Description

Mastodon is a free, open-source social network server based on ActivityPub. Prior to 4.4.19 and from 4.5.0 until 4.5.12, Mastodon's app/models/concerns/user/ldap_authenticable.rb mutates OpenSSL::SSL::SSLContext::DEFAULT_PARAMS when LDAP authentication uses LDAP_TLS_NO_VERIFY=true, disabling SSL and TLS certificate verification globally for requests made by puma web processes while sidekiq background jobs remain unaffected. This issue is fixed in versions 4.4.19 and 4.5.12.

Executive Summary

Technical Details

Mitigations

References

Additional Metadata

{
  "score": "7.4",
  "pubDate": "2026-08-18T15:16:55.623Z",
  "pubdate": "2026-08-18T15:16:55.623Z",
  "executiveSummary": "A global SSL and TLS certificate verification bypass vulnerability affects Mastodon, specifically residing within the LDAP authentication module handled by puma web processes. The vulnerability is triggered when LDAP_TLS_NO_VERIFY is set to true, causing the application to mutate OpenSSL::SSL::SSLContext::DEFAULT_PARAMS globally. This cryptographic misconfiguration impacts all outbound network connections made by the puma web process, effectively disabling secure certificate validation across the affected application context, while sidekiq background jobs remain unaffected. The risk implications are severe, as disabling global SSL/TLS verification exposes the application to machine-in-the-middle (MitM) attacks, allowing malicious actors positioned on the network path to intercept, decrypt, or tamper with sensitive data transmitted by the web server. Attacker capabilities include traffic interception and credential theft if administrative or user authentication data traverses unverified channels. Exploitation requirements involve the targeted deployment of LDAP authentication with the specific bypass flag enabled, combined with a network positioning capability to intercept or manipulate traffic originating from the affected puma worker processes.",
  "technicalDetails": "The vulnerability stems from improper state management and insecure global modification of cryptographic parameters within the Mastodon codebase. Specifically, the vulnerable component is located in app/models/concerns/user/ldap_authenticable.rb. The root cause of the vulnerability is the direct mutation of the global OpenSSL::SSL::SSLContext::DEFAULT_PARAMS hash when the configuration parameter LDAP_TLS_NO_VERIFY is explicitly set to true.\nBy modifying OpenSSL::SSL::SSLContext::DEFAULT_PARAMS globally, the application alters the default security posture of the underlying OpenSSL library for the entire Ruby process lifecycle. Consequently, any subsequent outbound TLS or SSL connection initiated by the puma web processes inherits these weakened parameters, leading to the complete omission of peer certificate verification. It is important to note that sidekiq background jobs operate in separate processes and remain unaffected by this specific global mutation in the web process space.\nThe affected versions of the product include Mastodon prior to version 4.4.19 and versions ranging from 4.5.0 up to 4.5.12. Network exposure encompasses environments where Mastodon is configured to utilize LDAP authentication alongside the insecure LDAP_TLS_NO_VERIFY=true setting. Privilege and authentication requirements for exploitation depend on the network architecture; an attacker must possess the ability to intercept network traffic between the Mastodon puma web processes and external endpoints relying on TLS.\nThe step-by-step attack flow proceeds as follows: First, an administrator or deployment script configures Mastodon with LDAP authentication and enables LDAP_TLS_NO_VERIFY=true. Second, during runtime initialization or execution of authentication flows within the puma web process, app/models/concerns/user/ldap_authenticable.rb executes and mutates OpenSSL::SSL::SSLContext::DEFAULT_PARAMS. Third, when the puma process initiates any outbound TLS connection, OpenSSL honors the globally weakened parameters, bypassing certificate chain validation and hostname verification. Fourth, an attacker positioned on the network path executes a machine-in-the-middle position, presenting an arbitrary or untrusted certificate. Fifth, the vulnerable Mastodon puma process accepts the untrusted connection without validation, enabling the attacker to successfully intercept, inspect, or modify transit data."
}