A bug in Thunderbird’s OpenPGP key retrieval over the Web Key Directory protocol caused the client to use incorrect padding when forming the key request. The padding difference exposed the length of the email address used in the lookup, allowing an attacker who could observe traffic to a WKD server to learn how long a victim’s email address is.

This flaw was present in all Thunderbird releases before 128.8 and 136, inclusive. Mozilla released a fix in Thunderbird 136 and the legacy 128.8 branch. Systems running earlier versions of Thunderbird are affected.

The CVSS score of 5.3 places this vulnerability in the medium range; the EPSS score of less than 1% indicates a low probability of exploitation. It is not listed in the CISA KEV catalog. The most likely attack path involves an adversary monitoring network traffic to a WKD endpoint and collecting length information, which can aid in user enumeration or targeted phishing. No elevated privileges or authentication are required.