Unpacking the Polyfill.io Security Flaw

Polyfills are pieces of programming code that provide developers with an efficient fix for a common problem and ensure that even users with outdated browsers can have the most up-to-date functionality. The usefulness of polyfills and their easy accessibility has led to their use on hundreds of thousands of sites.

So, what happens when a polyfill gets corrupted? Nearly 500,000 sites — including the streaming giant Hulu — recently found out. Polyfill code the sites had been relying on from the Polyfill.io domain was altered at the source level to maliciously redirect certain targeted users worldwide from intended sites to others promoting pornography and gambling.

The problem resulting in the Polyfill.io redirects was caused by a supply chain attack, which involves infiltrating websites by corrupting library code or other content delivery network (CDN) services provided by third-party vendors. The attack was triggered after the Polyfill.io domain providing the code was sold to a Chinese-owned company. Soon after the sale, warnings began to surface that the company was injecting malware into the code it provided.

Cybersecurity experts have warned that the attack reveals just how easy it has become for hackers to leverage polyfills to cause disruptions.

“Polyfill.io’s supply chain attack is the beginning of a new generation of supply chain attacks in which criminal groups no longer have to show technical prowess,” explains Yashin Manraj, CEO of Pvotal Technologies. “With a modest amount of cash or business acumen, they acquire fledgling businesses or abandoned projects widely used in the developer community due to their open-source nature or free licensing offers.”

Manraj is an expert in the cybersecurity space who brings a diverse background in computational chemistry and engineering to his efforts to provide businesses with stable, efficient, and secure infrastructure. Through Pvotal, Manraj empowers businesses with rapid change, seamless communication, top-notch security, and scalability to infinity. Pvotal provides deep technical knowledge in development, design, and coding that allows businesses to identify and solve gaps in their product pipeline.

The Polyfill attack was easily avoidable

In many cases, cyberattacks succeed because they are deployed in sophisticated ways that are difficult to detect. With the Polyfill attack, that was not the case.

“Compared to other major attacks in 2024, the Polyfill.io attack is not technically sophisticated and could have been easily avoided by proper development practices from its service developers or integrators,” Manraj reports. “In the case of Polyfill.io, developers should have included integrity checks such as CSP hashes to ensure CDN payloads they rely on were not tampered with.”

Integrity checks are a standard cybersecurity practice designed to ensure systems and the data they rely on have not been altered without authorization by identifying signs a system has been compromised before services are disrupted. Code injection attacks, such as the one carried out by Polyfill.io, are one of the main types of attacks integrity checks seek to identify.

Polyfill attacks are especially hard on smaller businesses

Polyfill CDN is popular because it streamlines the development process. Small companies with small development budgets can leverage it to reduce workloads and costs, but choosing that path also exposes companies to the risk of supply chain attacks.

“Unfortunately, many companies outsourcing their development relied on services like polyfill.io due to their ease of integration, easy-to-follow tutorials, and community activism,” Manraj says. “We identified thousands of scripts generated by developers on sites like Fiverr, Upwork, and other similar low-cost development sites to use unvetted scripts that included or relied on services like Polyfill.io to accelerate their release schedule and minimize their overheads.”

When an attack like the Polyfill.io attack occurs, many of the companies relying on the scripts lack the resources to identify the problem, let alone address it with a timely upgrade. 

“While larger companies like Disney, Intuit, and Atlassian can deploy fixes within hours to mitigate any potential damages from their localized use of Polyfill, we believe thousands of other companies will be unaware or unable to address this vulnerability until it is too late,” Manraj shares.

The Polyfill.io attack highlights the central role trust must play in building and maintaining a secure infrastructure for services. Companies must be wary of entrusting their operations to unvetted or unstable entities. Efficiency in programming is valuable, but not when it comes with the risk of major system disruption.

“When you lose control over the end-to-end infrastructure you are relying on, you put your operations and your reputation at considerable risk,” Manraj warns. “To develop a reliable infrastructure, growth and agility must go hand-in-hand with security.”