Klaviyo Password Leak Exposed: Signup Bug Shared Passwords With Dozens of Advertisers

TL;DR

• A bug in Klaviyo's signup flow inadvertently embedded new users' plaintext passwords into URLs and referral data, causing them to be shared with dozens of third-party advertising and analytics partners.
• The flaw was triggered during account creation and password reset, exposing credentials in browser history, logs, and to external trackers before Klaviyo patched it.
• Anyone who created a Klaviyo account or reset their password recently should immediately change their password, enable two-factor authentication, and change the same password anywhere else it was reused.

How The Signup Bug Leaked Passwords in Plain Text

The issue centered on how Klaviyo handled new user registration. According to initial disclosures and security researcher analysis, when a new user created an account and set a password, the website incorrectly appended the password as a plaintext query parameter to a URL. That URL was then loaded in the browser and automatically shared with external resources on the page.

Because Klaviyo's marketing site loads a large number of third-party trackers for advertising, analytics, and session replay, the password-containing URL was sent via referrer headers and network requests to those external partners. In effect, a sensitive credential was treated like a standard marketing UTM parameter and broadcast to the adtech ecosystem. The same flawed behavior reportedly occurred during some password reset flows.

Why This Is So Dangerous

Passwords should never appear in URLs. URLs are not considered secret - they are stored in browser history, saved in server logs, cached by CDNs, visible to browser extensions, and routinely passed to analytics platforms. Even over HTTPS, the full URL including query parameters is exposed to any script running on the page and is often included in the HTTP Referer when loading third-party resources.

In this case, that meant a plaintext password was not just logged internally but distributed outside Klaviyo's infrastructure. Security researchers noted that anyone with access to those advertising partner logs, or any intermediary that logged referrer data, could have potentially harvested the credentials without ever breaching Klaviyo directly.

Scope of the Exposure: Who Was Affected and For How Long

Klaviyo has not yet disclosed a precise number of affected users or the exact window the bug was live, but reports indicate the flaw was present on its public signup pages and was shared with dozens of advertising partners integrated into the site.

The exposure was not the result of a database breach or hack, but a client-side implementation error. That distinction matters because it means standard breach detection systems would not have flagged it, and the data was leaked passively through normal web traffic. Researchers who discovered the bug demonstrated that creating a test account resulted in the test password appearing in network traffic destined for third-party domains.

While Klaviyo's core email marketing and customer data platform does not appear to have been compromised, the incident highlights the privacy risk of loading extensive third-party tracking scripts on sensitive pages like login and signup.

How Klaviyo Responded

After being alerted to the issue by researchers, Klaviyo said it moved quickly to fix the flaw and remove passwords from URLs and referral data. The company has reportedly purged related logs where possible and is working with its advertising partners to request deletion of any inadvertently shared data.

Klaviyo has also stated it is conducting an internal review of how sensitive pages handle query parameters and third-party scripts, and is implementing additional safeguards to prevent sensitive data from being appended to URLs in the future. As of now, the company has not confirmed whether it will require a forced password reset for all potentially affected users, but it is notifying users believed to be impacted.

What You Should Do Right Now If You Have a Klaviyo Account

If you created a Klaviyo account or reset your Klaviyo password in recent weeks or months, take these steps immediately, even if you have not received a notification from the company:

1. Change Your Klaviyo Password Immediately
Log in and set a new, strong, unique password that you have not used anywhere else. If you used the same password on other sites, that is now critical to change as well, since those other accounts could be at risk from credential stuffing.

2. Enable Two-Factor Authentication
Turn on 2FA in your Klaviyo account security settings. This ensures that even if your old password was captured, it cannot be used alone to access your account.

3. Check for Suspicious Activity
Review your Klaviyo account for any unrecognized logins, API keys, user invites, or changes to campaigns and audience exports. If you manage a brand, also review connected integrations and e-commerce store permissions.

4. Clear Browser History and Consider a Password Manager
Since the password may have been saved in your browser history, clear it. Going forward, use a password manager to generate and store unique passwords so you never need to reuse them.

A Bigger Warning for the Web

This incident is a stark reminder of the hidden cost of the modern tracking-heavy web. Loading dozens of ad and analytics scripts on pages that handle authentication creates a single point of failure where one small coding mistake can turn a password into adtech data.

Security best practices have long dictated that sensitive pages should be stripped of unnecessary third-party scripts, and that passwords should only ever be transmitted in the body of a POST request, never in a URL. For users, it reinforces why password reuse is so dangerous - a bug on one site can compromise your accounts everywhere else.

AndroGuider Team

Articles written by the AndroGuider team. We try to make them thorough and informational while being easy to read.