OpenSSL Heartbeat Extension Buffer Over-read Vulnerability (Heartbleed, CVE-2014-0160)

1. Vulnerability Overview & Root Cause

This vulnerability, widely known as 'Heartbleed', resides in the TLS/DTLS Heartbeat Extension (RFC 6520) implementation within OpenSSL's d1_both.c and t1_lib.c. The root cause is a missing bounds check on a length field provided by the client. When a client sends a Heartbeat request, the server allocates a memory buffer based on the payload length specified in the request packet, rather than the actual size of the payload received. This leads to a buffer over-read (CWE-125), copying adjacent heap memory into the response packet.

2. Exploitation Mechanics & Attack Scenario

An attacker crafts a malicious Heartbeat request containing a small payload (e.g., 1 byte) but sets the payload length field to a high value, up to 65,535 bytes (64KB). Because OpenSSL fails to validate the actual payload size against the declared length, it copies the requested 64KB of data from its active process memory buffer back to the attacker. This leaked memory space often contains highly sensitive data, including active session cookies, user credentials, and the server's private cryptographic keys. The attack leaves no trace in standard application logs and can be executed repeatedly.

3. Infrastructure & Business Impact

With a CVSS score of 7.5, an EPSS score of 1.0, and its inclusion in the CISA KEV catalog, this vulnerability represents an extreme threat. If an attacker successfully extracts the server's private SSL/TLS key, they can decrypt past and future encrypted traffic via Man-in-the-Middle (MitM) attacks. This compromises the entire transport layer security, leading to widespread data breaches, session hijacking, and severe reputational damage.