Hash chaining degrades security at Facebook
Thomas Rivasseau
TL;DR
The paper analyzes a vulnerability in Facebook's historical password storage, which uses a layered hash-chain beginning with $MD5$ and extending through $SHA1$, $SHA256$, and a memory-hard function with salting. It shows that anchoring the chain to the initial $MD5$ stage creates a collision-enabled weakness, enabling two distinct passwords that share the same $MD5$ hash to yield identical final hashes and thus permit unauthorized login in a controlled demonstration. The authors demonstrate a functional hash-collision exploit using a known MD5 collision pair, illustrating how an attacker could bypass authentication by submitting a different password with the same MD5 output. Despite the presence of salting and a memory-hard function, the chain degrades security to the level of $MD5$-based schemes, underscoring the need for upgrading to modern, standard password-hashing approaches and highlighting limitations and ethical disclosure considerations.
Abstract
Modern web and digital application password storage relies on password hashing for storage and security. Ad-hoc upgrade of password storage to keep up with hash algorithm norms may be used to save costs but can introduce unforeseen vulnerabilities. This is the case in the password storage scheme used by Meta Platforms which services several billion monthly users worldwide. In this paper we present the first example of an exploit which demonstrates the security weakness of Facebook's password storage scheme, and discuss its implications. Proper ethical disclosure guidelines and vendor notification were followed.