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Cloudflare, a service that helps optimize the security and performance of more than 5.5 million websites, warned customers today that a recently fixed software bug exposed a range of sensitive information that could have included passwords, and cookies and tokens used to authenticate users.
A combination of factors made the bug particularly severe. First, the leakage may have been active since September 22, nearly five months before it was discovered, although the greatest period of impact was from February 13 and February 18. Second, some of the highly sensitive data that was leaked was cached by Google and other search engines. The result was that for the entire time the bug was active, hackers had the ability to access the data in real-time, by making Web requests to affected websites, and to access some of the leaked data later by crafting queries on search engines.
"The bug was serious because the leaked memory could contain private information and because it had been cached by search engines," Cloudflare CTO John Graham-Cumming wrote in a blog post published Thursday. "We are disclosing this problem now as we are satisfied that search engine caches have now been cleared of sensitive information. We have also not discovered any evidence of malicious exploits of the bug or other reports of its existence."
Cryptographic hash functions like SHA-1 are a cryptographer’s swiss army knife. You’ll find that hashes play a role in browser security, managing code repositories, or even just detecting duplicate files in storage. Hash functions compress large amounts of data into a small message digest. As a cryptographic requirement for wide-spread use, finding two messages that lead to the same digest should be computationally infeasible. Over time however, this requirement can fail due to attacks on the mathematical underpinnings of hash functions or to increases in computational power.
Today, more than 20 years after of SHA-1 was first introduced, we are announcing the first practical technique for generating a collision. This represents the culmination of two years of research that sprung from a collaboration between the CWI Institute in Amsterdam and Google. We’ve summarized how we went about generating a collision below. As a proof of the attack, we are releasing two PDFs that have identical SHA-1 hashes but different content.
For the tech community, our findings emphasize the necessity of sunsetting SHA-1 usage. Google has advocated the deprecation of SHA-1 for many years, particularly when it comes to signing TLS certificates. As early as 2014, the Chrome team announced that they would gradually phase out using SHA-1. We hope our practical attack on SHA-1 will cement that the protocol should no longer be considered secure.
We hope that our practical attack against SHA-1 will finally convince the industry that it is urgent to move to safer alternatives such as SHA-256.
What is a cryptographic hash collision?
A collision occurs when two distinct pieces of data—a document, a binary, or a website’s certificate—hash to the same digest as shown above. In practice, collisions should never occur for secure hash functions. However if the hash algorithm has some flaws, as SHA-1 does, a well-funded attacker can craft a collision. The attacker could then use this collision to deceive systems that rely on hashes into accepting a malicious file in place of its benign counterpart. For example, two insurance contracts with drastically different terms.
Finding the SHA-1 collision
In 2013, Marc Stevens published a paper that outlined a theoretical approach to create a SHA-1 collision. We started by creating a PDF prefix specifically crafted to allow us to generate two documents with arbitrary distinct visual contents, but that would hash to the same SHA-1 digest. In building this theoretical attack in practice we had to overcome some new challenges. We then leveraged Google’s technical expertise and cloud infrastructure to compute the collision which is one of the largest computations ever completed.
Here are some numbers that give a sense of how large scale this computation was:
- Nine quintillion (9,223,372,036,854,775,808) SHA1 computations in total
- 6,500 years of CPU computation to complete the attack first phase
- 110 years of GPU computation to complete the second phase
While those numbers seem very large, the SHA-1 shattered attack is still more than 100,000 times faster than a brute force attack which remains impractical.
Mitigating the risk of SHA-1 collision attacks
Moving forward, it’s more urgent than ever for security practitioners to migrate to safer cryptographic hashes such as SHA-256 and SHA-3. Following Google’s vulnerability disclosure policy, we will wait 90 days before releasing code that allows anyone to create a pair of PDFs that hash to the same SHA-1 sum given two distinct images with some pre-conditions. In order to prevent this attack from active use, we’ve added protections for Gmail and GSuite users that detects our PDF collision technique. Furthermore, we are providing a free detection system to the public.
You can find more details about the SHA-1 attack and detailed research outlining our techniques here.
About the team
This result is the product of a long-term collaboration between the CWI institute and Google’s Research security, privacy and anti-abuse group.
Marc Stevens and Elie Bursztein started collaborating on making Marc’s cryptanalytic attacks against SHA-1 practical using Google infrastructure. Ange Albertini developed the PDF attack, Pierre Karpman worked on the cryptanalysis and the GPU implementation, Yarik Markov took care of the distributed GPU code, Alex Petit Bianco implemented the collision detector to protect Google users and Clement Baisse oversaw the reliability of the computations.
Frank Abagnale is world-famous for pretending to be other people. The former teenage con man, whose exploits 50 years ago became a Leonardo DiCaprio film called Catch Me If You Can, has built a lifelong career as a security consultant and advisor to the FBI and other law enforcement agencies. So it's perhaps ironic that four and a half years ago, his identity was stolen—along with those of 3.6 million other South Carolina taxpayers.
"When that occurred," Abagnale recounted to Ars, "I was at the FBI office in Phoenix. I got a call from [a reporter at] the local TV news station, who knew that my identity was stolen, and they wanted a comment. And I said, 'Before I make a comment, what did the State Tax Revenue Office say?' Well, they said they did nothing wrong. I said that would be absolutely literally impossible. All breaches happen because people make them happen, not because hackers do it. Every breach occurs because someone in that company did something they weren't supposed to do, or somebody in that company failed to do something they were supposed to do." As it turned out (as a Secret Service investigation determined), a government employee had taken home a laptop that shouldn't have left the office and connected it—unprotected—to the Internet.
Government breaches of personal information have become all too common, as demonstrated by the impact of the hacking of the Office of Management and Budget's personnel records two years ago. But another sort of organization is now in the crosshairs of criminals seeking identity data to sell to fraudsters: doctors' offices. Abagnale was in Orlando this week to speak to health IT professionals at the 2017 HIMSS Conference about the rising threat of identity theft through hacking medical records—a threat made possible largely because of the sometimes haphazard adoption of electronic medical records systems by health care providers.