Web-Auth Webauthn predictable fake credentials in SimpleFakeCredentialGenerator

Severity: low · CVSS 2.9 · Published 2026-07-07

A security feature in the Webauthn library, designed to prevent attackers from guessing valid usernames, is ineffective when used with default settings. This component normally generates fake login data for non-existent users so that an attacker cannot tell the difference between a real account and a fake one. Because the default configuration uses an empty secret key, an attacker can predict exactly what the fake data will look like, allowing them to identify which usernames actually exist on the system.

Technologies: web-auth/webauthn-lib (Packagist). Vendors: Packagist.

Quick answers

Is This vulnerability exploited in the wild?
No. Junglewise Threat Intelligence has no report of This vulnerability being exploited in the wild.
How severe is This vulnerability?
Low severity, with a CVSS score of 2.9, published 7 July 2026.
Which products and versions are affected?
web-auth/webauthn-lib (Packagist) >= 4.9.0, < 5.3.5; Web-Auth Webauthn-Lib >= 4.9.0, < 5.3.5.
Is there a fix for This vulnerability?
A patch was published on 2026-07-07: Fixed in version 5.3.5 Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is This vulnerability attacked?
It is attacked over the network, in a software library.
Am I affected?
Check whether you run web-auth/webauthn-lib (Packagist) or Web-Auth Webauthn-Lib at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

Technical details

The `SimpleFakeCredentialGenerator` in `web-auth/webauthn-lib` is designed to mitigate username enumeration by returning stable decoy `PublicKeyCredentialDescriptor` objects for unknown users. However, the generator's constructor defaults to an empty string for its secret. Because the seed for generating these decoys is derived using a public algorithm (`hash('sha256', $username . $this->secret)`) and the secret is empty by default, an unauthenticated remote attacker can locally pre-compute the expected decoy list for any username. By comparing the server's response to their calculated decoys, the attacker can determine if a username is valid or not. This has been patched in version 5.3.5 by adding deprecation warnings for empty secrets, with a non-empty secret becoming mandatory in version 6.0.0.

Affected products

  • web-auth webauthn-lib >= 4.9.0, < 5.3.5

Timeline

  • 2026-05-31: disclosed
  • 2026-07-07: advisory: GitHub Advisory published
  • 2026-07-07: patched: Fixed in version 5.3.5

References

This page as Markdown. Free to reuse under CC BY 4.0 with a link back.