Address multiple field value header security discussion in draft 11

[kislyuk]

7.23.  Non-List Field Values

   When an HTTP field occurs multiple times in a single message, these
   values need to be combined into a single one-line string value to be
   included in the HTTP signature base, as described in Section 2.4.
   Not all HTTP fields can be combined into a single value in this way
   and still be a valid value for the field.  For the purposes of
   generating the signature base, the message component value is never
   meant to be read back out of the signature base string or used in the
   application.  Therefore it is considered best practice to treat the
   signature base generation algorithm separately from processing the
   field values by the application, particularly for fields that are
   known to have this property.  If the field values that are being
   signed do not validate, the signed message should also be rejected.

   If an HTTP field allows for unquoted commas within its values,
   combining multiple field values can lead to a situation where two
   semantically different messages produce the same line in a signature
   base.  For example, take the following hypothetical header field with
   an internal comma in its syntax, here used to define two separate
   lists of values:

   Example-Header: value, with, lots
   Example-Header: of, commas

   For this header field, sending all of these values as a single field
   value results in a single list of values:

   Example-Header: value, with, lots, of, commas

   Both of these messages would create the following line in the
   signature base:

   "example-header": value, with, lots, of, commas

   Since two semantically distinct inputs can create the same output in
   the signature base, special care has to be taken when handling such
   values.

   Specifically, the Set-Cookie field [COOKIE] defines an internal
   syntax that does not conform to the List syntax in
   [STRUCTURED-FIELDS].  In particular some portions allow unquoted
   commas, and the field is typically sent as multiple separate field
   lines with distinct values when sending multiple cookies.  When
   multiple Set-Cookie fields are sent in the same message, it is not
   generally possible to combine these into a single line and be able to
   parse and use the results, as discussed in [HTTP], Section 5.3.
   Therefore, all the cookies need to be processed from their separate
   header values, without being combined, while the signature base needs
   to be processed from the special combined value generated solely for
   this purpose.  If the cookie value is invalid, the signed message
   ought to be rejected as this is a possible padding attack as
   described in Section 7.24.

   To deal with this, an application can choose to limit signing of
   problematic fields like Set-Cookie, such as including the field in a
   signature only when a single field value is present and the results
   would be unambiguous.  Similar caution needs to be taken with all
   fields that could have non-deterministic mappings into the signature
   base.  Signers can also make use of the bs parameter to armor such
   fields, as described in Section 2.1.3.

7.24.  Padding Attacks with Multiple Field Values

   Since HTTP field values need to be combined in a single string value
   to be included in the HTTP signature base, as described in
   Section 2.4, it is possible for an attacker to inject an additional
   value for a given field and add this to the signature base of the
   verifier.

   In most circumstances, this causes the signature validation to fail
   as expected, since the new signature base value will not match the
   one used by the signer to create the signature.  However, it is
   theoretically possible for the attacker to inject both a garbage
   value to a field and a desired value to another field in order to
   force a particular input.  This is a variation of the collision
   attack described in Section 7.5, where the attacker accomplishes
   their change in the message by adding to existing field values.

   To counter this, an application needs to validate the content of the
   fields covered in the signature in addition to ensuring that the
   signature itself validates.  With such protections, the attacker's
   padding attack would be rejected by the field value processor, even
   in the case where the attacker could force a signature collision.