CVE-2025-51869 is an Insecure Direct Object Reference (IDOR) vulnerability identified in the Liner application, affecting versions up to June 3, 2025. The vulnerability arises from improper access control in the API endpoint v1/space/{space_id}/thread/{thread_id}/message/{message_id}, where crafted parameters for space_id, thread_id, and message_id can be manipulated by an attacker to gain unauthorized access to sensitive information. This vulnerability is classified under CWE-639, which relates to authorization bypass through direct object references. The CVSS v3.1 base score is 7.5, indicating a high severity level. The vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N indicates that the vulnerability can be exploited remotely over the network without any privileges or user interaction, and it results in a high impact on confidentiality without affecting integrity or availability. No known exploits are currently reported in the wild, and no patches have been publicly linked yet. The vulnerability allows attackers to bypass authorization checks and directly access sensitive data by manipulating URL parameters, which suggests a lack of proper validation or access control enforcement on these resource identifiers. This type of vulnerability can be particularly dangerous in collaborative or messaging platforms like Liner, where sensitive or private information may be exposed if unauthorized users can access message threads or spaces they should not have visibility into.

For European organizations using Liner, this vulnerability poses a significant risk to the confidentiality of sensitive communications and data. Unauthorized access to message threads and spaces could lead to exposure of intellectual property, personal data protected under GDPR, or confidential business information. This could result in regulatory penalties, reputational damage, and loss of customer trust. Since the vulnerability does not require authentication or user interaction, it can be exploited by remote attackers, increasing the risk of widespread data leakage. Organizations in sectors such as finance, healthcare, legal, and government, which often handle sensitive information, are particularly vulnerable. The lack of integrity and availability impact means the threat is primarily data exposure rather than disruption or data manipulation, but the confidentiality breach alone can have severe consequences under European data protection laws.

European organizations should immediately audit their use of the Liner platform and monitor for unusual access patterns to the affected API endpoints. Until an official patch is released, organizations can implement compensating controls such as network-level restrictions to limit access to the API endpoints only to trusted IP ranges or VPNs. Application-layer mitigations include implementing strict access control checks on the server side to verify that the requesting user is authorized to access the requested space, thread, and message resources. Logging and alerting should be enhanced to detect anomalous access attempts involving manipulation of space_id, thread_id, and message_id parameters. Additionally, organizations should conduct a thorough review of data exposure and consider notifying affected users if sensitive information has been potentially accessed. Once a patch is available, prompt application of the update is critical. Security teams should also educate developers and administrators about the risks of IDOR vulnerabilities and enforce secure coding practices to prevent similar issues in the future.