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Analysis of SSL Certificate Reissues and Revocations in the Wake

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Analysis of SSL Certificate Reissues and Revocations in the Wake Analysis of SSL Certificate Reissues and Revocations in the Wake of Heartbleed Liang Zhang David Choffnes Dave Levin Tudor Dumitra¸s Northeastern University Northeastern University University of Maryland University of Maryland [email protected] [email protected] [email protected] [email protected] Alan Mislove Aaron Schulman Christo Wilson Northeastern University Stanford University Northeastern University [email protected] [email protected] [email protected] ABSTRACT Categories and Subject Descriptors Central to the secure operation of a public key infrastruc- C.2.2 [Computer-Communication Networks]: Net- ture (PKI) is the ability to revoke certificates. While much work Protocols; C.2.3 [Computer-Communication Net- of users' security rests on this process taking place quickly, works]: Network Operations; E.3 [Data Encryption]: in practice, revocation typically requires a human to decide Public Key Cryptosystems, Standards to reissue a new certificate and revoke the old one. Thus, having a proper understanding of how often systems admin- istrators reissue and revoke certificates is crucial to under- Keywords standing the integrity of a PKI. Unfortunately, this is typi- Heartbleed; SSL; TLS; HTTPS; X.509; Certificates; Reissue; cally difficult to measure: while it is relatively easy to deter- Revocation; Extended validation mine when a certificate is revoked, it is difficult to determine whether and when an administrator should have revoked. In this paper, we use a recent widespread security vul- 1. INTRODUCTION nerability as a natural experiment. Publicly announced in Secure Sockets Layer (SSL) and Transport Layer Secu- April 2014, the Heartbleed OpenSSL bug, potentially (and rity (TLS)1 are the de-facto standards for securing Internet undetectably) revealed servers' private keys. Administrators transactions such as banking, e-mail and e-commerce. Along of servers that were susceptible to Heartbleed should have with a public key infrastructure (PKI), SSL provides trusted revoked their certificates and reissued new ones, ideally as identities via certificate chains and private communication soon as the vulnerability was publicly announced. via encryption. Central to these guarantees is that private Using a set of all certificates advertised by the Alexa Top 1 keys used in SSL are not compromised by third parties; if Million domains over a period of six months, we explore the so, certificates based on those private keys must be reissued patterns of reissuing and revoking certificates in the wake of and revoked to ensure that malicious third parties cannot Heartbleed. We find that over 73% of vulnerable certificates masquerade as a trusted entity. had yet to be reissued and over 87% had yet to be revoked Importantly, the PKI uses a default-valid model where three weeks after Heartbleed was disclosed. Moreover, our potentially compromised certificates remain valid until their results show a drastic decline in revocations on the weekends, expiration date or until they are revoked. Revocation, how- even immediately following the Heartbleed announcement. ever, is a process that requires manual intervention from cer- These results are an important step in understanding the tificate owners and cooperation from clients that use these manual processes on which users rely for secure, authenti- certificates. As a result, the practical security of the PKI is cated communication. dependent on the speed with which certificate owners and SSL clients update their revocation lists, operations that oc- cur at human timescales (hours or days) instead of computer ones (seconds or minutes). An important open question is: when private keys are compromised, how long are SSL clients exposed to potential attacks? In this paper, we address this question using a re- Permission to make digital or hard copies of all or part of this work for personal or cent widespread security vulnerability as a natural exper- classroom use is granted without fee provided that copies are not made or distributed iment. In mid-April 2014, an OpenSSL security vulnera- for profit or commercial advantage and that copies bear this notice and the full citation bility, Heartbleed, made it possible for attackers to inspect on the first page. Copyrights for components of this work owned by others than the servers' memory contents, thereby potentially (and unde- author(s) must be honored. Abstracting with credit is permitted. To copy otherwise, or tectably) revealing servers' private keys. Administrators of republish, to post on servers or to redistribute to lists, requires prior specific permission and/or a fee. Request permissions from [email protected]. 1 IMC’14, November 5–7, 2014, Vancouver, BC, Canada. TLS is the successor of SSL, but both use the same X.509 Copyright is held by the owner/author(s). Publication rights licensed to ACM. certificates. Throughout the paper, we refer to \SSL clients" ACM 978-1-4503-3213-2/14/11 ...$15.00. and \SSL certificates," but our findings apply equally to http://dx.doi.org/10.1145/2663716.2663758. servers using both protocols. 489 servers that were susceptible to Heartbleed should have op- The remainder of this paper is organized as follows. In erated under the assumption than an attacker had already the next section, we provide background about SSL/TLS, obtained their private keys, and therefore should have re- PKIs, and the Heartbleed vulnerability. In x3 we describe voked their certificates and reissued new ones [5], ideally as our dataset and methodology for extracting valid certifi- soon as the vulnerability was publicly announced. cates and determining Heartbleed vulnerability at servers. The scope of this vulnerability|it is estimated that up to x4 presents the results of our analysis, where we identify the 17% of all HTTPS web servers were vulnerable [22]|makes behavior of certificate reissuing and revocation on a large it an ideal case study for evaluating large-scale properties of dataset of Alexa's Top-1M web sites. We summarize related SSL security in the face of private key compromise. While work in x5 and conclude in x6. previous studies have measured how quickly and thoroughly administrators patch software vulnerabilities [25, 27, 35], we 2. BACKGROUND are, to the best of our knowledge, the first to study admin- Secure Sockets Layer (SSL) and Transport Layer Security istration of certificates in the wake of a vulnerability. In (TLS) offer application-layer confidentiality and integrity, particular, this paper focuses on certificate revocation and and are the basis of the vast majority of secure online com- reissues in response to the public announcement of Heart- munication. Through the use of a public key infrastructure bleed, both in terms of how quickly certificates are reissued (PKI), these protocols also allow clients to authenticate the and whether or not the certificates are eventually revoked. servers with whom they communicate. In this section, we Toward this goal, we make the following key contributions. provide a brief background of SSL/TLS and PKIs relevant to First, we conduct a large-scale measurement study of SSL our study, and describe the recent Heartbleed vulnerability. certificates in the wild using both data collected from public archives and through custom measurements conducted af- 2.1 Certificates ter Heartbleed was publicized. We focus on the Alexa Top A certificate is, at its core, a signed attestation binding 1 Million (Top-1M) domains, for which we find a total of a subject to a public key. Certificates are signed by a Cer- 628,692 valid SSL certificates from 166,124 unique domains. tificate Authority (CA), who in turn has its own certificate, Second, we conduct measurements to determine which and so on, terminating at self-signed root certificates. There servers remain vulnerable to Heartbleed and which ones is a logical chain of certificates|leading from a root certifi- were previously vulnerable but are now patched. We de- cate through zero or more intermediate certificates, to a leaf velop a new SSL implementation fingerprinting technique certificate|wherein the certificate at level i is signed with that is able to determine if a host is running a version of the private key corresponding to the certificate at level i−1, OpenSSL that was vulnerable in the past. We cross-validate with the exception of the self-signed certificate at the root. with direct measurements of the vulnerability (we find our In practice, the topology of certificates can be somewhat technique has a false positive rate of only 1.9%) and con- complex, with CAs signing one another's certificates [17], duct scans to compose a list of previously vulnerable hosts. but such details are not pertinent to the study performed in We find that the most popular web sites were more likely this paper. to have at least one host vulnerable to Heartbleed, likely When a client visits a site that supports, say, HTTPS, because they often have more hosts. that site sends its certificate chain to the client, who verifies Third, we develop novel heuristics to identify which cer- the signatures from leaf to root. If the client can success- tificates have been reissued in direct response to Heartbleed, fully validate each signature, and if the client trusts the root as opposed to other reasons such as certificate expiration or certificate|for instance by checking it against a set of certifi- periodic reissues. This allows us to understand how ad- cates pre-installed in the browser or operating system|then ministrators do (or do not) react to potential private key the client infers that the subject in the leaf certificate truly compromise. We observe that while vulnerable sites with is the owner of the public key. a higher Alexa rank were more likely to reissue their cer- The predominant format of certificates is X.509 [6], which tificates, the vast majority (73.3%) of vulnerable certificates includes considerably more information than just subject had not been reissued fully three weeks after the vulnerabil- and public key, including a unique (for that CA) serial num- ity was announced.
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