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go-safeweb

DISCLAIMER: This is not an officially supported Google product.

go-safeweb is a collection of libraries for writing secure-by-default HTTP servers in Go.

Contributing

This project is in an early stage. We are currently not accepting any contributions.

Overview

The flexibility of Go’s net/http package allows users to quickly implement HTTP servers.

Responses are then written simply as slices of bytes, headers can be arbitrarily manipulated and so on. This approach offers much needed flexibility for these who really need it.

Unfortunately, this approach leaves great space for introducing security vulnerabilities and even experienced developers tend to do so.

This document aims to design an HTTP API that eliminates whole classes of bugs, like Cross-Site Scripting (XSS) or Cross-Site Request Forgery (XSRF). This can be achieved by an approach known at Google as safe coding. Learn more at Securing the Tangled Web (Chistoph Kern, 2014) or Preventing Security Bugs through Software Design (Christoph Kern, 2016).

Goals and Non-Goals

Goals

G1: Secure-by-default

Security mechanisms are applied by default (opt-out, not opt-in).

G2: Unsafe Usage is Easy to Review, Track and Restrict

All opt-outs from security mechanisms are explicit. Wherever possible, they’re contained inside a package or an option that’s easy to restrict.

G3: Designed for Evolving Security Requirements

Enforcing new security measures is feasible through AST manipulation. Existing users can be migrated using static analysis and/or runtime monitoring. Read more here.

G4: High Compatibility with Go’s Standard Library and Existing Open-Source Frameworks

Whenever possible, keep existing layouts, function signatures and other API parts the same as the Go’s standard library. High compatibility enables wide adoption.

Non Goals

NG1: Safe API Completeness

Creating safe APIs for all the corner cases might result in a bloated codebase. Our experience shows that this isn’t necessary.

NG2: Full Compatibility with Go’s Standard Library and Existing Open-Source Frameworks

Existing open-source frameworks or the Go standard library need to support each developer scenario. This would have left us with limited options of creating safe-by-default HTTP servers.

NG3: Features That Are Not Security Critical

Go Safe Web aims to help you create a secure-by-default Go HTTP server and nothing more. Features that are not security critical will not be added. Focusing solely on security allows us to maintain high compatibility with the standard library and makes adoption easier.

Security Vulnerabilities and Mitigations

On a high level, we plan to address, or provide the needed infrastructure to address, following issues (not an exhaustive list):

• XSS (cross-site scripting) and XSSI (cross-site script inclusion) - e.g. by controlling how responses are generated
• XSRF (cross-site request forgery) - e.g. by using Fetch Metadata policies, supporting token-based XSRF protection
• CORS (cross-origin resource sharing) - e.g. by taking control of CORS response headers and handling CORS preflight requests
• CSP (content security policy) - e.g. by automatically adding script nonces to HTML responses, adding relevant security headers
• Transport Security - e.g. by enforcing HSTS support
• IFraming - e.g. by setting relevant HTTP headers to restrict framing or providing server-side support for origin selection
• Auth (access control) - e.g. by providing infrastructure for plugging in access control logic in an uniform, auditable way
• HTTP Request Parsing Bugs - e.g. by implementing strict and well documented parsing behavior
• Error responses - e.g. by providing infrastructure for uniform error handling (e.g. to prevent accidental leaks or XSS from error responses)
• Enforcement of other security specific HTTP headers - here

Appendix

Evolving Security Requirements (example)

Imagine an API for configuring access control. It features three types of rules:

• ALLOW(user) - allows a given user
• DENY(user) - denies a given user (has priority over ALLOW)
• REPORT(user) - reports that it has seen a request from a given user

Imagine now that at some point, security standards need to be increased and user = "frombulator" has been determined to not meet the desired bar.

How do we, for all the services running in our company, address this?

1. For existing services, we add a LegacyFrombulatorAccess option like so: security.AccessControl(rules, unsafe.LegacyFrombulatorAccess()).
2. We change the security.AccessControl() call to add by default a DENY("frombulator") rule. This rule is not added if unsafe.LegacyFrombulatorAccess is applied.
3. Instead, unsafe.LegacyFrombulatorAccess adds a REPORT("frombulator") rule.

This way, we have:

• Ensured that all new callers of security.AccessControl use the safe setting by default.
• Can monitor existing services dependence on calls from the frombulator. After a period of observation (let’s say, 30 days):
  • If the service doesn’t receive requests from the frombulator: prune the unsafe.LegacyFrombulatorAccess option.
  • If the service does receive requests from the frombulator: inform the service owners and plan a fix.

Crucially, only the last case (dependence on unsafe configuration) requires engineering work per service. The rest can be automated.

This approach is possible due to careful API design. A missing DENY or REPORT rule, or a single sink in the form of security.AccessControl would make this infeasible.

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Copyright 2020 Google LLC

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