Re: AD review of draft-ietf-imapapnd-appendlimit-extension-06

Jayantheesh S B <[email protected]> Fri, 18 Dec 2015 00:08:11 +0000

Newsgroups	gmane.ietf.imapext
Message-ID	<[email protected]>

Thanks Barry.

We have updated the Section 6 with the proposed text. 

Please find the latest one attached. If everyone is fine with this version, I will upload the same.

Regards,
Jay
-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of Barry Leiba
Sent: Thursday, December 17, 2015 6:21 PM
To: Jayantheesh S B
Cc: Narendra Bisht; Alexey Melnikov; [email protected]; S Moonesamy ([email protected]); S Moonesamy ([email protected])
Subject: Re: [imapext] AD review of draft-ietf-imapapnd-appendlimit-extension-06

> Say a server has a limit of 50 MB. Before this extension, an attacker 
> first tries to APPEND 25 MB and it succeeds.
> Then he tries  40MB and that too succeeds. Finally he tries 60 MB to 
> find the limit of server and use that as start of attack. With this 
> extension the attacker can find the limit in no time, making it easy 
> for him to attack.

OK, I see the point now.  It seems a little thin (but, then, the document does already say it is) -- I can just try to append 300MB, using non-synch literal, right from the start.

But perhaps this will be more satisfying:

OLD
   The IMAP APPENDLIMIT extension described in this document can
   conceivably be used to facilitate Denial-of-Service attacks.
   Specifically, the information contained in the APPENDLIMIT capability
   and use of the APPEND command make it somewhat quicker and easier to
   devise an efficacious Denial-of-Service attack.  However, unless
   implementations are very weak, these extensions do not create any
   vulnerability that has not always existed with IMAP.
NEW
   The IMAP APPENDLIMIT extension described in this document can
   conceivably be used to facilitate Denial-of-Service attacks by allowing
   an attacker to home in on a critical value right away.  The attacker
   might want to send a large data block to the server repeatedly,
   forcing the server to process the block, but would not want to limit
   the scope of its attack by filling an actual mailbox with successful
   appends.  Without this extension, the attacker needs to guess: a
   too-small guess results in an appended message that takes up the
   user's quota, while a far-too-large guess might simply cause the
   server to terminate the connection because of suspected abuse.

   But with this extension, the attacker can immediately choose a
   value that's a little too large, but not so much as to trigger an "abuse"
   response, making it easier to mount such an attack.

   To mitigate this extension's input to such an attack, a server might
   take a harder line on message sizes that are above the APPENDLIMIT
   value -- because the client knows the limit and should not even be
   trying to send such commands, a server might consider even a single
   attempt to be abusive, and terminate the IMAP connection straight
   away.
END

How's that work for you?

Barry

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draft-ietf-imapapnd-appendlimit-extension-07.xml (text/xml, 13.1 KB)

<?xml version="1.0" encoding="us-ascii"?>
<!DOCTYPE rfc SYSTEM "rfc2629.dtd" [
        <!ENTITY rfc2119 PUBLIC '' 'http://xml.resource.org/public/rfc/bibxml/reference.RFC.2119.xml'>
        <!ENTITY rfc3501 PUBLIC '' 'http://xml.resource.org/public/rfc/bibxml/reference.RFC.3501.xml'>
        <!ENTITY rfc5234 PUBLIC '' 'http://xml.resource.org/public/rfc/bibxml/reference.RFC.5234.xml'>
        <!ENTITY rfc5819 PUBLIC '' 'http://xml.resource.org/public/rfc/bibxml/reference.RFC.5819.xml'>
        <!ENTITY rfc4469 PUBLIC '' 'http://xml.resource.org/public/rfc/bibxml/reference.RFC.4469.xml'>
		<!ENTITY rfc4549 PUBLIC '' 'http://xml.resource.org/public/rfc/bibxml/reference.RFC.4549.xml'>
		<!ENTITY rfc4466 PUBLIC '' 'http://xml.resource.org/public/rfc/bibxml/reference.RFC.4466.xml'>
]>
<rfc category="std" ipr="trust200902" docName="draft-ietf-imapapnd-appendlimit-extension-07.txt">
	<?xml-stylesheet href="rfc5162_files/rfc2629.htm" type="text/xsl"?>
    <?rfc strict="yes" ?>
    <?rfc toc="yes" ?>
	<?rfc symrefs="yes" ?>
	<?rfc sortrefs="yes"?>
	<?rfc iprnotified="no" ?>
	<?rfc strict="yes" ?>
	<?rfc comments="yes" ?>
	<?rfc inline="yes" ?>
	<?rfc compact="yes"?>
	<?rfc subcompact="no"?>
	<front>
	<title>The IMAP APPENDLIMIT Extension</title>

    <author initials="" surname="Jayantheesh" fullname="Jayantheesh S B">
      <organization>Samsung Electronics America</organization>
      <address>
        <postal>
          <street>685 US Highway 202/206</street>
          <city>Bridgewater</city>
          <region>NJ</region>
          <code>08807</code>
          <country>USA</country>
        </postal>
        <email>[email protected]</email>
      </address>
    </author>
    <author initials="" surname="Narendra" fullname="Narendra Singh Bisht">
		<organization>Samsung Electronics America</organization>
		<address>
		<postal>
			<street>685 US Highway 202/206</street>
			<city>Bridgewater</city>
			<region>NJ</region>
			<code>08807</code>
			<country>USA</country>
		</postal>
		<email>[email protected]</email>
		</address>
	</author>
    <date year="2015"/>
    
    <keyword>IMAP</keyword>
    <keyword>APPENDLIMIT</keyword>

    <abstract>
	<t>
		This document defines an extension to the IMAP service whereby a server can
		inform the client about a maximum mail upload size, allowing the client
		to avoid sending APPEND commands that will fail because of the messages are
		too large.
    </t>
    </abstract>
	
	</front>
  
	<middle>
	<section title="Introduction">
	<t>
		Some IMAP servers have limits for mail upload size, and those limits
		are not published to the email client.  When the email client APPENDs
		a message with huge attachments, using non-synchronizing literals,
		the APPEND fails because of the upload limit, but the client has
		already sent the message data anyway. This results in unnecessary resource
		usage. Especially in the mobile device environment, appending mail with
		huge attachment consumes device resources like device battery power and
		mobile data.
	</t>
	<t>
		The IMAP APPENDLIMIT extension provides an ability to advertise maximum
		upload size allowed by the IMAP server, so that email client knows the
		size limitation beforehand.  By implementing this extension, IMAP server
		side processing of huge attachments above the maximum upload size can
		be avoided.
    </t>
		<section title="Conventions and Terminology">
    <t>
		The key words "MUST", "MUST NOT", "REQUIRED", "SHALL",
		"SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
		and "OPTIONAL" in this document are to be interpreted as
		described in <xref target="RFC2119"/>.
	</t>

	<t>In examples, "C:" and "S:" indicate lines sent by
		the client and server respectively. If a single "C:" or "S:"
		label applies to multiple lines, then the line breaks between
		those lines are for editorial clarity only and are not part
		of the actual protocol exchange.
	</t>	
	</section>
	</section>
	


	<section title="APPENDLIMIT Extension">
	<t>
		An IMAP server that supports APPENDLIMIT extension advertises this by
		including the name APPENDLIMIT in its capability list in authenticated
		state.  The server may also advertise this extension before the user has
		logged in. If this capability is omitted, no information is conveyed about
		the server's fixed maximum mail	upload size. IMAP server can publish the
		APPENDLIMIT capability in two formats. 
	</t>
    <t>
		(a) APPENDLIMIT=<![CDATA[<number>]]>
	</t>
	<t>
		This indicates that the IMAP server has the same upload limit for all
		the mailboxes. The following example, demonstrates the APPENDLIMIT
		capability with same upload limit for all mailboxes.
	</t>
	<t>
	
		C: t1 CAPABILITY<vspace />
		S: * CAPABILITY IMAP4rev1 ID APPENDLIMIT=257890<vspace />
		S: t1 OK foo<vspace />
	
	</t>
	<t>
		(b) APPENDLIMIT
	</t>
	<t>
		The APPENDLIMIT capability without any value indicates that
		the IMAP server supports this extension, and that the client
		will need to discover upload limits for each mailbox, which
		might differ from mailbox to mailbox. The following
		example, demonstrates the APPENDLIMIT capability without any value.
	</t>
	<t>
		C: t1 CAPABILITY<vspace />
		S: * CAPABILITY IMAP4rev1 ID APPENDLIMIT<vspace />
		S: t1 OK foo<vspace />
	</t>
	<t>
		In this case the client SHOULD get APPENDLIMIT value by issuing
		STATUS or LIST command. New response code APPENDLIMIT is added to get
		the mailbox specific limit.  Refer section 5 for response code syntax.
	</t>
	<t>
		IMAP client SHOULD be able to parse both kind of formats. By looking
		at the upload size advertised by the IMAP server, client MUST NOT try
		to APPEND mail more than advertised limit.
	</t>
	</section>
	<section title="Mailbox specific APPENDLIMIT">
	<t>
		An IMAP server can have mailbox-specific APPENDLIMIT values, which
		will not be advertised as part of CAPABILITY response.  The IMAP
		server can publish specific values for each mailbox, and can publish
		"NIL" for a mailbox to convey that there is no APPENDLIMIT for that
		mailbox. The following subsections describe the changes to the STATUS
		and LIST commands in support of	this situation.
	</t>
	
	<section title="STATUS response to the STATUS command">
	<t>
		A new attribute APPENDLIMIT is added to get the limit set by the server
		for a mailbox as part of STATUS command. IMAP client should issue STATUS
		command with APPENDLIMIT item to get the mailbox specific upload value.
		The following example, demonstrates its usage.
	</t>
	<t>
		C: t1 STATUS INBOX (APPENDLIMIT)<vspace />
		S: * STATUS INBOX (APPENDLIMIT 257890)<vspace />
		S: t1 OK STATUS completed<vspace />
	</t>
	<t>
		In the above example, APPENDLIMIT represents the maximum upload size for
		INBOX.
	</t>
	</section>
	<section title="STATUS response to the LIST command">
	<t>
		If the server advertises the LIST-STATUS capability <xref target="RFC5819"/>,
		the	client can issue LIST in combination with the STATUS return option
		to get the mailbox-specific upload value.  The following example, demonstrates
		its usage.
	</t>
	<t>
		C: t1 LIST "" % RETURN (STATUS (APPENDLIMIT))<vspace />
		S: * LIST () "."  "INBOX"<vspace />
		S: * STATUS "INBOX" (APPENDLIMIT 257890)<vspace />
		S: t1 OK List completed.<vspace />
	</t>
	<t>
	   The IMAP server MUST recognize the APPENDLIMIT attribute and include an
	   appropriate STATUS response for each matching mailbox. Refer to
	   <xref target="for_synx" /> for the syntax.
	</t>
	<t>
	    If the server does not support the STATUS return option on the LIST
		command, then the client should use the STATUS command instead.
	</t>
	</section>
	<section title="APPENDLIMIT behaviour">
	<t>
		Computing the APPENDLIMIT should be fast, and need not take ACLs, quotas,
		and other such information into account.  The APPENDLIMIT specifies one part
		of the policy, but an APPEND command can still fail due to issues related
		to ACLs and quotas issues, even if the message	being appended is smaller
		than the APPENDLIMIT.
	</t>
	</section>
	</section>
	
	<section title="APPEND response">
	<t>
		If client uploads a mail which exceeds the maximum upload size set
		to that mailbox, then server SHALL reject the APPEND command with a
		tagged TOOBIG response code.  Refer <xref target="RFC4469"/> Section (4) for various
		APPEND response codes and its handling.
	</t>
	<t>
		Client SHOULD avoid use of non-synchronizing literals <xref target="LITERAL-"/>,
		when maximum upload size supported by the IMAP server is unknown.
		Refer section 4.2.2.3 of <xref target="RFC4549" /> for usage of 
		non-synchronizing literals and its risk for disconnected IMAP clients.
	</t>
	</section>
	<section title="Formal syntax" anchor="for_synx">
	<t>
		The following syntax specification uses the Augmented Backus-Naur
		Form (ABNF) notation as specified in <xref target="RFC5234"/> including the core
		rules in Appendix B.1.  <xref target="RFC3501"/> defines the non-terminals
		"capability" and "status-att", and <xref target="RFC4466"/> defines "status-att-val".

	</t>
	<t>
		All alphabetic characters are case-insensitive.  The use of upper or
		lower case characters to define	token strings is for editorial clarity only.
		Implementations MUST accept these strings in a case-insensitive fashion.
	</t>
<figure>
<artwork><![CDATA[
capability /= "APPENDLIMIT" ["=" number]
            ;; capability is defined in RFC 3501

status-att /= "APPENDLIMIT"
            ;; status-att is defined in RFC 3501

status-att-val /= "APPENDLIMIT" SP (number / nil)
                ;; status-att-val is defined in RFC 4466
]]>
</artwork>
</figure>
	<t>
		The number indicates the fixed maximum message size in octets that
		the server will accept.  An APPENDLIMIT number of 0 indicates the
		server will not accept any APPEND commands at all for the affected mailboxes. 
	</t>
	</section>
	<section title="Security Considerations">
    <t>
		The IMAP APPENDLIMIT extension described in this document can
		conceivably be used to facilitate Denial-of-Service attacks by allowing
		an attacker to home in on a critical value right away.  The attacker
		might want to send a large data block to the server repeatedly,
		forcing the server to process the block, but would not want to limit
		the scope of its attack by filling an actual mailbox with successful
		appends.  Without this extension, the attacker needs to guess: a
		too-small guess results in an appended message that takes up the
		user's quota, while a far-too-large guess might simply cause the
		server to terminate the connection because of suspected abuse.

		But with this extension, the attacker can immediately choose a
		value that's a little too large, but not so much as to trigger an "abuse"
		response, making it easier to mount such an attack.

		To mitigate this extension's input to such an attack, a server might
		take a harder line on message sizes that are above the APPENDLIMIT
		value -- because the client knows the limit and should not even be
		trying to send such commands, a server might consider even a single
		attempt to be abusive, and terminate the IMAP connection straight
		away.

    </t>
	</section>
		
	<section title="IANA Considerations">
	<t>
	   IANA is asked to add "APPENDLIMIT" to the IMAP Capabilities
	   registry, using this document as its reference.
	</t>
    </section>
	<section title="Acknowledgements">
    <t>Thanks to Alexey Melnikov, Dave Cridland, Adrien de Croy,
		 Michael M Slusarz, Timo Sirainen, Chris Newman,
		 Pete Maclean, Jamie Nicolson, Stu Brandt, Bron Gondwana,
		 Arnt Gulbrandsen, Cyrus Daboo, Jan Kundrat, Brandon Long
         and Barry Leiba for providing valuable comments.
	</t>
    </section>
	</middle>
	<back>	
	<references title="Normative References">
	  &rfc4466;
      &rfc2119;
      &rfc3501;
      &rfc5234;
      &rfc5819;
	  &rfc4469;
	<reference anchor="LITERAL-">
		<front>
		<title>IMAP4 non-synchronizing literals, https://tools.ietf.org/html/draft-ietf-imapapnd-rfc2088bis-01 (work in progress)</title>
		<author initials="A." surname="Melnikov" fullname="Alexey Melnikov">
		<organization>Isode Ltd</organization>
		</author>
		<date year="2015" month="October"/>
		<abstract>
		<t>
		The Internet Message Access Protocol (RFC 3501) contains the
		"literal" syntactic construct for communicating strings.  When
		sending a literal from client to server, IMAP requires the client to
		wait for the server to send a command continuation request between
		sending the octet count and the string data.  This document specifies
		an alternate form of literal which does not require this network
		round trip
		</t>
		<t>
		This document specifies 2 IMAP extensions: LITERAL+ and LITERAL-.
		The former allows the alternate form of literals in all IMAP command.
		The latter is the same as LITERAL+, but disallow the alternate form
		in IMAP APPEND, unless they are 4096 bytes or less.
		</t>
		</abstract>
		</front>
	</reference>
	</references>
	
	<references title="Informative References">
	 &rfc4549;
	</references>	
  </back>
</rfc>

draft-ietf-imapapnd-appendlimit-extension-07.txt (text/plain, 14.4 KB)

Network Working Group                                        Jayantheesh
Internet-Draft                                                  Narendra
Intended status: Standards Track             Samsung Electronics America
Expires: June 19, 2016                                 December 17, 2015


                     The IMAP APPENDLIMIT Extension
            draft-ietf-imapapnd-appendlimit-extension-07.txt

Abstract

   This document defines an extension to the IMAP service whereby a
   server can inform the client about a maximum mail upload size,
   allowing the client to avoid sending APPEND commands that will fail
   because of the messages are too large.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on June 19, 2016.

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.




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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Conventions and Terminology . . . . . . . . . . . . . . .   2
   2.  APPENDLIMIT Extension . . . . . . . . . . . . . . . . . . . .   3
   3.  Mailbox specific APPENDLIMIT  . . . . . . . . . . . . . . . .   3
     3.1.  STATUS response to the STATUS command . . . . . . . . . .   4
     3.2.  STATUS response to the LIST command . . . . . . . . . . .   4
     3.3.  APPENDLIMIT behaviour . . . . . . . . . . . . . . . . . .   4
   4.  APPEND response . . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Formal syntax . . . . . . . . . . . . . . . . . . . . . . . .   5
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   6
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   Some IMAP servers have limits for mail upload size, and those limits
   are not published to the email client.  When the email client APPENDs
   a message with huge attachments, using non-synchronizing literals,
   the APPEND fails because of the upload limit, but the client has
   already sent the message data anyway.  This results in unnecessary
   resource usage.  Especially in the mobile device environment,
   appending mail with huge attachment consumes device resources like
   device battery power and mobile data.

   The IMAP APPENDLIMIT extension provides an ability to advertise
   maximum upload size allowed by the IMAP server, so that email client
   knows the size limitation beforehand.  By implementing this
   extension, IMAP server side processing of huge attachments above the
   maximum upload size can be avoided.

1.1.  Conventions and Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

   In examples, "C:" and "S:" indicate lines sent by the client and
   server respectively.  If a single "C:" or "S:" label applies to
   multiple lines, then the line breaks between those lines are for
   editorial clarity only and are not part of the actual protocol
   exchange.




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2.  APPENDLIMIT Extension

   An IMAP server that supports APPENDLIMIT extension advertises this by
   including the name APPENDLIMIT in its capability list in
   authenticated state.  The server may also advertise this extension
   before the user has logged in.  If this capability is omitted, no
   information is conveyed about the server's fixed maximum mail upload
   size.  IMAP server can publish the APPENDLIMIT capability in two
   formats.

   (a) APPENDLIMIT=<number>

   This indicates that the IMAP server has the same upload limit for all
   the mailboxes.  The following example, demonstrates the APPENDLIMIT
   capability with same upload limit for all mailboxes.

   C: t1 CAPABILITY
   S: * CAPABILITY IMAP4rev1 ID APPENDLIMIT=257890
   S: t1 OK foo

   (b) APPENDLIMIT

   The APPENDLIMIT capability without any value indicates that the IMAP
   server supports this extension, and that the client will need to
   discover upload limits for each mailbox, which might differ from
   mailbox to mailbox.  The following example, demonstrates the
   APPENDLIMIT capability without any value.

   C: t1 CAPABILITY
   S: * CAPABILITY IMAP4rev1 ID APPENDLIMIT
   S: t1 OK foo

   In this case the client SHOULD get APPENDLIMIT value by issuing
   STATUS or LIST command.  New response code APPENDLIMIT is added to
   get the mailbox specific limit.  Refer section 5 for response code
   syntax.

   IMAP client SHOULD be able to parse both kind of formats.  By looking
   at the upload size advertised by the IMAP server, client MUST NOT try
   to APPEND mail more than advertised limit.

3.  Mailbox specific APPENDLIMIT

   An IMAP server can have mailbox-specific APPENDLIMIT values, which
   will not be advertised as part of CAPABILITY response.  The IMAP
   server can publish specific values for each mailbox, and can publish
   "NIL" for a mailbox to convey that there is no APPENDLIMIT for that




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Internet-Draft       The IMAP APPENDLIMIT Extension        December 2015


   mailbox.  The following subsections describe the changes to the
   STATUS and LIST commands in support of this situation.

3.1.  STATUS response to the STATUS command

   A new attribute APPENDLIMIT is added to get the limit set by the
   server for a mailbox as part of STATUS command.  IMAP client should
   issue STATUS command with APPENDLIMIT item to get the mailbox
   specific upload value.  The following example, demonstrates its
   usage.

   C: t1 STATUS INBOX (APPENDLIMIT)
   S: * STATUS INBOX (APPENDLIMIT 257890)
   S: t1 OK STATUS completed

   In the above example, APPENDLIMIT represents the maximum upload size
   for INBOX.

3.2.  STATUS response to the LIST command

   If the server advertises the LIST-STATUS capability [RFC5819], the
   client can issue LIST in combination with the STATUS return option to
   get the mailbox-specific upload value.  The following example,
   demonstrates its usage.

   C: t1 LIST "" % RETURN (STATUS (APPENDLIMIT))
   S: * LIST () "."  "INBOX"
   S: * STATUS "INBOX" (APPENDLIMIT 257890)
   S: t1 OK List completed.

   The IMAP server MUST recognize the APPENDLIMIT attribute and include
   an appropriate STATUS response for each matching mailbox.  Refer to
   Section 5 for the syntax.

   If the server does not support the STATUS return option on the LIST
   command, then the client should use the STATUS command instead.

3.3.  APPENDLIMIT behaviour

   Computing the APPENDLIMIT should be fast, and need not take ACLs,
   quotas, and other such information into account.  The APPENDLIMIT
   specifies one part of the policy, but an APPEND command can still
   fail due to issues related to ACLs and quotas issues, even if the
   message being appended is smaller than the APPENDLIMIT.







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4.  APPEND response

   If client uploads a mail which exceeds the maximum upload size set to
   that mailbox, then server SHALL reject the APPEND command with a
   tagged TOOBIG response code.  Refer [RFC4469] Section (4) for various
   APPEND response codes and its handling.

   Client SHOULD avoid use of non-synchronizing literals [LITERAL-],
   when maximum upload size supported by the IMAP server is unknown.
   Refer section 4.2.2.3 of [RFC4549] for usage of non-synchronizing
   literals and its risk for disconnected IMAP clients.

5.  Formal syntax

   The following syntax specification uses the Augmented Backus-Naur
   Form (ABNF) notation as specified in [RFC5234] including the core
   rules in Appendix B.1.  [RFC3501] defines the non-terminals
   "capability" and "status-att", and [RFC4466] defines "status-att-
   val".

   All alphabetic characters are case-insensitive.  The use of upper or
   lower case characters to define token strings is for editorial
   clarity only.  Implementations MUST accept these strings in a case-
   insensitive fashion.

   capability /= "APPENDLIMIT" ["=" number]
               ;; capability is defined in RFC 3501

   status-att /= "APPENDLIMIT"
               ;; status-att is defined in RFC 3501

   status-att-val /= "APPENDLIMIT" SP (number / nil)
                   ;; status-att-val is defined in RFC 4466


   The number indicates the fixed maximum message size in octets that
   the server will accept.  An APPENDLIMIT number of 0 indicates the
   server will not accept any APPEND commands at all for the affected
   mailboxes.

6.  Security Considerations

   The IMAP APPENDLIMIT extension described in this document can
   conceivably be used to facilitate Denial-of-Service attacks by
   allowing an attacker to home in on a critical value right away.  The
   attacker might want to send a large data block to the server
   repeatedly, forcing the server to process the block, but would not
   want to limit the scope of its attack by filling an actual mailbox



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   with successful appends.  Without this extension, the attacker needs
   to guess: a too-small guess results in an appended message that takes
   up the user's quota, while a far-too-large guess might simply cause
   the server to terminate the connection because of suspected abuse.
   But with this extension, the attacker can immediately choose a value
   that's a little too large, but not so much as to trigger an "abuse"
   response, making it easier to mount such an attack.  To mitigate this
   extension's input to such an attack, a server might take a harder
   line on message sizes that are above the APPENDLIMIT value -- because
   the client knows the limit and should not even be trying to send such
   commands, a server might consider even a single attempt to be
   abusive, and terminate the IMAP connection straight away.

7.  IANA Considerations

   IANA is asked to add "APPENDLIMIT" to the IMAP Capabilities registry,
   using this document as its reference.

8.  Acknowledgements

   Thanks to Alexey Melnikov, Dave Cridland, Adrien de Croy, Michael M
   Slusarz, Timo Sirainen, Chris Newman, Pete Maclean, Jamie Nicolson,
   Stu Brandt, Bron Gondwana, Arnt Gulbrandsen, Cyrus Daboo, Jan
   Kundrat, Brandon Long and Barry Leiba for providing valuable
   comments.

9.  References

9.1.  Normative References

   [LITERAL-]
              Melnikov, A., "IMAP4 non-synchronizing literals,
              https://tools.ietf.org/html/draft-ietf-imapapnd-
              rfc2088bis-01 (work in progress)", October 2015.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC3501]  Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
              4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003,
              <http://www.rfc-editor.org/info/rfc3501>.

   [RFC4466]  Melnikov, A. and C. Daboo, "Collected Extensions to IMAP4
              ABNF", RFC 4466, DOI 10.17487/RFC4466, April 2006,
              <http://www.rfc-editor.org/info/rfc4466>.




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   [RFC4469]  Resnick, P., "Internet Message Access Protocol (IMAP)
              CATENATE Extension", RFC 4469, DOI 10.17487/RFC4469, April
              2006, <http://www.rfc-editor.org/info/rfc4469>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <http://www.rfc-editor.org/info/rfc5234>.

   [RFC5819]  Melnikov, A. and T. Sirainen, "IMAP4 Extension for
              Returning STATUS Information in Extended LIST", RFC 5819,
              DOI 10.17487/RFC5819, March 2010,
              <http://www.rfc-editor.org/info/rfc5819>.

9.2.  Informative References

   [RFC4549]  Melnikov, A., Ed., "Synchronization Operations for
              Disconnected IMAP4 Clients", RFC 4549,
              DOI 10.17487/RFC4549, June 2006,
              <http://www.rfc-editor.org/info/rfc4549>.

Authors' Addresses

   Jayantheesh S B
   Samsung Electronics America
   685 US Highway 202/206
   Bridgewater, NJ  08807
   USA

   Email: [email protected]


   Narendra Singh Bisht
   Samsung Electronics America
   685 US Highway 202/206
   Bridgewater, NJ  08807
   USA

   Email: [email protected]












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