I think this is a bug in libsoup. It was originally reported here: http://bugs.debian.org/583131

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I use the following header to set a cookie:
| Set-Cookie: auth="name=blank;sig=MZTJl0eYACEJB6L8ibIm4S6QK1k="; Secure

epiphany lists the cookie with name "auth" and value '"name=blank', aka
it splits it after the first ; within the value. However RFC 2109 and
the referenced 2068 specifies this headers the following way.

set-cookie    = "Set-Cookie:" cookies
cookies       = 1#cookie
cookie        = NAME "=" VALUE *(";" cookie-av)
VALUE         = value
value         = word
word          = token | quoted-string
quoted-string = ( <"> *(qdtext) <"> )
qdtext        = <any TEXT except <">>
TEXT          = <any OCTET except CTLs, but including LWS>

The most identical definition is given in RFC 2965 and 2616 for the
Set-Cookie2 header.

It even transmits it in this broken state (note the missing quote):
| Cookie: I18N_LANGUAGE="de"; auth="name=blank