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= Introduction  =
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The [[Pinning Cheat Sheet]] is a technical guide to implementing certificate and public key pinning as discussed at the Virginia chapter's presentation [[Media:Securing-Wireless-Channels-in-the-Mobile-Space.ppt|Securing Wireless Channels in the Mobile Space]]. This guide is focused on providing clear, simple, actionable guidance for securing the channel in a hostile environment where actors could be malicious and the conference of trust a liability.
A verbose article is available at [[Certificate_and_Public_Key_Pinning|Certificate and Public Key Pinning]]. The article includes additional topics, such as Alternatives to Pinning, Ephemeral Keys, Pinning Gaps, Revocation, and X509 Validation.
Please visit [ Pinning Cheat Sheet] to see the latest version of the cheat sheet.
== What's the problem? ==
Users, developers, and applications expect end-to-end security on their secure channels, but some secure channels are not meeting the expectation. Specifically, channels built using well known protocols such as VPN, SSL, and TLS can be vulnerable to a number of attacks.
== What Is Pinning? ==
Pinning is the process of associating a host with their ''expected'' X509 certificate or public key. Once a certificate or public key is known or seen for a host, the certificate or public key is associated or 'pinned' to the host. If more than one certificate or public key is acceptable, then the program holds a ''pinset'' (taking from [ Jon Larimer and Kenny Root Google I/O talk]). In this case, the advertised identity must match one of the elements in the pinset.
A host or service's certificate or public key can be added to an application at development time, or it can be added upon first encountering the certificate or public key. The former - adding at development time - is preferred since ''preloading'' the certificate or public key ''out of band'' usually means the attacker cannot taint the pin.
=== When Do You Pin? ===
You should pin anytime you want to be relatively certain of the remote host's identity or when operating in a hostile environment. Since one or both are almost always true, you should probably pin all the time.
=== When Do You Whitelist? ===
If you are working for an organization which practices "egress filtering" as part of a Data Loss Prevention (DLP) strategy, you will likely encounter ''Interception Proxies''. I like to refer to these things as '''"good" bad guys''' (as opposed to '''"bad" bad guys''') since both break end-to-end security and we can't tell them apart. In this case, '''do not''' offer to whitelist the interception proxy since it defeats your security goals. Add the interception proxy's public key to your pinset after being '''instructed''' to do so by the folks in Risk Acceptance.
=== How Do You Pin? ===
The idea is to re-use the exiting protocols and infrastructure, but use them in a hardened manner. For re-use, a program would keep doing the things it used to do when establishing a secure connection.
To harden the channel, the program would take advantage of the <tt>OnConnect</tt> callback offered by a library, framework or platform. In the callback, the program would verify the remote host's identity by validating its certificate or public key.
== What Should Be Pinned? ==
The first thing to decide is what should be pinned. For this choice, you have two options: you can (1) pin  the certificate; or (2) pin the public key. If you choose public keys, you have two additional choices: (a) pin the <tt>subjectPublicKeyInfo</tt>; or (b) pin one of the concrete types such as <tt>RSAPublicKey</tt> or <tt>DSAPublicKey</tt>.
[[File:random-org-der-dump.png|thumb| 100px |subjectPublicKeyInfo]]The three choices are explained below in more detail. I would encourage you to pin the <tt>subjectPublicKeyInfo</tt> because it has the public parameters (such as <tt>{e,n}</tt> for an RSA public key) '''and''' contextual information such as an algorithm and OID. The context will help you keep your bearings at times, and the figure to the right shows the additional information available.
=== Certificate ===
[[File:pin-cert.png|thumb|right|100px|Certificate]] The certificate is easiest to pin. You can fetch the certificate out of band for the website, have the IT folks email your company certificate to you, use <tt>openssl s_client</tt> to retrieve the certificate etc. At runtime, you retrieve the website or server's certificate in the callback. Within the callback, you compare the retrieved certificate with the certificate embedded within the program. If the comparison fails, then fail the method or function.
There is a downside to pinning a certificate. If the site rotates its certificate on a regular basis, then your application would need to be updated regularly. For example, Google rotates its certificates, so you will need to update your application about once a month (if it depended on Google services). Even though Google rotates its certificates, the underlying public keys (within the certificate) remain static.
=== Public Key ===
[[File:pin-pubkey.png|thumb|right|100px|Public Key]] Public key pinning is more flexible but a little trickier due to the extra steps necessary to extract the public key from a certificate. As with a certificate, the program checks the extracted public key with its embedded copy of the public key.
There are two downsides to public key pinning. First, it's harder to work with keys (versus certificates) since you must extract the key from the certificate. Extraction is a minor inconvenience in Java and .Net, buts it's uncomfortable in Cocoa/CocoaTouch and OpenSSL. Second, the key is static and may violate key rotation policies.
=== Hashing ===
While the three choices above used DER encoding, its also acceptable to use a hash of the information. In fact, the original sample programs were written using digested certificates and public keys. The samples were changed to allow a programmer to inspect the objects with tools like <tt>dumpasn1</tt> and other ASN.1 decoders.
Hashing also provides three additional benefits. First, hashing allows you to anonymize a certificate or public key. This might be important if you application is concerned about leaking information during decompilation and re-engineering. Second, a digested certificate fingerprint is often available as a native API for many libraries, so its convenient to use.
Finally, an organization might want to supply a reserve (or back-up) identity in case the primary identity is compromised. Hashing ensures your adversaries do not see the reserved certificate or public key in advance of its use. In fact, Google's IETF draft ''websec-key-pinning'' uses the technique.
== Examples of Pinning ==
This section discusses certificate and public key pinning in Android Java, iOS, .Net, and OpenSSL. Code has been omitted for brevity, but the key points for the platform are highlighted.
=== Android ===
Since Android N, the preferred way for implementing pinning is by leveraging Android's [ Network Security Configuration] feature, which lets apps customize their network security settings in a safe, declarative configuration file without modifying app code.
To enable pinning, [ the `<pin-set>` configuration setting] can be used.
If devices running a version of Android that is earlier than N need to be supported, a backport of the Network Security Configuration pinning functionality is available via the TrustKit Android library at
Lastly, the Android documentation provides an example of how SSL validation can be customized within the app's code (in order to implement pinning) in the [ Unknown CA implementation document]. However, implementing pinning validation from scratch should be avoided, as implementation mistakes are extremely likely and usually lead to severe vulnerabilities.
=== iOS ===
TrustKit, an open-source SSL pinning library for iOS and macOS is available at It provides an easy-to-use API for implementing pinning, and has been deployed in many apps.
Otherwise, more details regarding how SSL validation can be customized on iOS (in order to implement pinning) are available in the "HTTPS Server Trust Evaluation" technical note at However, implementing pinning validation from scratch should be avoided, as implementation mistakes are extremely likely and usually lead to severe vulnerabilities.
=== .Net ===
.Net pinning can be achieved by using <tt>ServicePointManager</tt>.
Download: [[|.Net sample program]].
=== OpenSSL ===
Pinning can occur at one of two places with OpenSSL. First is the user supplied <tt>verify_callback</tt>. Second is after the connection is established via <tt>SSL_get_peer_certificate</tt>. Either method will allow you to access the peer's certificate.
Though OpenSSL performs the X509 checks, you must fail the connection and tear down the socket on error. By design, a server that does not supply a certificate will result in <tt>X509_V_OK</tt> with a '''NULL''' certificate. To check the result of the customary verification: (1) you must call <tt>SSL_get_verify_result</tt> and verify the return code is <tt>X509_V_OK</tt>; and (2) you must call <tt>SSL_get_peer_certificate</tt> and verify the certificate is '''non-NULL'''.
Download: [[|OpenSSL sample program]].
== References ==
* OWASP [[Injection_Theory|Injection Theory]]
* OWASP [[Data_Validation|Data Validation]]
* OWASP [[Transport_Layer_Protection_Cheat_Sheet|Transport Layer Protection Cheat Sheet]]
* IETF [ RFC 1421 (PEM Encoding)]
* IETF [ RFC 4648 (Base16, Base32, and Base64 Encodings)]
* IETF [ RFC 5280 (Internet X.509, PKIX)]
* IETF [ RFC 3279 (PKI, X509 Algorithms and CRL Profiles)]
* IETF [ RFC 4055 (PKI, X509 Additional Algorithms and CRL Profiles)]
* IETF [ RFC 2246 (TLS 1.0)]
* IETF [ RFC 4346 (TLS 1.1)]
* IETF [ RFC 5246 (TLS 1.2)]
* RSA Laboratories [ PKCS#1, RSA Encryption Standard]
* RSA Laboratories [ PKCS#6, Extended-Certificate Syntax Standard]
= Authors and Editors  =
* Jeffrey Walton - jeffrey,
* John Steven - john,
* Jim Manico - jim,
* Kevin Wall - kevin,
* Ricardo Iramar - [email protected]
= Other Cheatsheets =

Revision as of 13:21, 14 February 2019


The Cheat Sheet Series project has been moved to GitHub!

Please visit Pinning Cheat Sheet to see the latest version of the cheat sheet.