updated to reflect dropping TLS client behavior specs, no gossip
protocol yet defined,
and no stand-alone Auditors.
comments welcome, but I'm abut to head off on vacation, so I won't be
able to respond immediately
Steve
------
*XX*. Attack Model and Discussion of Detection and Mitigation Options
Certificate mis-issuance may arise in one of several ways. The ways that
CT enables a Subject (or others) to detect and redress mis-issuance
depends on the context and the entities involved in the mis-issuance.
This attack model applies to the Web PKI context. If CT is applied to
other contexts, each will require its own attack model, although most of
the model described here is likely to be applicable.
Certificates are issued by CAs, so the top level differentiation is
whether the CA that mis-issued a certificate did so maliciously or not.
If not, then the next point of differentiation is whether the
mis-issuance was the result of an error, an attack, or malicious
behavior by a CA. In the case of an attack or malicious behavior by a
CA, it is necessary to consider whether the certificate was logged. If
the certificate was logged, it is necessary to consider whether the
log(s) may have conspired with the attacker. Finally, one must consider
the possibility that a third party Monitor might collude with an
attacker (or a malicious CA). Here’s an taxonomy of attacks and
assumptions about the attack environment:
1.CA
a.Non-malicious CA
i.Error
ii.Attack victim
1.Certificate logged
a.Benign log
i.Self-monitor
ii.Benign 3^rd party Monitor
iii.Conspiring 3^rd party Monitor
b.Conspiring log
2.Certificate not logged
b.Malicious CA
i.Certificate logged
1.Benign log
a.Self-monitor
b.Benign 3^rd party Monitor
c.Conspiring 3^rd party Monitor
2.Conspiring log
ii.Certificate not logged
The following sections examine each of these cases, for both syntactic
and semantic mis-issuance. As noted above, the focus here is on the Web
PKI context, although most of the analysis is applicable to other PKI
contexts.
*XX*.1 Non-malicious Web PKI CA context
If a pre-certificate is submitted to a log, prior to issuance of a
certificate, syntactic mis-issuance can (optionally) be detected, and
noted. This will happen only if the log performs syntactic checks in
general, and if the log is capable of performing the checks applicable
to the submitted (pre-) certificate. (A pre-certificate will be logged
even if it fails syntactic validation, thus logging takes precedence
over detection of syntactic mis-issuance.) If syntactic validation
fails, this will be noted in the SCT returned to the CA. Because this
case assumes a non-malicious CA, the CA will remedy the syntactic
problem and re-submit the pre-certificate to a log. In this case
syntactic checking by a log helps avoid issuance of a malformed certificate.
If a certificate is submitted by its Subject, we assume that the Subject
will contact the issuer and request a new certificate if a log indicates
that the certificate failed the applicable syntactic check. Here too
syntactic checking by a log enables a Subject to be informed that its
certificate is malformed and thus may hasten issuance of a replacement
certificate.
If the certificate was submitted by a third party, that party might
contact the Subject or the issuer, but because the party is not the
Subject of the certificate it is not clear how the issuer will respond.
Bottom line: Syntactic mis-issuance of a certificate can be avoided by a
CA if it makes use of logs that are capable of performing these checks
for the types of certificates that are submitted, and if the CA acts on
the feedback it receives. If a CA uses a log that does not perform such
checks, or if the CA requests checking relative to criteria not
supported by the log, then syntactic mis-issuance will not be detected
or avoided by this mechanism. Syntactic mis-issuance can be remedied if
a Subject submits a certificate to a log that performs syntactic checks,
and if the Subject requests the issuer to fix problems detected by the
log. (The issuer is presumed to be willing to re-issue the certificate,
correcting any problems, because the CA is not malicious.)
*XX*.1.1 A CA may issue the certificate to an unauthorized party
(semantic mis-issuance), as a result of an error or because it was the
victim of a social engineering attack. We will refer to such a
certificate as “bogus”. In this case the CA has a record of the bogus
certificate and it is prepared to revoke the bogus certificate once it
has confirmed its error. If the CA is submitting (pre-) certificates for
logging, there will be evidence of the mis-issuance in one or more logs.
If a Monitor is “protecting” the affected Subject, it will detect the
mis-issuance, and will alert the Subject. Because the CA has a record of
the mis-issuance, it should be able to revoke the bogus certificate,
after investigating, based on the information provided by the legitimate
certificate Subject. The presence of an embedded SCT in the bogus
certificate, or an SCT accompanying the bogus certificate is irrelevant
to the mitigation procedure in this case. (See Note 1 below.) Because
the mis-issuance was not malicious, there is no notion of a log operator
or a Monitor conspiring with the CA.
*XX*.1.2 A non-malicious Web PKI CA may be the victim of an undetected
attack (c.f., DigiNotar [cite]) which results in semantic mis-issuance
of a certificate. In this case the CA is not aware of the mis-issuance
and may have no record of the certificate content.
*XX*.1.2.1 The mis-issued certificate may have been submitted to one or
more logs prior to issuance, to acquire an embedded SCT, or
post-issuance to acquire a standalone SCT. In either case, a Monitor
that is protecting the targeted Subject will detect the bogus
certificate and can alert the Subject. The Subject, in turn, will
request the CA to revoke the bogus certificate. In this case, the CA
will make use of the log entry (supplied by the Subject) to determine
the serial number of the mis-issued certificate, and revoke it (after
investigation). (See Notes 1 + 2.) _Since there is no requirement for a
TLS client to reject a certificate when no SCT is provided, the
preferred strategy for an attacker is to not log bogus certificates_.
(See XX.1.2.2 and Note 3.)
*XX*.1.2.2 The bogus certificate may not have been submitted to any
logs. In this case, Monitors will not detect the bogus certificate.
_Since TLS clients are not required to reject a certificate that lacks
(or is not accompanied by) an SCT, or even to notify a user in such
situations, the attacker will not be thwarted in this case._ (See Note 3.)
XX.1.2.3 The bogus certificate may have been submitted to logs that are
conspiring with the attacker. In this case, Monitors will not detect the
bogus certificate because the logs will suppress a bogus certificate log
entry. TLS clients will not reject a bogus certificate in this case,
because it is accompanied by an SCT. In this scenario, unless Monitors
“gossip” to detect conspiring logs, the bogus certificate will not be
detected. _Because there are no requirements for such gossiping, an
attack of this sort can succeed based on the current CT design._
__
XX.1.2.4 If a semantically bogus certificate is submitted to
non-conspiring logs, a Subject performing self-monitoring will be able
to detect the bogus certificate and request revocation. If a Subject
relies on a third-party Monitor, and that Monitor conspires with the
attacker, the Subject will not be alerted and thus will not request
revocation.
*XX*.2 Malicious Web PKI CA context
*XX*.2.1 If a (pre-) certificate is submitted to a (non-conspiring) log,
syntactic mis-issuance can be detected, and noted. This will happen only
if the log performs syntactic checks in general, and if the log is
capable of performing the checks applicable to the submitted
certificate. A (pre-) certificate will be logged even if it fails
syntactic validation, thus logging takes precedence over detection of
syntactic mis-issuance.
Because the CA is presumed to be malicious, the CA may cause the log to
not perform checks, in one of several ways.
1.The CA may assert that the certificate is being issued w/o regard to
any guidelines (the “no guidelines” reserved CCID).
2.The CA may assert a CCID that has not been registered, and thus no log
will be able to perform a check.
3.the CA may check to see which CCIDs a log declares it can check, and
chose a registered CCID that is not checked by the log in question. In
this fashion the CA can prevent the log from performing checks, and the
SCT and log entry will not contain an indication of a failed check.
4.The CA may submit a (pre-) certificate to a log that is known to not
perform any syntactic checks, and thus avoid syntactic checking.
_Since there is no requirement that a TLS client can be configured to
reject a certificate that has not been syntactically checked (as
indicated by the SCT), a malicious CA need not worry about failing a
log-based check. Similarly, _
_since there is no requirement for a TLS client to reject a certificate
that was logged by an operator that does not perform syntactic checks,
the fourth approach noted above will succeed as well_. If a client were
configured to know which versions of certificate types are applicable to
its use of a certificate, the second and third strategies noted above
could be thwarted.
*XX*.2.2 Because the CA is presumed malicious, it may choose to not
submit a certificate to a log. This avoids detection of syntactic
mis-issuance by a log, but it also means there is no SCT for the
certificate. _Since there is no requirement for a TLS client to reject a
certificate that lacks (or is not accompanied by) an SCT, this form of
mis-issuance will succeed. (See Note 3.)_
*XX*.2.3 A malicious CA may submit a certificate to one or more logs
that collude with this CA to not perform syntactic checks, even though
they claim to do so. In this case syntactic mis-issuance will not be
detected by logs. The log entry and the SCT for a syntactically invalid
certificate will assert that the certificate syntax was verified. Unless
Monitors also perform syntactic checks, this form of mis-issuance will
not be thwarted. TLS clients will believe that the certificate has been
syntactically verified.
*XX*.2.4 A malicious CA may semantically mis-issue a certificate that is
syntactically valid. Because it is syntactically valid, logs will not
mark the bogus certificate as failing a syntax check. The semantically
bogus certificate may have been issued because the CA was bribed or was
compelled to issue the certificate. (A CA might be compelled to issue a
bogus certificate by a government agency or a criminal
organization.)This CA might be one or more tiers below a trust anchor
(aka root CA).
*XX*.2.4.1 A bogus certificate may not have been submitted to any logs.
In this case, Monitors will not detect the bogus certificate. _Since
there is no requirement for a TLS client to reject a certificate that
lacks (or is not accompanied by) an SCT, there is no motivation for an
attacker to submit the certificate in this case. (See Note 3.)_
*XX*.2.4.2 A bogus pre-certificate may have been submitted to one or
more logs, to acquire an embedded SCT, a certificate may have been
submitted post-issuance (to acquire a standalone SCT). In either case, a
(non-conspiring) Monitor protecting the targeted Subject will detect a
bogus certificate and alert that Subject. The Subject, in turn, will
request the CA to revoke the bogus certificate. In this case, the CA may
refuse, or substantially delay, to revoke the bogus certificate. It
could make excuses about inadequate proof that the certificate is bogus,
or argue that it cannot quickly revoke the certificate because of local,
legal concerns, etc. In this case, the CT mechanisms have detected
mis-issuance, but the information logged by CT does not help remedy the
problem. (See Note 4.)
*XX*.2.4.3 If a third-party Monitor conspires with the malicious CA,
then it will not notify the targeted Subject when a bogus certificate is
logged. If a Subject performs self-monitoring this sort of attack will
be avoided.
*XX*.2.5 A bogus certificate may have been submitted to one or more
conspiring logs. These logs will issue SCTs, but will hide the log
entries from some or all Monitors. Any Monitor (even a self-Monitor)
from which the log is hiding data will not detect the bogus certificate,
even if it is trying to protect the targeted Subject. If a client
accepts an SCT from a conspiring log, then the client will not reject
the bogus certificate on the basis of a missing SCT. In this case CT
will not detect the bogus certificate.
The audit function is intended to detect logs that conspire to suppress
log entries, based on consistency checking of logs and use of a “gossip”
protocol. It is assumed that Monitors will perform the audit functions
described in Section <*insert #*>.
A Monitor performing an audit function could alert its clients if the
Monitor detects evidence of malfeasant log operation. This would cause
Monitors to avoid using such a log, and clients would reject SCTs
generated by such a log. _Because there is no defined gossip protocol,
CT does not provide the necessary info to detect this form of
mis-issuance. _(See Note 5 below.)
Notes:
1.If a CA submits a bogus certificate to one or more logs, but these
logs are not watched by a Monitor that is protecting the targeted
Subject, CT will not mitigate this type of mis-issuance attack. It is
not clear whether every Monitor MUST offer to track every Subject that
requests protection. Absent such a guarantee, how do Subjects know which
set of Monitors will provide “sufficient” coverage? If a Subject acts as
its own Monitor, this problem is solved for that Subject. It also is not
clear how a Monitor becomes aware of all (relevant?) logs, including
newly created logs. The means by which Monitors become aware of new logs
MUST accommodate self-monitoring by a potentially very large number of
web site operators.
2.A CA being presented with evidence of a bogus certificate, in the form
of a log entry, will need to examine its records to determine if it has
knowledge of the certificate in question. It also will likely require
the targeted Subject to provide assurances that it is the authorized
entity representing the Subject name (subjectAltname) in question. Thus
a Subject should not expect immediate revocation of a contested
certificate. The time frame in which a CA will respond to a revocation
request usually is described in the CPS for the CA. Other certificate
fields and extensions may be of interest for forensic purposes, but are
not required to effect revocation nor to verify that the certificate to
be revoked is bogus, based on applicable criteria. The SCT and log
entry, because each contains a timestamp from a third party, is probably
valuable for forensic purposes (assuming a non-conspiring log operator).
3.If a TLS client were to reject a certificate that lacks an embedded
SCT, or is not accompanied by an SCT transported via the TLS handshake,
this behavior needs to be defined in a way that is compatible with
incremental deployment. Issuing a warning to a (human) user is probably
insufficient, based on experience with warnings displayed for expired
certificates, lack of certificate revocation status information, and
similar errors that violate RFC 5280 path validation rules. _Until a
mechanism is defined that accommodates incremental deployment of this
capability, attackers probably will avoid submitting bogus certificates
to (non-conspiring) logs as a means of evading detection._
4.A targeted Subject might request the parent of a malicious CA to
revoke the certificate of the non-cooperative CA. However, a request of
this sort may be rejected, e.g., because of the potential for
significant collateral damage. A browser might be configured to reject
all certificates issued by the malicious CA, e.g., using a CA hot list
distributed by a browser vendor. However, if the malicious CA has a
sufficient number of legitimate clients, treating all of them as bogus
still represents serious collateral damage. If this specification were
to require that a browser can be configured to reject a specific, bogus
certificate identified by a Monitor, then the bogus certificate could be
rejected in that fashion. This mitigation strategy calls for
communication between Monitors and browsers, or between Monitors and
browser vendors. Such communication has not been specified, i.e., there
are no standard ways to configure a browser to reject individual bogus
certificates based on info provided by an external entity such as a
Monitor. Moreover, the same or another malicious CA could issue new
bogus certificates for the targeted Subject, which would have to be
detected and rejected in this (as yet unspecified) fashion. Thus, for
now, CT does not seem to provide a way to mitigate this form of attack,
even though it provides a basis for detecting such attacks.
5.The combination of a malicious CA and one or more conspiring logs
motivates the definition of an audit function, to detect conspiring
logs. If a Monitor protecting s Subject does not see mis-issued
certificates, it cannot alert the Subject. If one or more SCTs are
present in a certificate, or passed via the TLS handshake, a client has
no way to know that the logged certificate is not visible to Monitors.
Only if Monitors and clients reject certificates that contains SCTs from
conspiring logs (based on info from an audit) will CT be able to deter
use of such logs. Thus the means by which a Monitor performing an audit
function detect such logs, and inform TLS clients must be specified for
this to be effective. Moreover, if a certificate (or TLS handshake)
contains more than one SCT, unless the client verifies all of them if it
is to counter the threat posed by conspiring logs.
Absent a “gossip” protocol that enables Monitors to verify that data
from logs are reported in a consistent fashion, CT does not provide
protection against logs that may conspire with, or are victims of,
attackers effecting certificate mis-issuance. When a gossip protocol is
defined and deployed, it will be necessary to describe how the CT system
will deal with a mis-behaving or compromised log. For example, will
there be a mechanism to alert all TLS clients to reject SCTs issued by
such a log? Absent a description of a mitigation strategy to deal with
mis-behaving or compromised logs, CT cannot ensure detection of
mis-issuance.
Monitors play a critical role in detecting semantic certificate
mis-issuance, for Subjects that have requested monitoring of their
certificates. A monitor (including a Subject performing self-monitoring)
examines logs for certificates associated with one or more Subjects. It
must obtain a list valid certificates for the Subject being monitored,
in a secure manner.
Note: A Monitor must not rely on a CA or RA database for this
information or use certificate discovery protocols; this information
must be acquired by the Monitor based on reference certificates provided
by a Subject. If a Monitor were to rely on a CA or RA database (for the
CA that issued a targeted certificate), the Monitor would not detect
mis-issuance due to malfeasance on the part of that CA or the RA, or due
to compromise of the CA or the RA. If a CA or RA database is used, it
does detect mis-issuance by an unauthorized CA. A Monitor must not rely
on certificate discovery mechanisms to build the list of valid
certificates since such mechanisms might result in mis-issued
certificates being added to the list.
As noted above, Monitors represent another target for adversaries who
wish to effect certificate mis-issuance. If a Monitor is compromised by,
or conspires with, an attacker, it will fail to alert a Subject to a
mis-issued certificate targeting that Subject, as noted above. It is
RECOMMENDED that a Subject request certificate monitoring from multiple
sources to guard against such failures. Operation of a Monitor by a
Subject, on its own behalf, avoids dependence on third party Monitors.
However, the burden of Monitor operation may be viewed as too great for
many web sites, and thus this mode of operation ought not be assumed to
be universal when evaluating protection against Monitor compromise.
A final note: Now that certificate pinning has been approved as a
standard (currently in the RFC Editor’s queue), it is appropriate to
factor in its use by TLS clients. It would appear that pinning will
dramatically reduce the set of TLS clients that are vulnerable to
mis-issuance; a client that pins a certificate for a web site would
reject a bogus certificate without use of any CT mechanisms. The
security considerations section of 6962-bis needs to note this, since
deployment of pinning appears to reduces the need for CT in the Web PKI
context.
_______________________________________________
Trans mailing list
[email protected]
https://www.ietf.org/mailman/listinfo/trans
