RFC 7230, "Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing", June 2014

Note: This RFC has been obsoleted by RFC 9110, RFC 9112

Note: This RFC has been updated by RFC 8615

Errata ID: 5257
Status: Held for Document Update
Type: Technical
Publication Format(s) : TEXT
Reported By: Erwin Pe
Date Reported: 2018-02-07
Held for Document Update by: Francesca Palombini
Date Held: 2021-08-23

Section 7 says:

For compatibility with legacy list rules, a recipient MUST parse and
ignore a reasonable number of empty list elements: enough to handle
common mistakes by senders that merge values, but not so much that
they could be used as a denial-of-service mechanism.  In other words,
a recipient MUST accept lists that satisfy the following syntax:

  #element => [ ( "," / element ) *( OWS "," [ OWS element ] ) ]

  1#element => *( "," OWS ) element *( OWS "," [ OWS element ] )

It should say:

For compatibility with legacy list rules, a recipient MUST parse and
ignore a reasonable number of empty list elements: enough to handle
common mistakes by senders that merge values, but not so much that
they could be used as a denial-of-service mechanism.  In other words,
a recipient MUST accept lists that satisfy the following syntax:

  #element => [ ( ("," OWS element) / element ) *( OWS "," [ OWS 
    element ] ) ]

  1#element => *( "," OWS ) element *( OWS "," [ OWS element ] )

Notes:

With the current ABNF rule for #element, and using token as an element, the construction:

", foobar"

cannot be derived from #element, but can be derived from 1#element. (legacy list rule)
Since #element is meant to be a superset of 1#element, lists derived from 1#element should satisfy the #element rule as well.

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