Heartbleed OpenSSL Vulnerability

Incident Overview

Discovered in April 2014, Heartbleed was a catastrophic vulnerability in OpenSSL 1.0.1. An attacker could craft a 1-byte payload with a claimed length of 65,535 bytes, prompting OpenSSL to copy 64KB of secret memory back to the client.

Technical Root Cause Analysis

OpenSSL failed to check whether the payload length declared in an incoming TLS Heartbeat request matched the actual buffer size, echoing back up to 64KB of contiguous process heap memory.

• Missing Bounds Check: `memcpy(bp, pl, payload)` executed without checking if `payload <= actual_buffer_bytes`.
• Memory Exposure: OpenSSL used a custom freelist memory pool (`freelist`), so heap allocations contained recently freed private keys, passwords, and cookies.
• Silent Exploitation: Attacks left no footprint in server error logs because valid Heartbeat responses were sent without crashing the process.

Vulnerable OpenSSL Heartbeat Implementation (C)

/* Vulnerable code in ssl/t1_lib.c */
hbtype = *p++;
n2s(p, payload); // Unchecked 16-bit payload length provided by attacker!
ptls2 = p;

/* Missing check: if (1 + 2 + payload + 16 > s->s3->rrec.length) return 0; */

buffer = OPENSSL_malloc(1 + 2 + payload + padding);
bp = buffer;

/* Over-reads 64KB of server heap memory! */
memcpy(bp, pl, payload);

Incident Timeline