Webauthn: SimpleFakeCredentialGenerator with an empty secret produces predictable fake credentials, weakening username enumeration protection

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS 4.0

2.9 · CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:L/VI:N/VA:N/SC:N/SI:N/SA:N/E:P · score source

EPSS

No dated EPSS signal available.

As published by the source; long descriptions are shortened.

Impact Webauthn\SimpleFakeCredentialGenerator is the library-provided default implementation of the FakeCredentialGenerator interface. It returns a stable list of decoy PublicKeyCredentialDescriptor objects for a given username so that an assertion request for an unknown user looks the same as a request for a real one, which mitigates username enumeration. The generator derives the whole decoy list from a single seed: When it is constructed without a secret (its constructor default, $secret = ''), the seed depends only on the username. The username is attacker-chosen and the algorithm is public, so an unauthenticated requester can recompute the exact, byte-for-byte decoy list the server returns for any username. The attacker then compares a probed username's response against the locally computed list and decides whether the account is real or fake, which is precisely the distinction the mechanism is meant to hide. With any non-empty secret the seed becomes a value the attacker cannot…

Published by the source

7 Jul 2026, 23:39 UTC

Last modified by the source

7 Jul 2026, 23:40 UTC

Source last checked

26 Oct 2026, 14:50 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-gq4g-fpc9-vjfq

Preview the exact text before you copy it. Commands are read-only suggestions; review them before running anything.

# Patch Next advisory brief: GHSA-gq4g-fpc9-vjfq

Advisory: Webauthn: SimpleFakeCredentialGenerator with an empty secret produces predictable fake credentials, weakening username enumeration protection
IDs: GHSA-gq4g-fpc9-vjfq
Record: https://patchnext.com/advisories/ghsa-gq4g-fpc9-vjfq (revision 1, source checked 26 Oct 2026, 14:50 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Low (CVSS 2.9)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: web-auth/webauthn-lib 5.3.5. A vendor fix does not mean a deployment is fixed.

## Affected components
- Packagist web-auth/webauthn-lib: affected >= 4.9.0, < 5.3.5; fixed in 5.3.5

## Suggested inspection (read-only)
- Packagist web-auth/webauthn-lib: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on web-auth/webauthn-lib.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-gq4g-fpc9-vjfq (GitHub-reviewed advisory GHSA-gq4g-fpc9-vjfq lists affected ranges: Packagist web-auth/webauthn-lib >=4.9.0 <5.3.5)

Commands are suggestions: review each one before running it.