A duplicate line of code crippled Apple's encryption
In 2014, security researchers discovered a catastrophic flaw in Apple's SecureTransport library. An accidental duplicate line—`goto fail;`—was indented inside an `if` statement but lacked enclosing braces. In C, that meant the second statement always executed, bypassing digital signature verification entirely. For months, iOS and OS X devices accepted forged certificates as genuine, allowing eavesdroppers on public Wi-Fi networks to intercept encrypted data undetected.
The Mechanics of the Duplicate Line
In February 2014, Apple issued a security update to address a critical vulnerability in SecureTransport, the cryptographic library responsible for handling SSL and TLS protocols across iOS and OS X. At the center of the issue was a single, duplicated line of C code inside a source file named sslKeyExchange.c. The vulnerability, designated CVE-2014-1266, completely compromised the library's ability to verify the authenticity of secure server connections, exposing millions of devices to silent interception.
The flaw resided within a function named SSLVerifySignedServerKeyExchange. When a client device connects to an encrypted server using TLS, the server provides cryptographic parameters signed with its private key to prove its identity. The client must verify this digital signature against the server's certificate. The function executed a sequence of checks, updating a SHA-1 cryptographic hash with the incoming parameters. After each hashing operation, the code checked whether an error had occurred. If an error was detected, execution was designed to abort by jumping directly to an exit and cleanup block labeled fail.
The bug was introduced when a second goto fail; statement was inserted immediately following an existing one. In the C programming language, an if statement that lacks curly braces applies only to the single statement directly following it. Although the duplicate goto fail; was formatted with matching indentation, giving the visual appearance of being enclosed within the conditional check, the compiler treated it as an independent, unconditional command. The indentation was purely cosmetic.