6. Examples

6. Examples

This section consists of some example security considerations sections, intended to give the reader a flavor of what's intended by this document.

The first example is a 'retrospective' example, applying the criteria of this document to an existing widely deployed protocol, SMTP. The second example is a good security considerations section clipped from a current protocol.

6.1. SMTP

When RFC 821 was written, Security Considerations sections were not required in RFCs, and none is contained in that document. [RFC 2821] updated RFC 821 and added a detailed security considerations section. We reproduce here the Security Considerations section from that document (with new section numbers). Our comments are indented and prefaced with 'NOTE:'. We also add a number of new sections to cover topics we consider important. Those sections are marked with [NEW] in the section header.

6.1.1. Security Considerations

6.1.1.1. Mail Security and Spoofing

SMTP mail is inherently insecure in that it is feasible for even fairly casual users to negotiate directly with receiving and relaying SMTP servers and create messages that will trick a naive recipient into believing that they came from somewhere else. Constructing such a message so that the "spoofed" behavior cannot be detected by an expert is somewhat more difficult, but not sufficiently so as to be a deterrent to someone who is determined and knowledgeable. Consequently, as knowledge of Internet mail increases, so does the knowledge that SMTP mail inherently cannot be authenticated, or integrity checks provided, at the transport level. Real mail security lies only in end-to-end methods involving the message bodies, such as those which use digital signatures (see [14] and, e.g., PGP [4] or S/MIME [31]).

NOTE: One bad approach to sender authentication is [IDENT] in which the receiving mail server contacts the alleged sender and asks for the username of the sender. This is a bad idea for a number of reasons, including but not limited to relaying, TCP connection hijacking, and simple lying by the origin server. Aside from the fact that IDENT is of low security value, use of IDENT by receiving sites can lead to operational problems. Many sending sites blackhole IDENT requests, thus causing mail to be held until the receiving server's IDENT request times out.

Various protocol extensions and configuration options that provide authentication at the transport level (e.g., from an SMTP client to an SMTP server) improve somewhat on the traditional situation described above. However, unless they are accompanied by careful

handoffs of responsibility in a carefully-designed trust environment, they remain inherently weaker than end-to-end mechanisms which use digitally signed messages rather than depending on the integrity of the transport system.

Efforts to make it more difficult for users to set envelope return path and header "From" fields to point to valid addresses other than their own are largely misguided: they frustrate legitimate applications in which mail is sent by one user on behalf of another or in which error (or normal) replies should be directed to a special address. (Systems that provide convenient ways for users to alter these fields on a per-message basis should attempt to establish a primary and permanent mailbox address for the user so that Sender fields within the message data can be generated sensibly.)

This specification does not further address the authentication issues associated with SMTP other than to advocate that useful functionality not be disabled in the hope of providing some small margin of protection against an ignorant user who is trying to fake mail.

NOTE: We have added additional material on communications security and SMTP in Section 6.1.2 In a final specification, the above text would be edited somewhat to reflect that fact.

6.1.1.2. Blind Copies

Addresses that do not appear in the message headers may appear in the RCPT commands to an SMTP server for a number of reasons. The two most common involve the use of a mailing address as a "list exploder" (a single address that resolves into multiple addresses) and the appearance of "blind copies". Especially when more than one RCPT command is present, and in order to avoid defeating some of the purpose of these mechanisms, SMTP clients and servers SHOULD NOT copy the full set of RCPT command arguments into the headers, either as part of trace headers or as informational or private-extension headers. Since this rule is often violated in practice, and cannot be enforced, sending SMTP systems that are aware of "bcc" use MAY find it helpful to send each blind copy as a separate message transaction containing only a single RCPT command.

There is no inherent relationship between either "reverse" (from MAIL, SAML, etc., commands) or "forward" (RCPT) addresses in the SMTP transaction ("envelope") and the addresses in the headers. Receiving systems SHOULD NOT attempt to deduce such relationships and use them

to alter the headers of the message for delivery. The popular "Apparently-to" header is a violation of this principle as well as a common source of unintended information disclosure and SHOULD NOT be used.

6.1.1.3. VRFY, EXPN, and Security

As discussed in section 3.5, individual sites may want to disable either or both of VRFY or EXPN for security reasons. As a corollary to the above, implementations that permit this MUST NOT appear to have verified addresses that are not, in fact, verified. If a site disables these commands for security reasons, the SMTP server MUST return a 252 response, rather than a code that could be confused with successful or unsuccessful verification.

Returning a 250 reply code with the address listed in the VRFY command after having checked it only for syntax violates this rule. Of course, an implementation that "supports" VRFY by always returning 550 whether or not the address is valid is equally not in conformance.

Within the last few years, the contents of mailing lists have become popular as an address information source for so-called "spammers." The use of EXPN to "harvest" addresses has increased as list administrators have installed protections against inappropriate uses of the lists themselves. Implementations SHOULD still provide support for EXPN, but sites SHOULD carefully evaluate the tradeoffs. As authentication mechanisms are introduced into SMTP, some sites may choose to make EXPN available only to authenticated requesters.

NOTE: It's not clear that disabling VRFY adds much protection, since it's often possible to discover whether an address is valid using RCPT TO.

6.1.1.4. Information Disclosure in Announcements

There has been an ongoing debate about the tradeoffs between the debugging advantages of announcing server type and version (and, sometimes, even server domain name) in the greeting response or in response to the HELP command and the disadvantages of exposing information that might be useful in a potential hostile attack. The utility of the debugging information is beyond doubt. Those who argue for making it available point out that it is far better to actually secure an SMTP server rather than hope that trying to conceal known vulnerabilities by hiding the server's precise identity will provide more protection. Sites are encouraged to evaluate the

tradeoff with that issue in mind; implementations are strongly encouraged to minimally provide for making type and version information available in some way to other network hosts.

6.1.1.5. Information Disclosure in Trace Fields

In some circumstances, such as when mail originates from within a LAN whose hosts are not directly on the public Internet, trace ("Received") fields produced in conformance with this specification may disclose host names and similar information that would not normally be available. This ordinarily does not pose a problem, but sites with special concerns about name disclosure should be aware of it. Also, the optional FOR clause should be supplied with caution or not at all when multiple recipients are involved lest it inadvertently disclose the identities of "blind copy" recipients to others.

6.1.1.6. Information Disclosure in Message Forwarding

As discussed in section 3.4, use of the 251 or 551 reply codes to identify the replacement address associated with a mailbox may inadvertently disclose sensitive information. Sites that are concerned about those issues should ensure that they select and configure servers appropriately.

6.1.1.7. Scope of Operation of SMTP Servers

It is a well-established principle that an SMTP server may refuse to accept mail for any operational or technical reason that makes sense to the site providing the server. However, cooperation among sites and installations makes the Internet possible. If sites take excessive advantage of the right to reject traffic, the ubiquity of email availability (one of the strengths of the Internet) will be threatened; considerable care should be taken and balance maintained if a site decides to be selective about the traffic it will accept and process.

In recent years, use of the relay function through arbitrary sites has been used as part of hostile efforts to hide the actual origins of mail. Some sites have decided to limit the use of the relay function to known or identifiable sources, and implementations SHOULD provide the capability to perform this type of filtering. When mail is rejected for these or other policy reasons, a 550 code SHOULD be used in response to EHLO, MAIL, or RCPT as appropriate.

6.1.1.8. Inappropriate Usage

SMTP itself provides no protection against unsolicited commercial mass e-mail (aka spam). It is extremely difficult to tell a priori whether a given message is spam or not. From a protocol perspective, spam is indistinguishable from other e-mail -- the distinction is almost entirely social and often quite subtle. (For instance, is a message from a merchant from whom you've purchased items before advertising similar items spam?) SMTP spam-suppression mechanisms are generally limited to identifying known spam senders and either refusing to service them or target them for punishment/disconnection. [RFC-2505] provides extensive guidance on making SMTP servers spam-resistant. We provide a brief discussion of the topic here.

The primary tool for refusal to service spammers is the blacklist. Some authority such as [MAPS] collects and publishes a list of known spammers. Individual SMTP servers then block the blacklisted offenders (generally by IP address).

In order to avoid being blacklisted or otherwise identified, spammers often attempt to obscure their identity, either simply by sending a false SMTP identity or by forwarding their mail through an Open Relay -- an SMTP server which will perform mail relaying for any sender. As a consequence, there are now blacklists [ORBS] of open relays as well.

6.1.1.8.1. Closed Relaying

To avoid being used for spam forwarding, many SMTP servers operate as closed relays, providing relaying service only for clients who they can identify. Such relays should generally insist that senders advertise a sending address consistent with their known identity. If the relay is providing service for an identifiable network (such as a corporate network or an ISP's network) then it is sufficient to block all other IP addresses). In other cases, explicit authentication must be used. The two standard choices for this are TLS [STARTTLS] and SASL [SASLSMTP].

6.1.1.8.2. Endpoints

Realistically, SMTP endpoints cannot refuse service to unauthenticated senders. Since the vast majority of senders are unauthenticated, this would break Internet mail interoperability. The exception to this is when the endpoint server should only be

receiving mail from some other server which can itself receive unauthenticated messages. For instance, a company might operate a public gateway but configure its internal servers to only talk to the gateway.

6.1.2. Communications security issues

SMTP itself provides no communications security, and therefore a large number of attacks are possible. A passive attack is sufficient to recover the text of messages transmitted with SMTP. No endpoint authentication is provided by the protocol. Sender spoofing is trivial, and therefore forging email messages is trivial. Some implementations do add header lines with hostnames derived through reverse name resolution (which is only secure to the extent that it is difficult to spoof DNS -- not very), although these header lines are normally not displayed to users. Receiver spoofing is also fairly straight-forward, either using TCP connection hijacking or DNS spoofing. Moreover, since email messages often pass through SMTP gateways, all intermediate gateways must be trusted, a condition nearly impossible on the global Internet.

Several approaches are available for alleviating these threats. In order of increasingly high level in the protocol stack, we have:

SMTP over IPSEC SMTP/TLS S/MIME and PGP/MIME

6.1.2.1. SMTP over IPSEC

An SMTP connection run over IPSEC can provide confidentiality for the message between the sender and the first hop SMTP gateway, or between any pair of connected SMTP gateways. That is to say, it provides channel security for the SMTP connections. In a situation where the message goes directly from the client to the receiver's gateway, this may provide substantial security (though the receiver must still trust the gateway). Protection is provided against replay attacks, since the data itself is protected and the packets cannot be replayed.

Endpoint identification is a problem, however, unless the receiver's address can be directly cryptographically authenticated. Sender identification is not generally available, since generally only the sender's machine is authenticated, not the sender himself. Furthermore, the identity of the sender simply appears in the From header of the message, so it is easily spoofable by the sender. Finally, unless the security policy is set extremely strictly, there is also an active downgrade to cleartext attack.

Another problem with IPsec as a security solution for SMTP is the lack of a standard IPsec API. In order to take advantage of IPsec, applications in general need to be able to instruct the IPsec implementation about their security policies and discover what protection has been applied to their connections. Without a standard API this is very difficult to do portably.

Implementors of SMTP servers or SMTP administrators MUST NOT assume that IPsec will be available unless they have reason to believe that it will be (such as the existence of preexisting association between two machines). However, it may be a reasonable procedure to attempt to create an IPsec association opportunistically to a peer server when mail is delivered. Note that in cases where IPsec is used to provide a VPN tunnel between two sites, this is of substantial security value, particularly to the extent that confidentiality is provided, subject to the caveats mentioned above. Also see [USEIPSEC] for general guidance on the applicability of IPsec.

6.1.2.2. SMTP/TLS

SMTP can be combined with TLS as described in [STARTTLS]. This provides similar protection to that provided when using IPSEC. Since TLS certificates typically contain the server's host name, recipient authentication may be slightly more obvious, but is still susceptible to DNS spoofing attacks. Notably, common implementations of TLS contain a US exportable (and hence low security) mode. Applications desiring high security should ensure that this mode is disabled. Protection is provided against replay attacks, since the data itself is protected and the packets cannot be replayed. [Note: The Security Considerations section of the SMTP over TLS document is quite good and bears reading as an example of how to do things.]

6.1.2.3. S/MIME and PGP/MIME

S/MIME and PGP/MIME are both message oriented security protocols. They provide object security for individual messages. With various settings, sender and recipient authentication and confidentiality may be provided. More importantly, the identification is not of the sending and receiving machines, but rather of the sender and recipient themselves. (Or, at least, of cryptographic keys corresponding to the sender and recipient.) Consequently, end-to-end security may be obtained. Note, however, that no protection is provided against replay attacks. Note also that S/MIME and PGP/MIME generally provide identifying marks for both sender and receiver. Thus even when confidentiality is provided, traffic analysis is still possible.

6.1.3. Denial of Service

None of these security measures provides any real protection against denial of service. SMTP connections can easily be used to tie up system resources in a number of ways, including excessive port consumption, excessive disk usage (email is typically delivered to disk files), and excessive memory consumption (sendmail, for instance, is fairly large, and typically forks a new process to deal with each message.)

If transport- or application-layer security is used for SMTP connections, it is possible to mount a variety of attacks on individual connections using forged RSTs or other kinds of packet injection.

6.2. VRRP

The second example is from VRRP, the Virtual Router Redundance Protocol ([VRRP]). We reproduce here the Security Considerations section from that document (with new section numbers). Our comments are indented and prefaced with 'NOTE:'.

6.2.1. Security Considerations

VRRP is designed for a range of internetworking environments that may employ different security policies. The protocol includes several authentication methods ranging from no authentication, simple clear text passwords, and strong authentication using IP Authentication with MD5 HMAC. The details on each approach including possible attacks and recommended environments follows.

Independent of any authentication type VRRP includes a mechanism (setting TTL=255, checking on receipt) that protects against VRRP packets being injected from another remote network. This limits most vulnerabilities to local attacks.

NOTE: The security measures discussed in the following sections only provide various kinds of authentication. No confidentiality is provided at all. This should be explicitly described as outside the scope.

6.2.1.1. No Authentication

The use of this authentication type means that VRRP protocol exchanges are not authenticated. This type of authentication SHOULD only be used in environments were there is minimal security risk and little chance for configuration errors (e.g., two VRRP routers on a LAN).

6.2.1.2. Simple Text Password

The use of this authentication type means that VRRP protocol exchanges are authenticated by a simple clear text password.

This type of authentication is useful to protect against accidental misconfiguration of routers on a LAN. It protects against routers inadvertently backing up another router. A new router must first be configured with the correct password before it can run VRRP with another router. This type of authentication does not protect against hostile attacks where the password can be learned by a node snooping VRRP packets on the LAN. The Simple Text Authentication combined with the TTL check makes it difficult for a VRRP packet to be sent from another LAN to disrupt VRRP operation.

This type of authentication is RECOMMENDED when there is minimal risk of nodes on a LAN actively disrupting VRRP operation. If this type of authentication is used the user should be aware that this clear text password is sent frequently, and therefore should not be the same as any security significant password.

NOTE: This section should be clearer. The basic point is that no authentication and Simple Text are only useful for a very limited threat model, namely that none of the nodes on the local LAN are hostile. The TTL check prevents hostile nodes off-LAN from posing as valid nodes, but nothing stops hostile nodes on-LAN from impersonating authorized nodes. This is not a particularly realistic threat model in many situations. In particular, it's extremely brittle: the compromise of any node the LAN allows reconfiguration of the VRRP nodes.

6.2.1.3. IP Authentication Header

The use of this authentication type means the VRRP protocol exchanges are authenticated using the mechanisms defined by the IP Authentication Header [AH] using [HMAC]. This provides strong protection against configuration errors, replay attacks, and packet corruption/modification.

This type of authentication is RECOMMENDED when there is limited control over the administration of nodes on a LAN. While this type of authentication does protect the operation of VRRP, there are other types of attacks that may be employed on shared media links (e.g., generation of bogus ARP replies) which are independent from VRRP and are not protected.

NOTE: It's a mistake to have AH be a RECOMMENDED in this context. Since AH is the only mechanism that protects VRRP against attack from other nodes on the same LAN, it should be a MUST for cases where there are untrusted nodes on the same network. In any case, AH should be a MUST implement.

NOTE: There's an important piece of security analysis that's only hinted at in this document, namely the cost/benefit tradeoff of VRRP authentication.

[The rest of this section is NEW material] The threat that VRRP authentication is intended to prevent is an attacker arranging to be the VRRP master. This would be done by joining the group (probably multiple times), gagging the master and then electing oneself master. Such a node could then direct traffic in arbitrary undesirable ways.

However, it is not necessary for an attacker to be the VRRP master to do this. An attacker can do similar kinds of damage to the network by forging ARP packets or (on switched networks) fooling the switch VRRP authentication offers no real protection against these attacks.

Unfortunately, authentication makes VRRP networks very brittle in the face of misconfiguration. Consider what happens if two nodes are configured with different passwords. Each will reject messages from the other and therefore both will attempt to be master. This creates substantial network instability.

This set of cost/benefit tradeoffs suggests that VRRP authentication is a bad idea, since the incremental security benefit is marginal but the incremental risk is high. This judgment should be revisited if the current set of non-VRRP threats are removed.