Mastodon instance administrators coordinated new shared blocklists in early 2023 to address harassment targeting activists on the platform. According to Mastodon’s official documentation, the blocklists allow admins to suspend or limit problematic servers in bulk, streamlining moderation efforts against hate speech and harassment.

According to Mastodon’s official documentation, the feature allows admins to upload CSV files containing lists of other servers to suspend or limit in bulk, streamlining moderation efforts against hate speech and harassment. The documentation outlines a standardized process where admins download a blocklist, navigate to Preferences → Moderation → Federation, import the list, and review entries before applying blocks. Admins can also export their own blocklists to share with others.

Starting in early 2023, Mastodon introduced an instance-level blocklist import and export feature designed to help administrators manage problematic servers more efficiently.

This feature was developed in response to the challenge of handling “hundreds of problematic instances,” which made manual blocking tedious and prone to error, especially for new administrators unfamiliar with which servers to block, according to Mastodon developers and community researchers. Shared blocklists maintained by individuals such as Rohan Kumar (known as Seirdy), Gardenfence, and Oliphant have gained significant adoption since the feature’s rollout, according to a 2023 research overview of fediverse moderation practices.

Several public blocklists have emerged with a focus on protecting marginalized users from harassment and hate speech. The Gardenfence blocklist, hosted as a GitHub project, describes itself as “a basic fediverse blocklist for Mastodon admins to block the most well known sources of hate speech and harassment,” targeting instances associated with racism, sexism, homophobia, transphobia, doxxing, stalking, and trolling. Another list, Fblocklist, launched in January 2024, similarly aims to block servers known for hate speech and harassment. The JoinFediverse wiki documents multiple public blocklists used by admins, including FediSeer by Rikudou, Gardenfence by Brook, Oliphant’s blocklists, Seirdy’s blocklist, and The Bad Space by Ro.

The FediBlock ecosystem exemplifies community coordination among instance administrators and users sharing information about abusive or malicious servers. Research shows that admins aggregate multiple sources—including stux’s list from mastodon.social, hashtag-driven reports like #fediblock, and older curated lists—to create composite blocklists published on platforms such as GitHub and Codeberg. Mastodon GitHub records dating back to August 2019 reveal early interest in sharing mutelists and blocklists between “friend instances” to facilitate coordinated moderation.

Harassment controversies targeting activists and marginalized groups have been a significant driver behind the adoption of shared blocklists. Research on fediverse moderation highlights campaigns involving the #asstodon hashtag, where activists became targets of coordinated harassment on Mastodon and other ActivityPub platforms. Commentators have warned that the fediverse can be “unsafe by design and unsafe by default” for marginalized communities, citing the presence of hundreds of problematic instances. Blocklists like Gardenfence explicitly state their goal of protecting “non-white, non-straight, non-binary folk” from servers known to cause drama and reduce safety on the platform. Oliphant’s blocklists categorize instances with reasons such as “Harassment” and “Hate speech,” suspending servers associated with stalking, doxxing, and attacks on marginalized users.

Technically, Mastodon’s blocklist feature uses a CSV format in which each row lists an instance domain alongside a moderation action—such as “suspend” or “silence”—and a reason, including categories like harassment, hate speech, or spam. For example, Oliphant’s blocklist includes multiple domains related to well-known harassment hubs such as kiwifarms.cc, marked for suspension due to harassment. The lists also classify entries under categories like “Third-party bots,” “Mirroring accounts without permission,” and “Spam,” allowing admins to review and select blocks based on detailed reasons. Research indicates that while many moderators share their blocklists publicly as references, others maintain private lists, reflecting a range of coordination styles.

Despite their widespread use, shared blocklists have raised governance concerns within the Mastodon community. Analysts caution that widely shared blocklists can introduce harms if they reflect biased judgments or are applied without local review, potentially affecting already marginalized communities. Academic research notes that a small number of list maintainers can exercise considerable influence over which communities are defederated, raising questions about delegated moderation and power asymmetries. Proposals for themed blocklists—such as those focused on racism—suggest a model where instances subscribe to curated lists to block servers for specific harms, but critics warn this may risk mislabeling or over-blocking activist spaces, some of which are run by marginalized groups.

Historically, early efforts to curate safer spaces on the fediverse began around 2019 with projects like FediHealth, which suggested instances to follow or block based on federation policies. Calls for shared blocklists intensified from 2022 onward as administrators sought collective tools to manage racist and harmful content more effectively. By 2023, blocklists maintained by Seirdy, Gardenfence, and Oliphant became key resources for defederating from poorly moderated or racist servers. In 2024, additional lists such as IFTAS’ Do Not Interact (DNI) and CARIAD were introduced, often emphasizing protection for marginalized and activist communities.

Overall, Mastodon instance administrators continue to coordinate through shared blocklists to respond rapidly to harassment campaigns targeting activists and marginalized users. This coordination occurs alongside ongoing debates about governance, fairness, and the balance between protecting users and preserving open federation.