Use origin (rather than URL) for security decisions.

URLs are often not sufficient for security decisions, since the origin may not be present in the URL (e.g., about:blank), may be tricky to parse (e.g., blob: or filesystem: URLs), or may be opaque despite a normal-looking URL (e.g., the security context may be sandboxed). Use origins whenever possible.

Use GetLastCommittedOrigin() or GetSecurityContext().

Good

// Example of browser-side code.
content::RenderFrameHost* frame = ...;
if (safelist.Matches(frame->GetLastCommittedOrigin()) {
  // ...
}

// Example of renderer-side code.  Note that browser-side checks are still
// needed to ensure that a compromised renderer cannot bypass renderer-side-only
// checks.
blink::Frame* frame = ...;
if (safelist.Matches(frame->GetSecurityContext()->GetSecurityOrigin()) {
  // ...
}

Bad

// Example of browser-side code.
content::RenderFrameHost* frame = ...;
if (safelist.Matches(frame->GetLastCommittedURL()) {
  // BUG: doesn't work for about:blank or sandboxed frames.
}

// Example of renderer-side code.  Note that browser-side checks are still
// needed to ensure that a compromised renderer cannot bypass renderer-side-only
// checks.
blink::LocalFrame* frame = ...;
if (safelist.Matches(frame->GetDocument()->Url()) {
  // BUG: doesn't work for about:blank or sandboxed frames.
  // BUG: doesn't work for RemoteFrame(s) which don't have a local Document
  //      object and don't know the URL (only the origin) of the frame.
}

Don't use the GURL type to store origins.

GURL origin is an anti-pattern - representing origins as a URL-focused data type means that 1) some information is lost (e.g., origin's nonce and precursor information) and 2) some unnecessary information may be present (e.g., URL path and/or query parts are never part of an origin).

Use the following datatypes to represent origins:

• C++: url::Origin or blink::SecurityOrigin (instead of GURL or blink::KURL).
• Mojo: url.mojom.Origin (instead of url.mojom.Url). Remember to validate data received over IPC from untrustworthy processes or even better avoid sending origins in the first place.
• Java: org.chromium.url.Origin (see also url::Origin::FromJavaObject and url::Origin::CreateJavaObject).

Avoid converting URLs to origins.

Good

url::Origin origin = GetLastCommittedOrigin();

Bad

GURL url = ...;
GURL origin = url.GetOrigin();
// BUG: |origin| will be empty if |url| is a blob: URL like
//      "blob:http://origin/guid-goes-here".
// BUG: |origin| will be incorrect if |url| is an "about:blank" URL
//      or if |url| came from a sandboxed frame.
// NOTE: |GURL origin| is also an anti-pattern; see the "Use correct type to
//       represent origins" section below.

Risky

// If you know what you are doing (e.g., don't care about about:blank and/or
// sandboxed frames) and really need to convert a GURL into url::Origin then
// url::Origin::Create will correctly handle filesystem: and/or blob: URLs
// (unlike the "bad" example above).
GURL url = ...;
url::Origin origin = url::Origin::Create(url);
// |origin| will be "http://origin/" if |url| is a blob: or filesystem: URL
// like "blob:http://origin/guid-goes-here".