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# Oh, the Places You’ll Go! Finding Our Way Back from the Web Platform’s Ill-conceived Jaunts

**Oh, the Places You’ll Go! Finding Our Way Back from the Web Platform’s Ill-conceived Jaunts** - Artur Janc, Mike West, secweb.work.

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# Oh, the Places You’ll Go! Finding Our Way Back from the Web Platform’s Ill-conceived Jaunts

--- page 1 ---

Oh, the Places You'll Go! Finding Our Way Back from the Web Platform's
Ill-conceived Jaunts
Artur Janc
Google, Inc
aaj@google.com
Mike West
Google, Inc
mkwst@google.com
Abstract
—In its transition from the original concept of a
mesh of hypertext documents [2] into the world's most
successful application ecosystem, the open web platform
[3] has steadily, iteratively, accumulated a large number
of unsafe features and behaviors. These features lead to
vulnerabilities in web applications, enable attacks on web
users, and often add signicant complexity to developers'
mental models of the web and to user-agent implementations.
In this paper, we start from a scattered list of concrete
grievances about the web platform based on informal dis-
cussions among browser– and web security engineers. After
reviewing the details of these issues, we work towards a
model of the root causes of the problems, categorizing them
based on the type of risk they introduce to the platform.
We then identify possible solutions for each class of issues,
dividing them by the most effective approach to address it.
In the end, we arrive at a general blueprint for backing
out of these dead ends. We propose a three-pronged approach
which includes changing web browser defaults, creating a
slew of features for web authors to opt out of dangerous
behaviors, and adding new security primitives. We then show
how this approach can be practically applied to address
each of the individual problems, providing a conceptual
framework for solving unsafe legacy web platform behaviors.
1. Introduction
OH! THE PLACES YOU'LL GO!
You'll be seeing great sights!
You'll join the high iers
who soar to high heights.
You won't lag behind, because you'll have the speed.
You'll pass the whole gang and you'll soon take the lead.
Wherever you y, you'll be best of the best.
Wherever you go, you will top all the rest.
Except when you don't.
Because, sometimes, you won't. [1]
The web is the world's most successful application
ecosystem, universally admired by users and authors. It
is also woefully inadequate from a security standpoint:
after decades of growth and accumulating features in a
seemingly haphazard manner, the platform nds itself at
risk from legacy behaviors that threaten to undermine
its basic security and privacy guarantees. The threats are
exacerbated by the web's openness and composability –
resulting in an outsized attack surface – and the high
amount and sensitivity of data which users have entrusted
to web applications.
In our attempt to nd a solution to this problem, we
start by enumerating a number of specic long-standing
sharp edges of the web platform which contribute to a
variety of security issues affecting web applications and
users. After listing the dangerous behaviors in Section 2
we group these seemingly unrelated problems into three
categories based on the type of threat caused by each
dangerous behavior in Section 3.
Crucially, this categorization assists us in evaluating
potential solutions to the problems, which we discuss in
Section 3.2. Specically, we analyze
where
a change could
be made to address a given problem pattern, identifying
three major locations for possible xes:
disabling the
behavior by default
in web browsers,
providing an opt-
out toggle
for web applications, and
creating new security
primitives
to offer more principled solutions.
The remainder of this paper expands upon this idea to
build a simple framework that can help the web security
community understand how to best remove each source
of insecurity from the platform. In Section 4 we review
past web security improvements and see where they t
within our model. Finally, we propose a concrete plan for
using modern mechanisms including the Reporting API
[51] and Document Policy [49] to present a blueprint for
a collaborative process for browser vendors and applica-
tion authors to address current and future web security
problems.
2. A Laundry List of Web Security Problems
It is the authors' strong belief that starting with a neat
model which systematizes and offers possible solutions
to long-standing problems is a surere way to alienate
the reader, have them immediately suspect the end result,
and begin harboring negative feelings towards the authors.
This is a particular risk when attempting to make sense
of a domain as complex as the web, which – almost as a
rule – dees elegant solutions and simple models.
Let's try to avoid this pitfall by rst embarking on a
survey of specic web behaviors which, with the benet
of hindsight, can be considered mis-features or have oth-
erwise been shown to be the cause of security headaches
for authors and users. Since agreeing on the problems is
easier than agreeing on the solutions, we hope to nd

--- page 2 ---

common ground with the reader by commiserating about
a number of problematic APIs that we expect the reader,
like us, has had the misfortune of dealing with.
This list in this section is certainly not objective,
nor is it exhaustive. However, we expect that the reader,
being well acquainted with the web platform's quirks, will
intuitively empathize with these problems and hope these
issues are a sufcient starting point for the discussion
further in this paper. The high-level grouping in this
section is provided for clarity only; we explain and expand
this categorization in Section 3.
2.1. Vulnerability-prone APIs
2.1.1.
document.domain
.
document.domain
[4]
allows documents to relax the same-origin policy [5] and
enables direct DOM access between documents hosted on
different subdomains within the same site.
Setting
document.domain
to the parent domain in
any document exposes all data in a given origin to attack-
ers with scripting access to a same-site origin, removing
a fundamental web security boundary [6].
2.1.2. Maintaining state over HTTP.
Documents de-
livered over an unencrypted channel (HTTP [9]) enjoy
access to most web APIs which allow persistence: cookies,
and all variants of local storage (e.g.
IndexedDB
or
localStorage
).
This allows developers to create full-featured applica-
tions with user sessions, which fail to enforce basic data
condentiality and integrity guarantees.
2.1.3. Cookie semantics.
As the canonical web mecha-
nism to persist state, cookies are surprisingly misaligned
with other fundamental web security mechanisms: their
scoping dees the same-origin policy in important ways
[10].
Importantly, cookies are generally unsafe by default:
they are available to HTTP origins even when set over
HTTPS, they are available to client-side scripts, and they
are attached by default even on cross-site requests [11].
2.1.4. Mixed content.
Browsers have historically not
enforced that secure contexts (HTTPS) load all of their
subresources securely. This allows sensitive applications
to mistakenly load resources which can be observed and
modied by network-level attackers, leading to loss of
condentiality and integrity for users [17].
2.1.5.
javascript:
URIs.
When a document initiates
a navigation (e.g. by loading a frame or pop-up, or
changing its own location), if the target of the navigation
happens to be a
javascript:
URL [18], this will
result in script execution in the context of the navigating
document.
This is a common cause of cross-site scripting vul-
nerabilities in applications which accept user-controlled
links without validating their protocols, and in cases where
untrusted input is interpolated by the application inside a
javascript:
URL [19] (the authors dare the reader to
identify the correct escaping required for this context).
2.2. Behaviors Enabling Attacks on Websites
2.2.1. MIME type snifng.
MIME type snifng [7] (also
known as
content snifng
) is a compatibility feature which
causes the browser to ignore the
Content-Type
value
provided by the server in order to process a resource
whose advertised MIME type would make it fail to load
or display.
This is a particular concern when the browser can
be forced to treat a server response with the attacker's
data as HTML or JavaScript, often as a result of allowing
users to upload les [8], leading to cross-site scripting
vulnerabilities.
2.2.2. Plugins;
<embed>
and
<object>
elements.
For unfortunate historical reasons, plugins [12] enjoyed
a decade as the
de facto
cross-browser standard to pro-
vide ”rich content” such as games, multimedia, and more
powerful runtimes with capabilities not provided by web
APIs.
This brought along a variety of security problems,
including memory corruption bugs in plugin implemen-
tations [13], a proliferation of custom security principals
incompatible with the same-origin policy [14], and extant
strangeness in how browsers handle resource loads via the
<object>
element.
2.2.3. DOM clobbering.
As a convenient shortcut for
accessing HTML elements in client-side scripts, browsers
create references to such DOM elements based on their
id
and
name
attributes [16] (for example
<a id=foo>
will create a reference to the element in the global
window.foo
variable).
This allows attacks which control a limited part of the
DOM of the application (for example, markup processed
by an HTML sanitizer, or injections mitigated by the
Content Security Policy) to affect the behavior of client-
side scripts, enabling a number of attacks [15].
2.2.4. Credentials in URL's
userinfo
.
Browsers allow
URLs to include credentials and supply these credentials
to servers (e.g. via HTTP 'Basic' Authentication [20] or
when connecting to non-web protocols such as FTP).
This enables social engineering attacks based on dis-
playing attacker-controlled messages in a trusted browser
UI, and gives attackers opportunities to create state in the
context of another origin.
2.2.5. Site-based security boundaries.
Most security
rules are enforced by the browser at the level of individual
origins [5]; however, there are important counterexamples
where APIs are more broadly scoped and information may
leak between different origins within the site boundary.
Examples include domain-scoped cookies, and –
driven primarily by performance considerations – HTTP
cache partitioning [21] and process isolation rules [22].
2.2.6.
window.frames
and frame tree access.
While
the browser prevents access to most DOM properties
of cross-origin windows, an important exception are the
window.length
and
window.frames
properties.
These properties reveal the number of iframes embed-
ded in any cross-origin document, and allow the attacker

--- page 3 ---

to interact with individual iframes (for example, send them
messages via
postMessage
). This enables cross-origin
information leaks [23].
2.2.7.
window.history
.
The
window.history
API [24] reveals the number of navigations that have pre-
viously been performed in an attacker-controlled window,
and allows navigation to any document in the window's
history. This enables cross-origin information leaks [23].
2.2.8. Unconstrained
<base>
URIs.
The
<base>
ele-
ment [26] allows applications to set a URL prex against
which all subresource loads are resolved. This allows
limited HTML injections (for example, those mitigated
by Content Security Policy, or in cases where the attacker
controls a small number of characters) to modify the
locations of resources loaded from relative paths.
This allows exploiting a subset of injection bugs [25].
2.2.9. Liberal parsing of malformed markup.
In true
spirit of Postel's law [27], browsers' HTML parsers are
liberal when processing markup, even when it's mal-
formed. This allows limited HTML injections to alter
the meaning of subsequent markup, possibly allowing
exltration of the contents of the page.
In the specic example of
dangling markup
at-
tacks [28], an attacker-controlled element (such as a
<textarea>
or an
<img>
tag with an unterminated src
attribute) can swallow the remaining parts of the document
and include them in a request to an attacker-controlled
destination.
2.2.10. Unlimited recursive
@import
in CSS.
The
@import
rule [35] in CSS allows the loading of arbitrary
styles, and can be used recursively to include subsequent
stylesheets.
This allows a single stylesheet injection to reliably
exitrate data from the DOM by using advanced CSS
features [36].
2.2.11. Scrolling to URL fragments.
When a docu-
ment is navigated to a URL containing a
location frag-
ment
, the browser will automatically scroll to an element
whose
id
matches the fragment value [37]. For example
site.example/#foo
will scroll to an element with
id=foo
if it exists.
Because the consequences of a scroll can
be inferred cross-origin (for example by using
IntersectionObserver
in any iframes loaded
by the document, or by inspecting the state of the HTTP
cache), this reveals whether an element with a given
id
is present on the page [38]. This enables cross-origin
information leaks.
2.2.12.
load
/
error
events on external resources.
Cross-origin requests by default attach cookies and the
browser provides APIs that reveal both whether the re-
quest succeeded and the accurate time when the response
was received.
This allows cross-origin timing attacks and reveals
whether the user has access to an arbitrary cross-origin
resource, allowing deanonimization attacks and informa-
tion leaks [39].
2.3. Behaviors Enabling Attacks on Users
2.3.1. Styling of visited links in CSS.
The CSS
:visited
pseudoclass allows any document to style a
link present in the user's browsing history differently from
other links, creating a distinction which can reveal the list
of pages visited by the user [29].
Despite mitigations that prevent the page from directly
reading the styles of links, multiple side-channels have
been demonstrated to allow attackers to infer the contents
of users' browsing histories [30] [31].
2.3.2. Non-partitioned HTTP cache.
The fact that the
HTTP cache has historically constituted global state,
shared between cross-origin applications, creates oppor-
tunities for any origin to determine the state of other
applications based on their cached resources [32].
The HTTP cache also reveals information about the
sites previously visited by the user and may allow
deanonymization attacks [33].
2.3.3. Requests to RFC1918 addresses.
Because
browsers lack a distinction between public and private
destinations, any website visited by a user can issue HTTP
requests to any destination, even if the target address is
local to the user and unreachable externally [34].
Attackers can task the user's browser with making re-
quests to the loopback address or the user's local network,
using the browser as a proxy. This enables ngerprinting
the user's network environment and exploiting vulnerabil-
ities in services not reachable directly by the attacker.
2.3.4. Nested contexts control top-level browser UI.
Subresources, such as iframes, can initiate actions which
affect browser UI visible at the top level, which can be
easily misattributed by the user as being generated by the
embedding site.
This allows crafting convincing UI spoong attacks by
showing prompts (
alert()
or
prompt()
), initiating
downloads or requesting permissions in windows where
an attacker controls an iframe [40].
The reader has likely noticed that even this incomplete
enumeration of sharp edges in the web platform contains
problems that are varied in kind, scope and impact. Is there
a way to think about them that, even if not demonstrably
correct
, is at least
useful
?
We attempt to answer this question in the next section.
3. Categorizing the Web's Faults
You'll look up and down streets. Look 'em over with care.
About some you will say, ”I don't choose to go there.”;
With your head full of brains and your shoes full of feet,
you're too smart to go down any not-so-good street.
And you may not nd any
you'll want to go down.
In that case, of course,
you'll head straight out of town. [1]
In our attempt to create a useful model of the security
shortcomings of the web, we will depart from general

--- page 4 ---

approaches traditionally used for understanding software
anti-patterns [41], vulnerability classication [42] and
fault analysis [43]. Readers looking for references to the
Bell-LaPadula model [44] throughout this paper will nd
themselves sorely disappointed.
Instead, true to the principles of the web, and es-
chewing rigor and theoretical purity, we will base our
categorization on the simplest approach that has a chance
of working; like the problems, our categorizations in this
section are inherently web-specic.
3.1. A Summary of Problems
We posit that it is an interesting thought experiment to
understand the fundamental nature of each of the problems
listed above. In the previous section we outlined
what
patterns reduce security on the web; here we will attempt
to understand
why
they do so.
3.1.1. Vulnerability-prone API.
A fairly evident subset
of the problems arises because the use of certain web APIs
in an application is likely to introduce vulnerabilities. For
example, setting
document.domain
is by itself very
frequently a security bug; using cookies in their default
conguration, or composing
javascript:
URIs with
user input similarly leads to security bugs.
The existence of such APIs isn't
inherently
a security
problem. However, in practice, application authors who
use such APIs will inevitably do so unsafely and make
their code vulnerable. We posit that this is
de facto
unde-
sirable for the ecosystem.
3.1.2. Enabling attacks on websites.
Another class of
problems stems from the fact that some web behaviors
provide attackers with capabilities to attack unrelated ap-
plications to which the user is logged in. The existence of
MIME snifng, DOM clobbering, cross-origin access to
window.frames
or the ability to scroll other documents
to a URL fragment, all create opportunities to conduct
injection attacks or leak cross-origin information.
This category of issues can manifest itself in the
presence of a seemingly benign pattern in the vulnerable
application; for example, an application that correctly san-
itizes user-provided HTML but doesn't account for DOM
clobbering may be vulnerable to XSS as a result. Other
behaviors, such as the ability to traverse a cross-origin
document's frames, affect any web application, even in
the absence of any programming mistakes.
3.1.3. Enabling attacks on the user.
Some web security
problems are based on an untrusted website attacking the
user directly, even if the user doesn't have any authenti-
cated web sessions. Revealing web browsing history with
the
:visited
selector or by querying the HTTP cache,
or the ability to craft requests to the user's local network,
all subvert users' security and privacy expectations.
As a result, even a user who never logs into websites
and doesn't store any of their data on the web can be
affected by these behaviors.
3.1.4. Bonus criterion: Complexity.
In some cases, a
web feature doesn't
directly
cause security problems, but
its presence introduces complexity in user agents and
TABLE 1. A
PARTIAL LIST OF UNSAFE WEB FEATURESFeature Problem Solutiondocument.domain
* Vulnerability-prone API Site opt-outState over HTTP Vulnerability-prone API New primitiveCookie semantics* Vulnerability-prone API New primitiveMixed content Vulnerability-prone API Default disablejavascript:
URIs* Vulnerability-prone API Site opt-outMIME snifng* Enables attacks on websites Site opt-outPlugins* Enables attacks on websites Default disableDOM clobbering Enables attacks on websites Site opt-outuserinfo
in URLs* Enables attacks on websites Default disableSite-based boundaries Enables attacks on websites Default disablewindow.frames
Enables attacks on websites Site opt-outwindow.history
Enables attacks on websites Site opt-outUnrestricted
<base>
* Enables attacks on websites Site opt-outDangling markup Enables attacks on websites Site opt-outLiberal HTML parsing Enables attacks on websites Site opt-out@import
in CSS Enables attacks on websites Default disableFragment scrolling Enables attacks on websites Site opt-outonload
/
onerror
Enables attacks on websites Default disable:visited
in CSS Enables attacks on users Default disableGlobal HTTP cache Enables attacks on users Default disableRFC1918 requests Enables attacks on users Default disableUI for nested contexts Enables attacks on users Default disableComplex features (as dened in Section 3.1.4) are marked with *.
adds cognitive load for applications and library authors,
increasing the risk of implementation bugs.
For example, the ability to carry credentials in URLs
increases the difculty of parsing links, and the pres-
ence of
document.domain
signicantly complicates
browsers' same-origin logic.
3.2. A Summary of Solutions
Having proposed a summary of root causes for the
problems, let's now categorize them based on a different
criterion: the nature of a potential solution. Specically,
let's consider
where
an ideal solution to the unsafety could
lie. We outline the categories below and come back to a
more detailed discussion in Section 4.
3.2.1. Disable by default in web browsers.
In an ideal
case, a given source of unsafety could be completely
removed by web browsers. Addressing problems in the
browser improves the security posture of the platform,
protecting users without requiring action on part of web
developers.
In practice, such security improvements must gener-
ally weigh the security benet against other criteria, e.g.
compatibility or performance concerns.
3.2.2. Provide an opt-out toggle for applications.
When
a certain feature cannot be removed from the platform,
but is not necessary for most applications, browsers can
provide developers to ability to opt their sites out of a
given unsafe behavior. This allows sensitive applications
to reduce the risk for their users, while allowing the
behavior for sites with less strict security requirements.
3.2.3. Create new primitives.
In practice, some unsafe
behaviors cannot be removed either by default or on an
opt-in basis: certain APIs are simply too ubiquitous and
lack a suitable replacement.
In this case, the web platform can provide new security
mechanisms that preserve the use of the feature while

--- page 5 ---

mitigating its risk; we provide several examples of security
features which ll this niche in Section 4.3. Alternatively,
the platform can create APIs that serve the same use cases
as the problematic feature in a safer way, and provide a
well-lit path to help developers migrate their existing code.
3.2.4. Solutions as a spectrum.
An astute reader will
notice that there is nothing inherent to each problem
that would naturally match it with the most appropriate
location of the solution. After all, browser vendors could
decide to remove all sources of unsafety by default (for
example: disable cookies and disallow HTTP trafc) or
provide opt-in features for developers to disable them for
their origins.
We propose that an important distinction is the de-
ployment and criticality of a given source of unsafety.
On one end of the spectrum lie issues which can be
addressed by default by the browser; the opposite of the
spectrum contains mechanisms which are ubiquitous and
have no suitable replacement, and require browsers to
provide security primitives to reduce their risk. In between
the two extremes lies the ”opt-out territory” where features
can be disabled by individual applications, but cannot be
fully removed from the platform.
We also propose that, in time, features can shift along
this spectrum from being critical for most sites, through
being discouraged and allowing developers to disable
them on their sites, to being removed from the platform
by browser vendors. The work on HTTP and plugins is
illustrative of this journey.
3.2.5. Other classications.
In addition to understanding
the essence of the problem and the most effective location
for the remedy, the patterns in Section 2 can be categorized
along a number of other useful axes. For example, it could
be instructive to assess each pattern in terms of its severity,
the type of attack it allows (some vendors' threat models
may prioritize protecting users against injections, others
may prefer mitigating information leaks and attacks on
user privacy), the web compatibility impact of removing
the behavior, or the amount of community consensus for
addressing the given risk.
This may be particularly useful as web browser ven-
dors' threat models diverge, or as input towards the prior-
itization of web platform security work by each vendor.
4. Solving Web Problems: A Discussion
And when you're in a Slump,
you're not in for much fun.
Un-slumping yourself
is not easily done.
You will come to a place where the streets are not marked.
Some windows are lighted. But mostly they're darked.
A place you could sprain both your elbow and chin!
Do you dare to stay out? Do you dare to go in? [1]
We nd that the location of the most effective so-
lution outlined in the previous section is a particularly
compelling axis alongside which we can evaluate potential
approaches for a given problem. Its primary benet is that
it gives us a blueprint for structuring the web platform
changes to address the problem; specically:

If the problem can be addressed directly by the
browser, vendors should aim to solve it by default.
This is discussed in more detail in Section 4.1.

Issues solvable with application opt-outs depend
on browsers to implement such opt-outs. They
would benet from common deployment and re-
porting infrastructure, described in Section 4.2.

Problematic web features which are in widespread
use and can neither be xed by default, nor can
they be realistically abandoned by web developers
require new APIs to let developers achieve the
same goals more safely. Section 4.3 goes into more
detail about this approach.
4.1. Fixes in Web Browsers
Several elements of the web platform have an infa-
mous track record of creating security headaches for im-
plementers, web authors and users. Plugins and the global
HTTP cache are two examples: the former has introduced
an inordinate number of implementation vulnerabilities
leading to remote code execution issues in browsers [13],
and the latter has been a canonical information disclosure
vector [32], allowing leaks of users' browsing habits and
enabling the exploitation of a number of XS-leaks [23].
In such cases, we argue that the correct approach is
to remove the feature from the platform. The two major
criteria to evaluate are the risk posed by the feature, and
the web compatibility impact of removing it (or enabling
its safe subset where possible). Features whose complete
removal would incur a substantial compatibility cost could
be selectively re-enabled, either per-user (for example, by
administrators in control of the browser's enterprise policy
[45]) or per-site as a Reverse Origin Trial [46].
An informed reader may notice that such browser-
side work has been in progress; for example, the work to
remove the reliance on plugins [47] and to prevent loading
mixed content [48] has advanced to the point that support
for these (mis-)features may soon be removed in major
browsers.
We express hope that the global HTTP cache and
visited link styling can be similarly addressed in the near
future.
4.2. Opt-outs in Web Applications
When a feature poses a danger to the security of
the web platform, but cannot be completely removed for
compatibility reasons, we propose that the platform should
allow web authors to disable it in their applications.
This applies particularly to problems in the
“vulnerability-prone API” and “allows attacks on
websites” categories – an application which exempts
itself from a given dangerous behavior will be protected
from mistakenly using error-prone APIs and from the
attackers abusing unsafe web defaults to launch attacks
on the application.
This is not a novel idea: features to opt out of dan-
gerous defaults have long been present in the platform;
notable examples include cookie attributes (
Secure
,

--- page 6 ---

HttpOnly
,
SameSite
) and headers to prevent con-
tent snifng (
X-Content-Type-Options
) and em-
bedding (
X-Frame-Options
).
Because a lot of the problems listed in Table 1 are
amenable to this approach, we review this idea in more
detail below.
4.2.1. Opting Applications out of Unsafety: A Plan.
We suggest that addressing dangerous web behaviors on
a per-application basis requires, for each feature:

A web-facing toggle to disable the behavior (e.g.
an HTTP response header)

A reporting mode that will notify authors about
incompatibilities in their applications.
While the presence of a toggle is a
sine qua non
to this
approach, the more interesting element is the reporting ca-
pability, which is in practice the true enabler of application
adoption. Building on the successful mode introduced by
Content Security Policy, we propose for each toggle to be
accompanied by a
report-only
and
report-while-enforcing
modes. This allows developers to both understand if their
application can safely disable the feature and be alerted
about potential regressions in the gure..
Two upcoming web platform features provide the in-
frastructure that browser vendors can build on to enable
this approach. Document Policy [49], an offshoot of Per-
missions Policy [50] scoped to same-origin boundaries,
is a general mechanism that controls the use of web
APIs; we posit that adding features to disable unsafe
behaviors would be a natural extension to this proposal.
The Reporting API [51] is a mechanism which provides
an abstraction for other browser features that allows them
to notify the server of potential problems or feature-
dependent error conditions. We propose that combining
these features would be a promising approach for a variety
of voluntary security restrictions.
An interesting aspect of this approach is that it offers
immediate benets for a subset of behaviors. As soon
as one browser implements reporting for unsafe APIs,
developers will be notied of any incompatibilities. For
the
vulnerability-prone APIs
class of problems, this allows
authors to identify and remove any uses of the offending
pattern, improving security for users of all browsers, with-
out requiring the use of polylls.
In the long-term, we envision that browsers could
simplify the disabling of unsafe features by bundling
them together in simpler
security best practices
modes
(for example, origin-wide via the Origin Policy [52]).
We anticipate that browsers can foster the adoption of
such modes by upgrading the UI of compatible sites, or
enforcing the restrictions for privileged sites (e.g. those to
which the user logs in, or which request permissions).
One more approach to help the ecosystem move for-
ward and opt out of unsafe features could be similar to
what was already applied for HTTP. Gating some powerful
APIs like media access behind a
Secure Context
[53]
helped speed up the adoption of HTTPS, so a similar
concept of an even
Securer Context
could be introduced
to leave some old and unsafe knobs behind.
4.3. New Security Primitives
Realistically, some elements of the web platform can-
not be removed either by default or even on an opt-in
basis: many authors cannot build their applications without
common features such as cookies.
In such situations, we recommend an approach of
building replacement APIs which will address authors' use
cases in a safer way, or providing new security features
which will protect applications from the sharp edges of
these mechanisms.
Interestingly, many modern web security features have
been implicitly following this approach:

Content Security Policy
[54] doesn't address the
root causes of markup injections (mixing code and
data), but mitigates their impact by introducing
ne-grained controls over script execution.

Trusted Types
[55] allow developers to keep us-
ing potentially unsafe DOM APIs, but prevent in-
jections by requiring their arguments to be created
safely by central policy code.

SameSite cookies
and
Fetch Metadata Request
Headers
don't change the web platform default
of attaching cookies as ambient authority to cross-
origin requests, but allow developers to change this
behavior for individual cookies, or to make ne-
grained per-endpoint decisions based on additional
security information included in HTTP headers.

Cross-Origin Opener Policy
lets developers dis-
able cross-origin interactions with their top-level
windows.
We anticipate that new opt-in security features can
help address several problems outlined in Section 2 as
well as other emerging web platform risks. However, the
authors want to caution that there are diminishing returns
to compensating for legacy insecurity by engineering new
features. Solving long-standing web problems will likely
require an increase emphasis on xing the root causes of
insecurity, as outlined in the previous sections.
5. Future Work
We believe this paper to be only an initial exploration
of an area that's arguably more practical than academic:
the idea of initiating a concerted effort to remove patterns
that have proven to undermine the security of the web
ecosystem. As such, there are several natural extensions
to the concepts discussed here.
First, the individual problems in Section 2 would
benet from examination and expansion by the security
community, in order to more completely capture the set
of known unsafe behaviors of the web platform.
Second, considering additional criteria for categorizing
web security problems, as outlined in Section 3.2.5, could
identify promising approaches for prioritizing specic im-
provements.
Third, the web is sorely lacking in meaningful mea-
surement studies which would shed light on the real web
compatibility impact of removing existing features. We
strongly encourage the academic community to develop
better models and infrastructure in this space.

--- page 7 ---

Finally, we believe that the most meaningful test of
the ideas presented in this paper is whether a discussion
among browser vendors and the security community can
lead to concrete actions resulting in addressing the issues
discussed in Section 2. We plan to initiate discussion
within the W3C Web Application Security working group
to evaluate and improve upon our approach.
6. Conclusion
You'll get mixed up, of course,
as you already know.
You'll get mixed up
with many strange birds as you go.
So be sure when you step.
Step with care and great tact
and remember that Life's
a Great Balancing Act.
Just never foget to be dexterous and deft.
And never mix up your right foot with your left.
And will you succeed?
Yes! You will, indeed!
(98 and 3/4 percent guaranteed.) [1]
In this paper we started by curmudgeonly complaining
about long-standing unsafe patterns in the web platform,
and explaining the problems they lead to. This provided
us with an opportunity to review the root causes of the
problems and divide them into three different groups
based on the most promising solution: issues that can be
addressed by default by web browsers, problems that can
be solved at the application level by opting out of unsafe
platform behavior, and the middle ground of issues that
require replacing unsafe APIs with new ones, or providing
additional security mechanisms to reduce their risk.
We then proposed a set of practical approaches for
each class of issues, which can serve as a starting point for
active work to address them. Importantly, we outlined an
approach for disabling legacy unsafe behaviors and APIs
based on Document Policy and the Reporting API, and
explained how browsers can help developers deploy these
security improvements in their applications.
We believe that approach lays the groundwork for
meaningful improvements that will allow the web platform
out of the legacy dark corners it explored over the past
three decades.
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