- This approach allows a single cookie to hold multiple name-value combinations without going over the browser's cookie limit. The downside to creating cookies in this format is that custom parsing is needed to extract the values rather than relying on the much simplier cookie format.
- The format is Netscape format as stated in the man page and this format is: The layout of Netscape's cookies.txt file is such that each line contains one name-value pair. An example cookies.txt file may have an entry that looks like this:.netscape.com TRUE / FALSE 946684799 NETSCAPEID 100103. Each line represents a single piece of stored.
- What is a Cookie? Cookies are essentially pieces of code saved by websites onto the user's web browser when a session is initiated. Cookies have a lot of uses but the most important ones are session management, user personalization, and tracking.
Picture instruments sort n rename pro 2 0 8. Mostly for my own future reference…
The Cookie header is included by the client application with an HTTP request sent to a server, if there is a cookie that has a matching domain and path. Set-Cookie Header. The Set-Cookie response header uses the following format: Set-Cookie: =; =. The Set-Cookie HTTP response header is used to send a cookie from the server to the user agent, so the user agent can send it back to the server later. To send multiple cookies, multiple Set-Cookie headers should be sent in the same response. Warning: Browsers block frontend JavaScript code from accessing the Set Cookie header, as.
This applies to permanent cookies only, as session cookies (cleared when you quit the browser) are not saved to a text file. One complete cookie per line, and each piece is separated by a tab character (‘t' in Python), not a standard space character as shown here:
Domain: the domain that set & can subsequently read the cookie. This could include subdomains, e.g., .google.com means that local.google.com, news.google.com, whatever.google.com could possibly read the cookie, based on the next flag.
Flag: either TRUE or FALSE, whether or not all machines under that domain can read the cookie's information.
Path: the root path under the domain where the cookie is valid. If this is /, the cookie is valid for the entire domain.
Secure Flag: either TRUE or FALSE, whether or not a secure connection (HTTPS) is required to read the cookie.
Expiration Timestamp: the 'Unix Time' in seconds when the cookie is set to expire.
Name: the name of the value that the cookie is storing/saving.
Value: the value
Cookies are small strings of data that are stored directly in the browser. They are a part of the HTTP protocol, defined by the RFC 6265 specification.
Cookies are usually set by a web-server using the response Set-Cookie
HTTP-header. Then, the browser automatically adds them to (almost) every request to the same domain using the Cookie
HTTP-header.
One of the most widespread use cases is authentication:
- Upon sign in, the server uses the
Set-Cookie
HTTP-header in the response to set a cookie with a unique 'session identifier'. - Next time when the request is sent to the same domain, the browser sends the cookie over the net using the
Cookie
HTTP-header. - So the server knows who made the request.
We can also access cookies from the browser, using document.cookie
property.
There are many tricky things about cookies and their options. In this chapter we'll cover them in detail.
Reading from document.cookie
Does your browser store any cookies from this site? Let's see:
The value of document.cookie
consists of name=value
pairs, delimited by ;
. Each one is a separate cookie.
To find a particular cookie, we can split document.cookie
by ;
, and then find the right name. We can use either a regular expression or array functions to do that.
We leave it as an exercise for the reader. Also, at the end of the chapter you'll find helper functions to manipulate cookies.
Writing to document.cookie
We can write to document.cookie
. But it's not a data property, it's an accessor (getter/setter). An assignment to it is treated specially.
A write operation to document.cookie
updates only cookies mentioned in it, but doesn't touch other cookies.
For instance, this call sets a cookie with the name user
and value John
:
If you run it, then probably you'll see multiple cookies. That's because the document.cookie=
operation does not overwrite all cookies. It only sets the mentioned cookie user
.
Technically, name and value can have any characters. To keep the valid formatting, they should be escaped using a built-in encodeURIComponent
function:
There are few limitations:
- The
name=value
pair, afterencodeURIComponent
, should not exceed 4KB. So we can't store anything huge in a cookie. - The total number of cookies per domain is limited to around 20+, the exact limit depends on the browser.
Cookies have several options, many of them are important and should be set.
The options are listed after key=value
, delimited by ;
, like this:
path
path=/mypath
The url path prefix must be absolute. It makes the cookie accessible for pages under that path. By default, it's the current path.
If a cookie is set with path=/admin
, it's visible at pages /admin
and /admin/something
, but not at /home
or /adminpage
.
Usually, we should set path
to the root: path=/
to make the cookie accessible from all website pages.
domain
domain=site.com
A domain defines where the cookie is accessible. In practice though, there are limitations. We can't set any domain.
By default, a cookie is accessible only at the domain that set it. So, if the cookie was set by site.com
, we won't get it at other.com
.
…But what's more tricky, we also won't get the cookie at a subdomain forum.site.com
!
There's no way to let a cookie be accessible from another 2nd-level domain, so other.com
will never receive a cookie set at site.com
.
It's a safety restriction, to allow us to store sensitive data in cookies, that should be available only on one site.
…But if we'd like to allow subdomains like forum.site.com
to get a cookie, that's possible. When setting a cookie at site.com
, we should explicitly set the domain
option to the root domain: domain=site.com
:
For historical reasons, domain=.site.com
(a dot before site.com
) also works the same way, allowing access to the cookie from subdomains. That's an old notation and should be used if we need to support very old browsers.
So, the domain
option allows to make a cookie accessible at subdomains.
expires, max-age
By default, if a cookie doesn't have one of these options, it disappears when the browser is closed. Such cookies are called 'session cookies'
To let cookies survive a browser close, we can set either the expires
or max-age
option.
expires=Tue, 19 Jan 2038 03:14:07 GMT
The cookie expiration date defines the time, when the browser will automatically delete it.
The date must be exactly in this format, in the GMT timezone. We can use date.toUTCString
to get it. For instance, we can set the cookie to expire in 1 day:
If we set expires
to a date in the past, the cookie is deleted.
max-age=3600
Is an alternative to expires
and specifies the cookie's expiration in seconds from the current moment.
If set to zero or a negative value, the cookie is deleted:
secure
secure
The cookie should be transferred only over HTTPS.
By default, if we set a cookie at http://site.com
, then it also appears at https://site.com
and vice versa.
That is, cookies are domain-based, they do not distinguish between the protocols.
With this option, if a cookie is set by https://site.com
, then it doesn't appear when the same site is accessed by HTTP, as http://site.com
. So if a cookie has sensitive content that should never be sent over unencrypted HTTP, the secure
flag is the right thing.
samesite
That's another security attribute samesite
. It's designed to protect from so-called XSRF (cross-site request forgery) attacks. Bbedit 10 5 1 – powerful text and html editor.
To understand how it works and when it's useful, let's take a look at XSRF attacks.
XSRF attack
Imagine, you are logged into the site bank.com
. That is: you have an authentication cookie from that site. Your browser sends it to bank.com
with every request, so that it recognizes you and performs all sensitive financial operations.
Now, while browsing the web in another window, you accidentally come to another site evil.com
. That site has JavaScript code that submits a form
bank.com
with fields that initiate a transaction to the hacker's account.The browser sends cookies every time you visit the site bank.com
, even if the form was submitted from evil.com
. So the bank recognizes you and actually performs the payment.
That's a so-called 'Cross-Site Request Forgery' (in short, XSRF) attack.
Real banks are protected from it of course. All forms generated by bank.com
have a special field, a so-called 'XSRF protection token', that an evil page can't generate or extract from a remote page. It can submit a form there, but can't get the data back. The site bank.com
checks for such token in every form it receives.
Such a protection takes time to implement though. We need to ensure that every form has the required token field, and we must also check all requests.
Enter cookie samesite option
The cookie samesite
option provides another way to protect from such attacks, that (in theory) should not require 'xsrf protection tokens'.
It has two possible values:
samesite=strict
(same assamesite
without value)
A cookie with samesite=strict
is never sent if the user comes from outside the same site.
In other words, whether a user follows a link from their mail or submits a form from evil.com
, or does any operation that originates from another domain, the cookie is not sent.
If authentication cookies have the samesite
option, then a XSRF attack has no chances to succeed, because a submission from evil.com
comes without cookies. So bank.com
will not recognize the user and will not proceed with the payment.
The protection is quite reliable. Only operations that come from bank.com
will send the samesite
cookie, e.g. a form submission from another page at bank.com
.
Although, there's a small inconvenience.
When a user follows a legitimate link to bank.com
, like from their own notes, they'll be surprised that bank.com
does not recognize them. Indeed, samesite=strict
cookies are not sent in that case.
We could work around that by using two cookies: one for 'general recognition', only for the purposes of saying: 'Hello, John', and the other one for>.
Along with the banner, the remote server at ads.com
may set the Set-Cookie
header with a cookie like id=1234
. Such a cookie originates from the ads.com
domain, and will only be visible at ads.com
:
Next time when ads.com
is accessed, the remote server gets the id
cookie and recognizes the user:
What's even more important is, when the user moves from site.com
to another site other.com
, which also has a banner, then ads.com
gets the cookie, as it belongs to ads.com
, thus recognizing the visitor and tracking him as he moves between sites:
Third-party cookies are traditionally used for tracking and ads services, due to their nature. They are bound to the originating domain, so ads.com
can track the same user between different sites, if they all access it.
Naturally, some people don't like being tracked, so browsers allow to disable such cookies.
Also, some modern browsers employ special policies for such cookies:
Fortune Cookie Format
- Safari does not allow third-party cookies at all.
- Firefox comes with a 'black list' of third-party domains where it blocks third-party cookies.
If we load a script from a third-party domain, like </code>, and that script uses <code>document.cookie</code> to set a cookie, then such cookie is not third-party.</p><p>If a script sets a cookie, then no matter where the script came from – the cookie belongs to the domain of the current webpage.</p><h2>Appendix: GDPR</h2><p>This topic is not related to JavaScript at all, just something to keep in mind when setting cookies.</p><p>There's a legislation in Europe called GDPR, that enforces a set of rules for websites to respect the users' privacy. One of these rules is to require an explicit permission for tracking cookies from the user.</p><p>Please note, that's only about tracking/identifying/authorizing cookies.</p><p>So, if we set a cookie that just saves some information, but neither tracks nor identifies the user, then we are free to do it.</p><p>But if we are going to set a cookie with an authentication session or a tracking id, then a user must allow that.</p>
Websites generally have two variants of following GDPR. You must have seen them both already in the web:
Videopad 7 33. If a website wants to set tracking cookies only for authenticated users.
To do so, the registration form should have a checkbox like 'accept the privacy policy' (that describes how cookies are used), the user must check it, and then the website is free to set auth cookies.
If a website wants to set tracking cookies for everyone.
To do so legally, a website shows a modal 'splash screen' for newcomers, and requires them to agree to the cookies. Then the website can set them and let people see the content. That can be disturbing for new visitors though. No one likes to see such 'must-click' modal splash screens instead of the content. But GDPR requires an explicit agreement.
GDPR is not only about cookies, it's about other privacy-related issues too, but that's too much beyond our scope.
Summary
Cookie Format
document.cookie
provides access to cookies
- write operations modify only cookies mentioned in it.
- name/value must be encoded.
- one cookie must not exceed 4KB, 20+ cookies per site (depends on the browser).
Cookie options:
Cookie Formatter
path=/
, by default current path, makes the cookie visible only under that path.domain=site.com
, by default a cookie is visible on the current domain only. If the domain is set explicitly, the cookie becomes visible on subdomains.expires
ormax-age
sets the cookie expiration time. Without them the cookie dies when the browser is closed.secure
makes the cookie HTTPS-only.samesite
forbids the browser to send the cookie with requests coming from outside the site. This helps to prevent XSRF attacks.
Additionally:
Http Cookie Format
- Third-party cookies may be forbidden by the browser, e.g. Safari does that by default.
- When setting a tracking cookie for EU citizens, GDPR requires to ask for permission.