What is a zero day vulnerability?

Owned by Eliot Muir
Nov 08, 2023
2 min read

A zero-day vulnerability refers to a security flaw in software that is unknown to the party or parties responsible for patching or otherwise fixing the flaw. The term "zero-day" denotes that the developers have "zero days" to work on a fix because the vulnerability has already been discovered and potentially exploited by attackers. The period before the developers address the vulnerability is critical because attackers can exploit it to harm systems, steal data, or gain unauthorized access.

Zero-day vulnerabilities are particularly dangerous because they are often exploited by attackers before the public is aware of the issue, which means there are no existing patches or workarounds to prevent the exploits. These vulnerabilities can be found in any software, from operating systems to applications, including widely used libraries like OpenSSL.

Examples of zero-day vulnerabilities that have affected OpenSSL include:

  1. Heartbleed (CVE-2014-0160): Perhaps the most notorious OpenSSL vulnerability, Heartbleed was a serious bug in OpenSSL's implementation of the TLS heartbeat extension. It allowed attackers to read the memory of servers running affected versions of OpenSSL, potentially exposing sensitive data such as private keys, usernames, passwords, and personal information. Despite being introduced into the codebase in 2012, it was not discovered until April 2014, thus it was a zero-day vulnerability for approximately two years.

  2. CCS Injection Vulnerability (CVE-2014-0224): Another critical vulnerability discovered in 2014 was the ChangeCipherSpec (CCS) injection vulnerability. It allowed attackers to perform a man-in-the-middle attack and decrypt and modify traffic from the affected client and server. The bug was in the code for several years before it was discovered and patched.

These examples highlight the potential impact of zero-day vulnerabilities in essential cryptographic software like OpenSSL. When such vulnerabilities are discovered, they prompt urgent responses from developers and rapid patching from all affected parties to minimize potential damages.