Steve, Looks good to me. See a few comments editorial and substantive in-line below starting at Note 4.
From: Trans [mailto:[email protected]] On Behalf Of Stephen Kent Sent: Tuesday, October 14, 2014 12:43 PM To: [email protected] Subject: [Trans] attack analysis, revised 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 ------- 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 [Santosh] (Add "is" here) 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 [Santosh] (replace "it" with "subject") 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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