Hackers are posting massive lists of domains vulnerable to the infamous Heartbleed bug, security researchers warn.

The warning comes amidst other evidence that the vulnerability is under active attack from hackers possibly based in China and elsewhere, targeting financial services firms among others.

Fraud protection firm Easy Solutions reports that black hats are posting huge lists of 10,000+ domains that have been run through the automated web-based Heartbleed vulnerability checking tools. These lists reveal whether the websites are vulnerable or patched, as well as noting whether or not SSL is present.

A blog post by Easy Solutions featuring a partially redacted list of captured domains, sourced from automated scans run by hackers, can be found here. "These scans might lead to automated attacks that harvest login credentials en masse," warns Easy Solutions, which came across the lists during brand intelligence work for its financial services clients.

"Since we still live in a world filled with single-factor authentication and an over-reliance on out-of-wallet questions, we can expect an increase in account takeover attacks by simply pulling credentials from the memory of vulnerable servers and automatically testing them against other sites."

The Heartbleed vulnerability (CVE-2014-0160) in OpenSSL affects an open source library widely used for website encryption and many other applications.

The bug means TLS “heartbeat” data packets are passed without checking their length, a missing bounds check flaw that opens the door wide open to exploits based on crafting deceptive packets of arbitrary length that return up to 64K chunks from the memory of a secure server.

The flaw - neatly explained in a cartoon strip by xkcd here - allows hackers to steal information located in the memory of each server, which can include passwords and private encryption keys.

A more technically detailed description can be found in a comprehensive analysis of the flaw by El Reg.

"Initially, it was reported that private keys could be disclosed via this bug, basically allowing attackers to decrypt captured SSL sessions," Robert Erbes, a security researcher at IOActive explains in a blog post.

"But as more people start looking at different sites, other issues have been revealed – servers are leaking information ranging from user sessions to encrypted search queries (duckduckgo) and passwords.

"The type of information accessible to an attacker is entirely a function of what happens to be in the target server’s memory at the time the attacker sends the request," Erbes adds.

IVPN.net, a VPN service provider, has put together a checklist on all the main consumer-focused sites affected by the Heartbleed bug, and whether or not punters need a change of password, in a blog post here.

The Heartbleed vulnerability affects five per cent of a sample of top level domains from the top one million, according to an analysis put together by Trend Micro on Thursday. Many will soon be patched (if they haven't been already), clearing the way for keys to be revoked and users (consumers) to change their passwords.

The first wave of coverage about Heartbleed has focused on how several internet giants (Google, Yahoo) were all vulnerable to Heartbleed but this is just one aspect of a problem that extended to networking kit, smartphones running Android 4.1 (other versions of the mobile OS are not affected) and, most importantly, commercial websites. The risk exists for any certificate deployed on OpenSSL over the past two years (the vulnerability has been around since March 2012).

“The HeartBleed vulnerability is easy to exploit and there are already many proof-of-concept tools available that one can use in minutes,” warned Ivan Ristic, director of engineering at cloud security firm Qualys, and an SSL technology expert.

El Reg's sources in the penetration testing community are telling us that financial services firms are "getting hammered" with Heartbleed exploit attempts, mostly from Chinese source IP addresses.

A list of the top Heartbleed exploit attempt sources that provides further evidence to substantiate these concerns has been posted on Pastebin here. ®