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Analysis of SSL Certificate Reissues and Revocations in the Wake of Heartbleed

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Heartbleed bug, illustration

Credit: Nimrod Flores

A properly managed public key infrastructure (PKI) is critical to ensure secure communication on the Internet. Surprisingly, some of the most important administrative steps—in particular, reissuing new X.509 certificates and revoking old ones—are manual and remained unstudied, largely because it is difficult to measure these manual processes at scale.

We use Heartbleed, a widespread OpenSSL vulnerability from 2014, as a natural experiment to determine whether administrators are properly managing their certificates. All domains affected by Heartbleed should have patched their software, revoked their old (possibly compromised) certificates, and reissued new ones, all as quickly as possible. We find the reality to be far from the ideal: over 73% of vulnerable certificates were not reissued and over 87% were not revoked three weeks after Heartbleed was disclosed. Our results also show a drastic decline in revocations on the weekends, even immediately following the Heartbleed announcement. These results are an important step in understanding the manual processes on which users rely for secure, authenticated communication.

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1. Introduction

Server authentication is the cornerstone of secure communication on the Internet; it is the property that allows client applications such as online banking, email, and e-commerce to ensure the servers with whom they communicate are truly who they say they are. In practice, server authentication is made possible by the globally distributed Public Key Infrastructure (PKI). The PKI leverages cryptographic mechanisms and X.509 certificates to establish the identities of popular websites. This mechanism works in conjunction with other network protocols—particularly Secure Sockets Layer (SSL) and Transport Layer Security (TLS)—to provide secure communications, but the PKI plays a key role: without it, a browser could establish a secure connection with an attacker that impersonates a trusted website.

The secure operation of the web's PKI relies on responsible administration. When a software vulnerability is discovered, administrators must act quickly and deploy the patch to prevent attackers from exploiting the vulnerability. Similarly, after a potential key compromise, website administrators must revoke the corresponding certificates to prevent attackers from intercepting encrypted communications between browsers and servers. A recent study suggests 0.2% of SSL connections to Facebook correspond to such man-in-the-middle attacks.10 After considerable research into understanding and improving the speed at which software is patched,14,22 much of software patching has become automated. However, the web's PKI requires a surprising amount of manual administration. To revoke a certificate, website administrators must send a request to their Certificate Authority (CA), and this request may be manually reviewed before the certificates are finally added to a list that browsers (are supposed to) check. Such operations occur 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?


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