What are allowed characters in cookies?

this one's a quickie:

You might think it should be, but really it's not at all!

What are the allowed characters in both cookie name and value?

According to the ancient Netscape cookie_spec the entire NAME=VALUE string is:

a sequence of characters excluding semi-colon, comma and white space.

So - should work, and it does seem to be OK in browsers I've got here; where are you having trouble with it?

By implication of the above:

  • = is legal to include, but potentially ambiguous. Browsers always split the name and value on the first = symbol in the string, so in practice you can put an = symbol in the VALUE but not the NAME.

What isn't mentioned, because Netscape were terrible at writing specs, but seems to be consistently supported by browsers:

  • either the NAME or the VALUE may be empty strings

  • if there is no = symbol in the string at all, browsers treat it as the cookie with the empty-string name, ie Set-Cookie: foo is the same as Set-Cookie: =foo.

  • when browsers output a cookie with an empty name, they omit the equals sign. So Set-Cookie: =bar begets Cookie: bar.

  • commas and spaces in names and values do actually seem to work, though spaces around the equals sign are trimmed

  • control characters (\x00 to \x1F plus \x7F) aren't allowed

What isn't mentioned and browsers are totally inconsistent about, is non-ASCII (Unicode) characters:

  • in Opera and Google Chrome, they are encoded to Cookie headers with UTF-8;
  • in IE, the machine's default code page is used (locale-specific and never UTF-8);
  • Firefox (and other Mozilla-based browsers) use the low byte of each UTF-16 code point on its own (so ISO-8859-1 is OK but anything else is mangled);
  • Safari simply refuses to send any cookie containing non-ASCII characters.

so in practice you cannot use non-ASCII characters in cookies at all. If you want to use Unicode, control codes or other arbitrary byte sequences, the cookie_spec demands you use an ad-hoc encoding scheme of your own choosing and suggest URL-encoding (as produced by JavaScript's encodeURIComponent) as a reasonable choice.

In terms of actual standards, there have been a few attempts to codify cookie behaviour but none thus far actually reflect the real world.

  • RFC 2109 was an attempt to codify and fix the original Netscape cookie_spec. In this standard many more special characters are disallowed, as it uses RFC 2616 tokens (a - is still allowed there), and only the value may be specified in a quoted-string with other characters. No browser ever implemented the limitations, the special handling of quoted strings and escaping, or the new features in this spec.

  • RFC 2965 was another go at it, tidying up 2109 and adding more features under a ‘version 2 cookies’ scheme. Nobody ever implemented any of that either. This spec has the same token-and-quoted-string limitations as the earlier version and it's just as much a load of nonsense.

  • RFC 6265 is an HTML5-era attempt to clear up the historical mess. It still doesn't match reality exactly but it's much better then the earlier attempts—it is at least a proper subset of what browsers support, not introducing any syntax that is supposed to work but doesn't (like the previous quoted-string).

In 6265 the cookie name is still specified as an RFC 2616 token, which means you can pick from the alphanums plus:

!#$%&'*+-.^_`|~

In the cookie value it formally bans the (filtered by browsers) control characters and (inconsistently-implemented) non-ASCII characters. It retains cookie_spec's prohibition on space, comma and semicolon, plus for compatibility with any poor idiots who actually implemented the earlier RFCs it also banned backslash and quotes, other than quotes wrapping the whole value (but in that case the quotes are still considered part of the value, not an encoding scheme). So that leaves you with the alphanums plus:

!#$%&'()*+-./:<=>?@[]^_`{|}~

In the real world we are still using the original-and-worst Netscape cookie_spec, so code that consumes cookies should be prepared to encounter pretty much anything, but for code that produces cookies it is advisable to stick with the subset in RFC 6265.


In ASP.Net you can use System.Web.HttpUtility to safely encode the cookie value before writing to the cookie and convert it back to its original form on reading it out.

// Encode
HttpUtility.UrlEncode(cookieData);

// Decode
HttpUtility.UrlDecode(encodedCookieData);

This will stop ampersands and equals signs spliting a value into a bunch of name/value pairs as it is written to a cookie.


I think it's generally browser specific. To be on the safe side, base64 encode a JSON object, and store everything in that. That way you just have to decode it and parse the JSON. All the characters used in base64 should play fine with most, if not all browsers.

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