Strict Mode for HTTP Public Key Pinning
draft-landau-websec-key-pinning-really-latest

Abstract

The HTTP Public Key Pinning Extension permits a User Agent to forego Pin Validation for policy reasons, for example when a server’s certificate chain terminates at a user-defined trust anchor. This facilitates “corporate” man-in-the-middle attacks which website operators may not wish to facilitate.

This specification specifies an extension to the HTTP Public Key Pinning specification allowing website operators to request that User Agents impose Denial of Service in relation to that website when a “policy” man-in-the-middle attack is detected. This allows such website operators to, by virtue of their plurality multiplied by their userbase, discourage the use of “policy” man-in-the-middle attacks.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on May 20, 2016.

Copyright Notice

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Table of Contents

• 1. Introduction
• 2. The “strict” Keyword
• 3. Security Considerations
• 4. IANA Considerations
• 5. Normative References
• Appendix A. Pin Examples
• Author's Address

1. Introduction

The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in [RFC2119].

2. The “strict” Keyword

The “strict” keyword is introduced to the Public-Key-Pinning and Public-Key-Pinning-Report-Only HTTP headers specified in [RFC7469].

Where the “strict” keyword appears, a User Agent shall record this fact. Where a User Agent attempts connection to a website which has a pin set recorded incorporating the “strict” keyword, a User Agent MUST perform Pin Validation as specified in [RFC7469], and MUST NOT skip Pin Validation for policy reasons as is otherwise permitted by [RFC7469] section 2.6.

Ordinarily, a pin set is not recorded if the connection it is delivered over would not pass Pin Validation in relation to that pin set. When the “strict” keyword is used, a pin set is recorded even if it would not pass Pin Validation. Since there is not necessarily any way of distinguishing between a man-in-the-middle attack performed for policy reasons and a website misconfiguration, this allows website operators to cause total Denial of Service on their websites. Thus the “strict” keyword has a mandatory parameter which acts as a max-age value for the “strict” keyword only. This allows the potential auto-DoS period due to a misconfiguration to be limited.

Because each page load will renew a pin with the “strict” keyword, even a short “strict” max-age value of e.g. 10 minutes will have the effect of making a site unusable for victims of “policy” man-in-the-middle attacks.

The parameter value for the “strict” keyword MUST be included.

3. Security Considerations

Entities performing man-in-the-middle attacks may trivially strip Public-Key-Pinning headers, defeating this mechanism. Many such entities may neglect to perform this practice, or may not have the operational expertise to configure such measures. The use of preload lists bundled with User Agents could ensure that strict mode is effective even in the face of such measures.

4. IANA Considerations

None.

5. Normative References

Appendix A. Pin Examples

The following shows an example pinning header, which also enables strict mode for an hour.

Public-Key-Pins: max-age=2592000;
  pin-sha256="E9CZ9INDbd+2eRQozYqqbQ2yXLVKB9+xcprMF+44U1g=";
  pin-sha256="LPJNul+wow4m6DsqxbninhsWHlwfp0JecwQzYpOLmCQ=";
  strict=3600

Author's Address