CVE-2022-46405 is a high-severity denial of service (DoS) vulnerability affecting Mastodon instances up to version 4.0.2. Mastodon is an open-source, federated social networking platform widely used for decentralized social media. The vulnerability arises from the way Mastodon handles Sidekiq job queues related to account follow relationships across federated servers. Specifically, an attacker can create numerous bot accounts that follow attacker-controlled accounts hosted on certain other servers associated with a wildcard DNS A record. This setup triggers uncontrolled recursion of attacker-generated messages, which causes the Sidekiq pull queue to grow excessively large. Sidekiq is a background job processing system used by Mastodon to handle asynchronous tasks. The excessive queue growth leads to resource exhaustion, effectively causing a denial of service by overwhelming the server's capacity to process legitimate requests. The vulnerability is classified under CWE-674 (Uncontrolled Recursion), indicating that the recursive processing of messages is not properly bounded or validated. The CVSS v3.1 base score is 7.5, reflecting a high severity with network attack vector (AV:N), low attack complexity (AC:L), no privileges required (PR:N), no user interaction (UI:N), unchanged scope (S:U), no impact on confidentiality or integrity (C:N/I:N), but high impact on availability (A:H). No known exploits in the wild have been reported, and no vendor patches or mitigations are explicitly referenced in the provided data. This vulnerability can be exploited remotely without authentication or user interaction, making it a significant risk for Mastodon instances that are publicly accessible and federated with other servers, especially those using wildcard DNS configurations that facilitate the recursive message generation.

For European organizations running Mastodon instances, this vulnerability poses a significant risk of service disruption. Mastodon is popular among various communities, including activists, journalists, and niche social groups in Europe, making affected instances critical communication platforms. A successful exploitation can lead to denial of service, rendering the platform unavailable to legitimate users, damaging reputation, and potentially disrupting communication channels. Since the attack requires no authentication and can be launched remotely, it increases the risk of widespread abuse. The uncontrolled recursion can also lead to increased server resource consumption (CPU, memory, and network bandwidth), potentially causing collateral damage to other services hosted on the same infrastructure. For organizations relying on Mastodon for public engagement or internal communication, this could result in operational downtime and loss of trust. Additionally, the federated nature of Mastodon means that an exploited instance could affect other interconnected servers, amplifying the impact across the federation. Given the lack of patches and the complexity of mitigating recursive message flows, European organizations must be vigilant to prevent exploitation and maintain service availability.

1. Implement rate limiting and throttling on account creation and follow requests to prevent mass creation of bot accounts and excessive follow relationships. 2. Monitor Sidekiq queue lengths and processing times closely; set alerts for abnormal queue growth to detect early signs of exploitation. 3. Restrict or carefully configure federation with servers that use wildcard DNS A records, as these facilitate the recursive message generation exploited by this vulnerability. 4. Employ network-level controls such as Web Application Firewalls (WAFs) or reverse proxies to detect and block suspicious traffic patterns indicative of recursive follow requests or bot activity. 5. Consider temporarily disabling federation with untrusted or unknown servers until a patch or official mitigation is available. 6. Engage with the Mastodon community and maintain awareness of updates or patches addressing this vulnerability; apply them promptly once released. 7. Harden server resources by isolating Mastodon instances and Sidekiq workers to prevent resource exhaustion from impacting other critical services. 8. Use anomaly detection tools to identify unusual patterns in account behavior, such as rapid follow/unfollow cycles or mass bot account creation. These targeted mitigations go beyond generic advice by focusing on the specific attack vector (recursive follow relationships via wildcard DNS) and the operational characteristics of Mastodon and Sidekiq.