An attacker could have exploited a timing attack by sending a large number of allowCredential entries and detecting the difference between invalid key handles and cross-origin key handles. This could have led to cross-origin account linking in violation of WebAuthn goals. This vulnerability affects Thunderbird < 91.10, Firefox < 101, and Firefox ESR < 91.10. A flaw was found in Mozilla. The Mozilla Foundation Security Advisory describes the issue as an attacker could have exploited a timing attack by sending a large number of allowCredential entries and detecting the difference between invalid key handles and cross-origin key handles. This could have led to cross-origin account linking in violation of WebAuthn goals.
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– Scott Cheney, Manager of Information Security, Sierra View Medical Center