The text below is my current take on an attack analysis for CT, revised to
reflect the goal of a broader, long-term scope, but emphasizing the current,
Web PKI-centric focus of the charter. It also has been changed based on
several
comments from folks over the past week or so. I think this analysis
belongs in
6962-bis. It raises some questions that need to be answered as revisions
proceed,
and suggests some topics that should be discussed in the security
considerations section.
Comments (especially revised text) welcome,
Steve
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*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 accident or an attack. In the case of
an attack, it is necessary to consider whether the certificate was
logged, and if so, whether the log conspired with the attacker.
If the CA acted maliciously, it is necessary to ask whether it logged a
mis-issued certificate. If so, then one must consider whether the CA
conspired with one or more log operators, or conspired with one of more
Monitors (for not self-monitoring Subjects).
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 A non-malicious Web PKI CA
If a certificate is submitted to a log, either prior to issuance (a
pre-certificate), or post-issuance, 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 (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, and
because the CA is non-malicious, it will remedy the syntactic problem
and re-submit the (pre-) certificate to a log again.
Thus syntactic mis-issuance 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, assuming that it 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.
*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 targeted Subject, it will detect the
mis-issuance, and will alert the Subject. Because the CA has a record of
the mis-issuance, it should 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 alert it. 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.) If TLS clients reject a certificate when no SCT is
provided, this might motivate the attacker to log the bogus
certificates, which helps justify such client behavior. (See 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. If
TLS clients reject a certificate that lacks (or is not accompanied by)
an SCT, the attacker will 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
or Auditors "gossip" to detect conspiring logs, the bogus certificate
will not be detected.
*XX*.2 A Malicious Web PKI CA
*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 two ways. The CA may assert that the
certificate is being issued w/o regard to any guidelines (the "no
guidelines" reserved CCID), or it may assert a CCID that has not been
registered. 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. Alternatively, the CA may submit a (pre-) certificate to a
log that is known to not perform syntactic checks, and thus avoid
syntactic checking.
If a client rejects a certificate that has not been syntactically
checked (as indicated by the SCT), the first approach to subverting
syntax checking will fail. If a client rejects a certificate that was
logged by an operator that does not perform syntactic checks, the third
approach noted above will fail. If a client is configured to know which
versions of CABF guidelines exist, the second approach to avoiding
syntactic checking also can 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. If clients reject a certificate that lacks (or is not
accompanied by) an SCT, this form of mis-issuance will be thwarted in
this case. (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 the 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 in this case. 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
reject the bogus certificate. This situation might result because the CA
was bribed or was compelled to issue the bogus 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. If clients
reject a certificate that lacks (or is not accompanied by) an SCT, the
attacker will be thwarted in this case. (See Note 3.)
*XX*.2.4.2 A bogus 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 protecting the
targeted Subject will detect a bogus certificate and alert the targeted
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*.2.5 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 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
semantically mis-issued certificate.
Auditors are intended to detect logs that conspire to suppress log
entries, based on consistency checking of logs and use of a "gossip"
protocol. Until such checks are clearly defined and a gossip protocol is
defined and deployed, CT does not provide the necessary info to detect
this form of mis-issuance. When Auditors can detect a conspiring log,
Monitors and clients can be alerted to it (how?). This would cause
Monitors to avoid using such a log, and clients would reject SCTs
generated by such a log. (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 is not
clear how a Monitor becomes aware of all (relevant?) logs, including
newly added 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 is 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 should not be expected to submit bogus
certificates to (non-conspiring) logs.
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 one can configure a
browser 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 no standard ways to configure a
browser to reject individual bogus certificates based on info provided
by 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 use of Auditors, 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 Audotirs) will CT be able to deter use of such logs.
Thus the means by which Auditors detect such logs, and inform TLS
clients and Monitors must be specified. Moreover, if a certificate (or
TLS handshake) contains more than one SCT, it appears that the client
MUST verify all of them if it is to counter the threat posed by
conspiring logs.
Absent a "gossip" protocol that enables Auditors to verify that data
from logs are reported in a consistent fashion to many (all?) Monitors,
CT does not provide protection against logs that may conspire with, or
be 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. 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 it has obtained and installed. 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.
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. This suggests that a Subject SHOULD
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, but 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.
Now that certificate pinning has been approved as a standard, 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 even without use of any CT
mechanisms. The security considerations section of 6962-bis needs to
note this, as it suggests that deployment of pinning reduces the need
for CT.
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