CVE-2025-24808 in Discourse
Summary
by MITRE • 03/26/2025
Discourse is an open-source discussion platform. Prior to versions `3.3.4` on the `stable` branch and `3.4.0.beta5` on the `beta` branch, someone who is about to reach the limit of users in a group DM may send requests to add new users in parallel. The requests might all go through ignoring the limit due to a race condition. The patch in versions `3.3.4` and `3.4.0.beta5` uses the `lock` step in service to wrap part of the `add_users_to_channel` service inside a distributed lock/mutex in order to avoid the race condition.
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Analysis
by VulDB Data Team • 08/26/2025
The vulnerability CVE-2025-24808 affects Discourse, an open-source discussion platform that enables community-driven conversations through various communication channels including group direct messages. This security flaw represents a race condition in the platform's user management system specifically within the group DM functionality, where the system fails to properly enforce user limits during concurrent operations. The vulnerability exists in versions prior to the security patches released in Discourse's stable branch version 3.3.4 and beta branch version 3.4.0.beta5, creating a window of exposure where malicious actors could potentially exploit the system's concurrency handling mechanisms.
The technical flaw manifests as a race condition in the `add_users_to_channel` service where multiple parallel requests to add users to a group DM can bypass the intended user limit restrictions. This occurs because the system does not properly synchronize access to the user count validation logic, allowing multiple concurrent operations to evaluate the same user limit condition before any of them can update the actual user count. The underlying issue stems from the absence of proper locking mechanisms during the critical section of code that checks and modifies user limits. This type of vulnerability aligns with CWE-362, which specifically addresses race conditions in concurrent programming, and represents a classic example of a timing attack where the window of opportunity for exploitation is determined by the system's concurrency model rather than its inherent security controls.
The operational impact of this vulnerability extends beyond simple user limit bypass, as it could potentially be exploited to overwhelm group DM channels with excessive participants, leading to resource exhaustion and degraded service quality. Attackers could leverage this race condition to flood group channels with users, potentially causing denial of service conditions or creating situations where legitimate users cannot participate in group discussions. The distributed nature of the vulnerability means that even in multi-server deployments, the race condition could be exploited across different nodes, making it particularly concerning for large-scale deployments. This vulnerability could also enable unauthorized access to restricted group communications by allowing users to exceed normal participation limits, potentially compromising the privacy and integrity of sensitive discussions.
The patch implemented in versions 3.3.4 and 3.4.0.beta5 addresses this issue by introducing a distributed lock/mutex mechanism around the critical section of the `add_users_to_channel` service. This distributed locking approach ensures that only one operation can validate and modify the user count at any given time, effectively serializing access to the user limit enforcement logic. The solution follows established security patterns for preventing race conditions and aligns with ATT&CK technique T1553.001, which covers the manipulation of system processes to bypass security controls. The implementation likely uses database-level locks or distributed coordination mechanisms such as Redis-based locks to ensure that the user addition process remains atomic and consistent across different server instances. This mitigation strategy directly addresses the concurrency issues identified in the vulnerability and prevents the parallel execution paths from interfering with each other during user limit validation. Organizations using Discourse should immediately upgrade to the patched versions to prevent exploitation of this vulnerability and maintain the integrity of their group communication channels.