurs in most browsers and will only occur on transitions between two TLS sites.
For example, a user following a link on https://example.com which leads to https://someOtherexample.com would expose the full URL of https://example.com (including URL arguments) in the referral header (within most browsers). This would not be the case if the user followed a link on https://example.com to http://someHTTPexample.com
Rule - Prevent Caching of Sensitive Data
The TLS protocol provides confidentiality only for data in transit but it does not help with potential data leakage issues at the client or intermediary proxies. As a result, it is frequently prudent to instruct these nodes not to cache or persist sensitive data. One option is to add anticaching headers to relevant HTTP responses, (for example, "Cache-Control: no-cache, no-store" and "Expires: 0" for coverage of many modern browsers as of 2013). For compatibility with HTTP/1.0 (i.e., when user agents are really old or the webserver works around quirks by forcing HTTP/1.0) the response should also include the header "Pragma: no-cache". More information is available in HTTP 1.1 RFC 2616, section 14.9.
Rule - Use HTTP Strict Transport Security
Rule - Use Public Key Pinning
Note: If using a FIPS 140-2 cryptomodule disregard the following rules and defer to the recommended configuration for the particular cryptomodule. Nevertheless we recommend to use this rules to audit your configuration.
Rule - Use Strong Keys & Protect Them
The private key used to generate the cipher key must be sufficiently strong for the anticipated lifetime of the private key and corresponding certificate. The current best practice is to select a key size of at least 2048 bits. Additional information on key lifetimes and comparable key strengths can be found in , NIST SP 800-57. In addition, the private key must be stored in a location that is protected from unauthorized access.
Rule - Use a Certificate That Supports Required Domain Names
A user should never be presented with a certificate error, including prompts to reconcile domain or hostname mismatches, or expired certificates. If the application is available at both https://www.example.com and https://example.com then an appropriate certificate, or certificates, must be presented to accommodate the situation. The presence of certificate errors desensitizes users to TLS error messages and increases the possibility an attacker could launch a convincing phishing or man-in-the-middle attack.
For example, consider a web application accessible at https://abc.example.com and https://xyz.example.com. One certificate should be acquired for the host or server abc.example.com; and a second certificate for host or server xyz.example.com. In both cases, the hostname would be present in the Subject's Common Name (CN).
Alternatively, the Subject Alternate Names (SANs) can be used to provide a specific listing of multiple names where the certificate is valid. In the example above, the certificate could list the Subject's CN as example.com, and list two SANs: abc.example.com and xyz.example.com. These certificates are sometimes referred to as "multiple domain certificates".
Rule - Use Fully Qualified Names in Certificates
Use fully qualified names in the DNS name field, and do not use unqualifed names (e.g., 'www'), local names (e.g., 'localhost'), or private IP addresses (e.g., 192.168.1.1) in the DNS name field. Unqualifed names, local names, or private IP addresses violate the certificate specification.
Rule - Do Not Use Wildcard Certificates
You should refrain from using wildcard certificates. Though they are expedient at circumventing annoying user prompts, they also violate the principal of least privilege and asks the user to trust all machines, including developer's machines, the secretary's machine in the lobby and the sign-in kiosk. Obtaining access to the private key is left as an exercise for the attacker, but its made much easier when stored on the file system unprotected.
Statistics gathered by Qualys for Internet SSL Survey 2010 indicate wildcard certificates have a 4.4% share, so the practice is not standard for public facing hosts. Finally, wildcard certificates violate EV Certificate Guidelines.
Rule - Do Not Use RFC 1918 Addresses in Certificates
Certificates should not use private addresses. RFC 1918 is Address Allocation for Private Internets. Private addresses are Internet Assigned Numbers Authority (IANA) reserved and include 192.168/16, 172.16/12, and 10/8.
Certificates issued with private addresses violate EV Certificate Guidelines. In addition, Peter Gutmann writes in in Engineering Security: "This one is particularly troublesome because, in combination with the router-compromise attacks... and ...OSCP-defeating measures, it allows an attacker to spoof any EV-certificate site."
Rule - Use an Appropriate Certification Authority for the Application's User Base
An application user must never be presented with a warning that the certificate was signed by an unknown or untrusted certificate authority (CA). The application's user population must have access to the public certificate of the CA which issued the server's certificate. For Internet accessible websites, the most effective method of achieving this goal is to purchase the TLS certificate from a recognized CA. Popular browsers already contain the public certificates of these recognized CAs.
Internal applications with a limited user population can use an internal CA provided its public certificate is securely distributed to all users. However, remember that all certificates issued by such CAs will be trusted by the users. Therefore, utilize controls to protect the private key and ensure that only authorized individuals have the ability to sign certificates.
The use of self-signed certificates is never acceptable. Self-signed certificates negate the benefit of end-point authentication and also significantly decrease the ability for an individual to detect a man-in-the-middle attack.
Rule - Always Provide All Needed Certificates
Clients attempt to solve the problem of identifying a server or host using PKI and X509 certificate. When a user receives a server or host's certificate, the certificate must be validated back to a trusted root certification authority. This is known as path validation.
There can be one or more intermediate certificates in between the end-entity (server or host) certificate and root certificate. In addition to validating both endpoints, the user will also have to validate all intermediate certificates. Validating all intermediate certificates can be tricky because the user may not have them locally. This is a well-known PKI issue called the “Which Directory?" problem.
To avoid the “Which Directory?" problem, a server should provide the user with all required certificates used in a path validation.
Rule - Be aware of and have a plan for the SHA-1 deprecation plan
In order to avoid presenting end users with progressive certificate warnings, organizations must proactively address the browser vendor's upcoming SHA-1 deprecation plans. The Google Chrome plan is probably the most specific and aggressive at this point: Gradually sunsetting SHA-1
If your organization has no SHA256 compatibility issues then it may be appropriate to move your site to a SHA256 signed certificate/chain. If there are, or may be, issues - you should ensure that your SHA-1 certificates expire before 1/1/2017.
Following vendors have rolled-out timelines for SHA-1 deprecation:
Server Protocol and Cipher Configuration
Note: If using a FIPS 140-2 cryptomodule disregard the following rules and defer to the recommended configuration for the particular cryptomodule. Nevertheless we recommend to use this rules to audit your configuration.
Rule - Only Support Strong Protocols
SSL/TLS is a collection of protocols. Weaknesses have been identified with earlier SSL protocols, including SSLv2 and SSLv3, hence SSL versions 1, 2, and 3 should not longer be used. The best practice for transport layer protection is to only provide support for the TLS protocols - TLS 1.0, TLS 1.1 and TLS 1.2. This configuration will provide maximum protection against skilled and determined attackers and is appropriate for applications handling sensitive data or performing critical operations.
Nearly all modern browsers support at least TLS 1.0. As of February 2014, contemporary browsers (Chrome v20+, Firefox v27+, IE v8+, Opera v10+, and Safari v5+) support TLS 1.1 and TLS 1.2. You should provide support for TLS 1.1 and TLS 1.2 to accommodate clients that support these protocols. The client and server (usually) negotiate the best protocol that is supported on both sides.
TLS 1.0 is still widely used as the 'best' protocol by a lot of browsers that are not patched to the very latest version. It suffers from CBC Chaining attacks and Padding Oracle attacks. TLSv1.0 should only be used after risk analysis and acceptance. PCI DSS 3.2 prohibits the use of TLS 1.0 after June 30, 2018 (earlier requirements to do so by June 30, 2016 were postponed).
Support the extension 'TLS_FALLBACK_SCSV' to prevent all unnecessary fallbacks from a higher to a lower protocol version, see RFC7507.
Under no circumstances either SSLv2 or SSLv3 should be enabled as a protocol selection:
Rule - Prefer Ephemeral Key Exchanges
Ephemeral key exchanges are based on Diffie-Hellman and use per-session, temporary keys during the initial SSL/TLS handshake. They provide perfect forward secrecy (PFS), which means a compromise of the server's long term signing key does not compromise the confidentiality of past session (see following rule). When the server uses an ephemeral key, the server will sign the temporary key with its long term key (the long term key is the customary key available in its certificate).
Use cryptographic parameters (like DH-parameter) that use a secure length that match to the supported keylength of your certificate (>=2048 bits or equivalent Elliptic Curves). As some middleware had some issues with this, upgrade to the latest version. Note: There are some legacy browsers or old Java versions that are not capable to cope with DH-Params >1024 bits, please read the following rule how this can be solved.
Do *not* use standardized DH-parameters like they are defined by RFCs 2409, 3526, or 5114. Generate your individual DH-parameters to get unique prime numbers (this may take a long time):
Set the path to use this parameter file, e.g. when using Apache:
If you have a server farm and are providing forward secrecy, then you might have to disable session resumption. For example, Apache writes the session id's and master secrets to disk so all servers in the farm can participate in resuming a session (there is currently no in-memory mechanism to achieve the sharing). Writing the session id and master secret to disk undermines forward secrecy.
Rule - Only Support Strong Cryptographic Ciphers
Each protocol (TLSv1.0, TLSv1.1, TLSv1.2, etc) provides cipher suites. As of TLS 1.2, there is support for over 300 suites (320+ and counting), including national vanity cipher suites. The strength of the encryption used within a TLS session is determined by the encryption cipher negotiated between the server and the browser. In order to ensure that only strong cryptographic ciphers are selected the server must be modified to disable the use of weak ciphers and to configure the ciphers in an adequate order. It is recommended to configure the server to only support strong ciphers and to use sufficiently large key sizes. In general, the following should be observed when selecting CipherSuites:
Additional information can be obtained within the TLS 1.2 RFC 5246, SSL Labs: 'SSL/TLS Deployment Best Practices', BSI: 'TR-02102 Part 2 (German)', ENISA: 'Algorithms, Key Sizes and Parameters Report', RFC 7525: Recommendations for Secure Use of Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS) and FIPS 140-2 IG.
Rule - Support TLS-PSK and TLS-SRP for Mutual Authentication
When using a shared secret or password offer TLS-PSK (Pre-Shared Key) or TLS-SRP (Secure Remote Password), which are known as Password Authenticated Key Exchange (PAKEs). TLS-PSK and TLS-SRP properly bind the channel, which refers to the cryptographic binding between the outer tunnel and the inner authentication protocol. IANA currently reserves 79 PSK cipehr suites and 9 SRP cipher suites.
Basic authentication places the user's password on the wire in the plain text after a server authenticates itself. Basic authentication only provides unilateral authentication. In contrast, both TLS-PSK and TLS-SRP provide mutual authentication, meaning each party proves it knows the password without placing the password on the wire in the plain text.
Finally, using a PAKE removes the need to trust an outside party, such as a Certification Authority (CA).
Rule - Only Support Secure Renegotiations
A design weakness in TLS, identified as CVE-2009-3555, allows an attacker to inject a plaintext of his choice into a TLS session of a victim. In the HTTPS context the attacker might be able to inject his own HTTP requests on behalf of the victim. The issue can be mitigated either by disabling support for TLS renegotiations or by supporting only renegotiations compliant with RFC 5746. All modern browsers have been updated to comply with this RFC.
Rule - Disable Compression
Compression Ratio Info-leak Made Easy (CRIME) is an exploit against the data compression scheme used by the TLS and SPDY protocols. The exploit allows an adversary to recover user authentication cookies from HTTPS. The recovered cookie can be subsequently used for session hijacking attacks.
Rule - Update your Crypto Libraries
Test your overall TLS/SSL setup and your Certificate
This section shows the most common references only. For more tools and such, please refer to Tools.
Client (Browser) Configuration
The validation procedures to ensure that a certificate is valid are complex and difficult to correctly perform. In a typical web application model, these checks will be performed by the client's web browser in accordance with local browser settings and are out of the control of the application. However, these items do need to be addressed in the following scenarios:
In these situations extensive certificate validation checks must occur in order to establish the validity of the certificate. Consult the following resources to assist in the design and testing of this functionality. The NIST PKI testing site includes a full test suite of certificates and expected outcomes of the test cases.
As specified in the above guidance, if the certificate can not be validated for any reason then the connection between the client and server must be dropped. Any data exchanged over a connection where the certificate has not properly been validated could be exposed to unauthorized access or modification.
Extended Validation Certificates
Extended validation certificates (EV Certificates) proffer an enhanced investigation by the issuer into the requesting party due to the industry's race to the bottom. The purpose of EV certificates is to provide the user with greater assurance that the owner of the certificate is a verified legal entity for the site. Browsers with support for EV certificates distinguish an EV certificate in a variety of ways. Internet Explorer will color a portion of the URL in green, while Mozilla will add a green portion to the left of the URL indicating the company name.
High value websites should consider the use of EV certificates to enhance customer confidence in the certificate. It should also be noted that EV certificates do not provide any greater technical security for the TLS. The purpose of the EV certificate is to increase user confidence that the target site is indeed who it claims to be.
Client side certificates can be used with TLS to prove the identity of the client to the server. Referred to as "two-way TLS", this configuration requires the client to provide their certificate to the server, in addition to the server providing their's to the client. If client certificates are used, ensure that the same validation of the client certificate is performed by the server, as indicated for the validation of server certificates above. In addition, the server should be configured to drop the TLS connection if the client certificate cannot be verified or is not provided.
The use of client side certificates is relatively rare currently due to the complexities of certificate generation, safe distribution, client side configuration, certificate revocation and reissuance, and the fact that clients can only authenticate on machines where their client side certificate is installed. Such certificates are typically used for very high value connections that have small user populations.
Certificate and Public Key Pinning
Hybrid and native applications can take advantage of certificate and public key pinning. Pinning associates a host (for example, server) with an identity (for example, certificate or public key), and allows an application to leverage knowledge of the pre-existing relationship. At runtime, the application would inspect the certificate or public key received after connecting to the server. If the certificate or public key is expected, then the application would proceed as normal. If unexpected, the application would stop using the channel and close the connection since an adversary could control the channel or server.
Pinning still requires customary X509 checks, such as revocation, since CRLs and OCSP provides real time status information. Otherwise, an application could possibly (1) accept a known bad certificate; or (2) require an out-of-band update, which could result in a lengthy App Store approval.
For more information, please see the Pinning Cheat Sheet.
Providing Transport Layer Protection for Back End and Other Connections
Although not the focus of this cheat sheet, it should be stressed that transport layer protection is necessary for back-end connections and any other connection where sensitive data is exchanged or where user identity is established. Failure to implement an effective and robust transport layer security will expose sensitive data and undermine the effectiveness of any authentication or access control mechanism.
Secure Internal Network Fallacy
The internal network of a corporation is not immune to attacks. Many recent high profile intrusions, where thousands of sensitive customer records were compromised, have been perpetrated by attackers that have gained internal network access and then used sniffers to capture unencrypted data as it traversed the internal network.
Protocol and Cipher Configuration for Back End and Other Connections
It is important to provide TLS for server-to-server communication in addition to client-to-server communication. Secure the 'client side' configuration of your server that is used for backend and other connections according to Server Protocol and Cipher Configuration. Be sure to deactivate insecure protocols and ciphers. (e.g. Only support a minimum strong configuration when your server acts as 'client').
Authors and Primary Editors
Torsten Gigler - torsten.gigler[at]owasp.org