Internet Engineering Task Force (IETF) R. Gellens
Request for Comments: 6856 QUALCOMM Incorporated
Obsoletes: 5721 C. Newman
Category: Standards Track Oracle
ISSN: 2070-1721 J. Yao
March 2013 Post Office Protocol Version 3 (POP3) Support for UTF-8
This specification extends the Post Office Protocol version 3 (POP3)
to support international strings encoded in UTF-8 in usernames,
passwords, mail addresses, message headers, and protocol-level text
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
This document forms part of the Email Address Internationalization
protocols described in the Email Address Internationalization
Framework document [RFC6530]. As part of the overall Email Address
Internationalization work, email messages can be transmitted and
delivered containing a Unicode string encoded in UTF-8 in the header
and/or body, and maildrops that are accessed using POP3 [RFC1939]
might natively store Unicode characters.
This specification extends POP3 using the POP3 extension mechanism
[RFC2449] to permit un-encoded UTF-8 [RFC3629] in headers and bodies
(e.g., transferred using 8-bit content-transfer-encoding) as
described in "Internationalized Email Headers" [RFC6532]. It also
adds a mechanism to support login names and passwords containing a
UTF-8 string (see Section 1.1 below), a mechanism to support UTF-8
strings in protocol-level response strings, and the ability to
negotiate a language for such response strings.
This specification also adds a new response code to indicate that a
message was not delivered because it required UTF-8 mode (as
discussed in Section 2) and the server was unable or unwilling to
create and deliver a surrogate form of the message as discussed in
Section 7 of "IMAP Support for UTF-8" [RFC6855].
This specification replaces an earlier, experimental, approach to the
same problem [RFC5721].
1.1. Conventions Used in This Document
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 "Key words for use in
RFCs to Indicate Requirement Levels" [RFC2119].
The terms "UTF-8 string" or "UTF-8 character" are used to refer to
Unicode characters, which may or may not be members of the ASCII
repertoire, encoded in UTF-8 [RFC3629], a standard Unicode encoding
form. All other specialized terms used in this specification are
defined in the Email Address Internationalization framework document.
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
Note that examples always use ASCII characters due to limitations of
the RFC format; otherwise, some examples for the "LANG" command would
have appeared incorrectly.
2. "UTF8" Capability
This specification adds a new POP3 Extension [RFC2449] capability
response tag and command to specify support for header field
information outside the ASCII repertoire. The capability tag and new
command and functionality are described below.
Arguments with CAPA tag:
Standard commands affected:
USER, PASS, APOP, LIST, TOP, RETR
Announced states / possible differences:
both / no
Commands valid in states:
This capability adds the "UTF8" command to POP3. The "UTF8" command
switches the session from the ASCII-only mode of POP3 [RFC1939] to
UTF-8 mode. The UTF-8 mode means that all messages transmitted
between servers and clients are UTF-8 strings, and both servers and
clients can send and accept UTF-8 strings.
2.1. The "UTF8" Command
The "UTF8" command enables UTF-8 mode. The "UTF8" command has no
UTF-8 mode has no effect on messages in an ASCII-only maildrop.
Messages in native Unicode maildrops can be encoded in UTF-8 using
internationalized headers [RFC6532], in 8bit
content-transfer-encoding (see Section 2.8 of MIME [RFC2045]), in
ASCII, or in any combination of these options. In UTF-8 mode, if the
character encoding format of maildrops is UTF-8 or ASCII, the
messages are sent to the client as is; if the character encoding
format of maildrops is a format other than UTF-8 or ASCII, the
messages' encoding format SHOULD be converted to be UTF-8 before they
are sent to the client. When UTF-8 mode has not been enabled,
character strings outside the ASCII repertoire MUST NOT be sent to
the client as is. If a client requests a UTF-8 message when UTF-8
mode is not enabled, the server MUST either send the client a
surrogate message that complies with unextended POP and Internet Mail
Format without UTF-8 mode support, or fail the request with an -ERR
response. See Section 7 of "IMAP Support for UTF-8" [RFC6855] for
information about creating a surrogate message and for a discussion
of potential issues. Section 5 of this document discusses "UTF8"
response codes. The server MAY respond to the "UTF8" command with an
Note that even in UTF-8 mode, MIME binary content-transfer-encoding
as defined in Section 6.2 of MIME [RFC2045] is still not permitted.
MIME 8bit content-transfer-encoding (8BITMIME) [RFC6152] is obviously
The octet count (size) of a message reported in a response to the
"LIST" command SHOULD match the actual number of octets sent in a
"RETR" response (not counting byte-stuffing). Sizes reported
elsewhere, such as in "STAT" responses and non-standardized,
free-form text in positive status indicators (following "+OK") need
not be accurate, but it is preferable if they are.
Normal operation for maildrops that natively support non-ASCII
characters will be for both servers and clients to support the
extension discussed in this specification. Upgrading both clients
and servers is the only fully satisfactory way to support the
capabilities offered by the "UTF8" extension and SMTPUTF8 mail more
generally. Servers must, however, anticipate the possibility of a
client attempting to access a message that requires this extension
without having issued the "UTF8" command. There are no completely
satisfactory responses for this case other than upgrading the client
to support this specification. One solution, unsatisfactory because
the user may be confused by being able to access the message through
some means and not others, is that a server MAY choose to reject the
command to retrieve the message as discussed in Section 5. Other
alternatives, including the possibility of creating and delivering a
surrogate form of the message, are discussed in Section 7 of "IMAP
Support for UTF-8" [RFC6855].
Clients MUST NOT issue the "STLS" command [RFC2595] after issuing
UTF8; servers MAY (but are not required to) enforce this by rejecting
with an -ERR response an "STLS" command issued subsequent to a
successful "UTF8" command. (Because this is a protocol error as
opposed to a failure based on conditions, an extended response code
[RFC2449] is not specified.)
2.2. USER Argument to "UTF8" Capability
If the USER argument is included with this capability, it indicates
that the server accepts UTF-8 usernames and passwords.
Servers that include the USER argument in the "UTF8" capability
response SHOULD apply SASLprep [RFC4013] or one of its Standards
Track successors to the arguments of the "USER" and "PASS" commands.
A client or server that supports APOP and permits UTF-8 in usernames
or passwords MUST apply SASLprep or one of its Standards Track
successors to the username and password used to compute the APOP
When applying SASLprep, servers MUST reject UTF-8 usernames or
passwords that contain a UTF-8 character listed in Section 2.3 of
SASLprep. When applying SASLprep to the USER argument, the PASS
argument, or the APOP username argument, a compliant server or client
MUST treat them as a query string [RFC3454]. When applying SASLprep
to the APOP password argument, a compliant server or client MUST
treat them as a stored string [RFC3454].
If the server includes the USER argument in the UTF8 capability
response, the client MAY use UTF-8 characters with a "USER", "PASS",
or "APOP" command; the client MAY do so before issuing the "UTF8"
command. Clients MUST NOT use UTF-8 characters when authenticating
if the server did not include the USER argument in the UTF8
The server MUST reject UTF-8 usernames or passwords that fail to
comply with the formal syntax in UTF-8 [RFC3629].
Use of UTF-8 strings in the "AUTH" command is governed by the POP3
SASL [RFC5034] mechanism.
3. "LANG" Capability
This document adds a new POP3 extension [RFC2449] capability response
tag to indicate support for a new command: "LANG".
The capability tag and new command are described below.
Arguments with CAPA tag:
Standard commands affected:
Announced states / possible differences:
both / no
Commands valid in states:
POP3 allows most +OK and -ERR server responses to include human-
readable text that, in some cases, might be presented to the user.
But that text is limited to ASCII by the POP3 specification
[RFC1939]. The "LANG" capability and command permit a POP3 client to
negotiate which language the server uses when sending human-readable
The "LANG" command requests that human-readable text included in all
subsequent +OK and -ERR responses be localized to a language matching
the language range argument (the "basic language range" as described
by the "Matching of Language Tags" [RFC4647]). If the command
succeeds, the server returns a +OK response followed by a single
space, the exact language tag selected, and another space. Human-
readable text in the appropriate language then appears in the rest of
the line. This, and subsequent protocol-level human-readable text,
is encoded in the UTF-8 charset.
If the command fails, the server returns an -ERR response and
subsequent human-readable response text continues to use the language
that was previously used.
If the client issues a "LANG" command with the special "*" language
range argument, it indicates a request to use a language designated
as preferred by the server administrator. The preferred language MAY
vary based on the currently active user.
If no argument is given and the POP3 server issues a positive
response, that response will usually consist of multiple lines.
After the initial +OK, for each language tag the server supports, the
POP3 server responds with a line for that language. This line is
called a "language listing".
In order to simplify parsing, all POP3 servers are required to use a
certain format for language listings. A language listing consists of
the language tag [RFC5646] of the message, optionally followed by a
single space and a human-readable description of the language in the
language itself, using the UTF-8 charset. There is no specific order
to the listing of languages; the order may depend on configuration or
Examples for "LANG" capability usage are shown below.
Note that some examples do not include the correct character
accents due to limitations of the RFC format.
C: USER karen
S: +OK Hello, karen
C: PASS password
S: +OK karen's maildrop contains 2 messages (320 octets)
Client requests deprecated MUL language [ISO639-2]. Server
replies with -ERR response.
C: LANG MUL
S: -ERR invalid language MUL
A LANG command with no parameters is a request for
a language listing.
S: +OK Language listing follows:
S: en English
S: en-boont English Boontling dialect
S: de Deutsch
S: it Italiano
S: es Espanol
S: sv Svenska
A request for a language listing might fail.
S: -ERR Server is unable to list languages
Once the client selects the language, all responses will be in
that language, starting with the response to the "LANG" command.
C: LANG es
S: +OK es Idioma cambiado
If a server returns an -ERR response to a "LANG" command
that specifies a primary language, the current language
for responses remains in effect.
C: LANG uga
S: -ERR es Idioma <<UGA>> no es conocido
C: LANG sv
S: +OK sv Kommandot "LANG" lyckades
C: LANG *
S: +OK es Idioma cambiado
4. Non-ASCII Character Maildrops
When a POP3 server uses a native non-ASCII character maildrop, it is
the responsibility of the server to comply with the POP3 base
specification [RFC1939] and Internet Message Format [RFC5322] when
not in UTF-8 mode. When the server is not in UTF-8 mode and the
message requires that mode, requests to download the message MAY be
rejected (as specified in the next section) or the various
alternatives outlined in Section 2.1 above, including creation and
delivery of surrogates for the original message, MAY be considered.
5. "UTF8" Response Code
Per "POP3 Extension Mechanism" [RFC2449], this document adds a new
response code: UTF8, described below.
Complete response code:
Valid for responses:
Valid for commands:
LIST, TOP, RETR
Response code meaning and expected client behavior:
The "UTF8" response code indicates that a failure is due to a
request for message content that contains a UTF-8 string when the
client is not in UTF-8 mode.
The client MAY reissue the command after entering UTF-8 mode.
6. IANA Considerations
Sections 2 and 3 of this specification update two capabilities
("UTF8" and "LANG") in the POP3 capability registry [RFC2449].
Section 5 of this specification adds one new response code ("UTF8")
to the POP3 response codes registry [RFC2449].
7. Security Considerations
The security considerations of UTF-8 [RFC3629], SASLprep [RFC4013],
and the Unicode Format for Network Interchange [RFC5198] apply to
this specification, particularly with respect to use of UTF-8 strings
in usernames and passwords.
The "LANG *" command might reveal the existence and preferred
language of a user to an active attacker probing the system if the
active language changes in response to the "USER", "PASS", or "APOP"
commands prior to validating the user's credentials. Servers are
strongly advised to implement a configuration to prevent this
It is possible for a man-in-the-middle attacker to insert a "LANG"
command in the command stream, thus, making protocol-level diagnostic
responses unintelligible to the user. A mechanism to protect the
integrity of the session can be used to defeat such attacks. For
example, a client can issue the "STLS" command [RFC2595] before
issuing the "LANG" command.
As with other internationalization upgrades, modifications to server
authentication code (in this case, to support non-ASCII strings) need
to be done with care to avoid introducing vulnerabilities (for
example, in string parsing or matching). This is particularly
important if the native databases or mailstore of the operating
system use some character set or encoding other than Unicode in
8.1. Normative References
[RFC1939] Myers, J. and M. Rose, "Post Office Protocol - Version
3", STD 53, RFC 1939, May 1996.
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, November 1996.
[RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions)
Part Three: Message Header Extensions for Non-ASCII
Text", RFC 2047, November 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2449] Gellens, R., Newman, C., and L. Lundblade, "POP3
Extension Mechanism", RFC 2449, November 1998.
[RFC3454] Hoffman, P. and M. Blanchet, "Preparation of
Internationalized Strings ("stringprep")", RFC 3454,
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
[RFC4013] Zeilenga, K., "SASLprep: Stringprep Profile for User
Names and Passwords", RFC 4013, February 2005.
[RFC4647] Phillips, A. and M. Davis, "Matching of Language Tags",
BCP 47, RFC 4647, September 2006.
[RFC5198] Klensin, J. and M. Padlipsky, "Unicode Format for Network
Interchange", RFC 5198, March 2008.
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
[RFC5646] Phillips, A. and M. Davis, "Tags for Identifying
Languages", BCP 47, RFC 5646, September 2009.
[RFC6152] Klensin, J., Freed, N., Rose, M., and D. Crocker, "SMTP
Service Extension for 8-bit MIME Transport", STD 71,
RFC 6152, March 2011.
[RFC6530] Klensin, J. and Y. Ko, "Overview and Framework for
Internationalized Email", RFC 6530, February 2012.
[RFC6532] Yang, A., Steele, S., and N. Freed, "Internationalized
Email Headers", RFC 6532, February 2012.
[RFC6855] Resnick, P., Newman, C., and S. Shen, "IMAP Support for
UTF-8", RFC 6855, March 2013.
8.2. Informative References
[ISO639-2] International Organization for Standardization, "ISO
639-2:1998. Codes for the representation of names of
languages -- Part 2: Alpha-3 code", October 1998.
[RFC2231] Freed, N. and K. Moore, "MIME Parameter Value and Encoded
Character Sets, Languages, and Continuations", RFC 2231,
[RFC2595] Newman, C., "Using TLS with IMAP, POP3 and ACAP",
RFC 2595, June 1999.
[RFC5034] Siemborski, R. and A. Menon-Sen, "The Post Office
Protocol (POP3) Simple Authentication and Security Layer
(SASL) Authentication Mechanism", RFC 5034, July 2007.
[RFC5721] Gellens, R. and C. Newman, "POP3 Support for UTF-8",
RFC 5721, February 2010.
Appendix A. Design Rationale
This non-normative section discusses the reasons behind some of the
design choices in this specification.
Due to interoperability problems with the MIME Message Header
Extensions [RFC2047] and limited deployment of the extended MIME
parameter encodings [RFC2231], it is hoped these 7-bit encoding
mechanisms can be deprecated in the future when UTF-8 header support
The USER capability (Section 2.2) and hence the upgraded "USER"
command and additional support for non-ASCII credentials, are
optional because the implementation burden of SASLprep [RFC4013] is
not well understood, and mandating such support in all cases could
negatively impact deployment.
Appendix B. Acknowledgments
Thanks to John Klensin, Joseph Yee, Tony Hansen, Alexey Melnikov, and
other Email Address Internationalization working group participants
who provided helpful suggestions and interesting debate that improved
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