A certificate spamming attack was discovered in the last week of June against two high-profile contributors in the OpenPGP community involved in the GnuPG (GNU Privacy Guard or GPG) project. The attack impacts to various degrees the encryption protocol implementations for encryption for signing packages and for JavaScript, causing them to slow their operations or even break them. An attacker can add as many as 150,000 signatures for a certificate in the keyserver network – the maximum it can handle. The number of deliberately poisoned certificates will only rise over time.
Source: https://www.bleepingcomputer.com/news/security/public-certificate-poisoning-can-break-some-openpgp-implementations/