CVE-2015-8542 in OX Guard
Summary
by MITRE
An issue was discovered in Open-Xchange Guard before 2.2.0-rev8. The "getprivkeybyid" API call is used to download a PGP Private Key for a specific user after providing authentication credentials. Clients provide the "id" and "cid" parameter to specify the current user by its user- and context-ID. The "auth" parameter contains a hashed password string which gets created by the client by asking the user to enter his or her OX Guard password. This parameter is used as single point of authentication when accessing PGP Private Keys. In case a user has set the same password as another user, it is possible to download another user's PGP Private Key by iterating the "id" and "cid" parameters. This kind of attack would also be able by brute-forcing login credentials, but since the "id" and "cid" parameters are sequential they are much easier to predict than a user's login name. At the same time, there are some obvious insecure standard passwords that are widely used. A attacker could send the hashed representation of typically weak passwords and randomly fetch Private Key of matching accounts. The attack can be executed by both internal users and "guests" which use the external mail reader.
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Analysis
by VulDB Data Team • 10/08/2022
The vulnerability identified as CVE-2015-8542 represents a critical authorization flaw in Open-Xchange Guard versions prior to 2.2.0-rev8, specifically within the "getprivkeybyid" API endpoint. This weakness stems from inadequate authentication mechanisms that fail to properly validate user permissions when accessing PGP private keys. The system's design relies on a single authentication parameter "auth" which contains a hashed password, while the "id" and "cid" parameters serve as identifiers for user and context selection. The fundamental flaw lies in the system's inability to enforce proper access controls between different user accounts, allowing unauthorized access to private cryptographic keys through parameter manipulation.
The technical implementation of this vulnerability demonstrates a classic case of insufficient authorization checks and predictable parameter structures. When users authenticate using the "getprivkeybyid" endpoint, the system accepts the "id" and "cid" parameters without verifying that the authenticated user has legitimate access rights to the specified user context. This creates a scenario where attackers can systematically iterate through sequential user identifiers to access private keys belonging to other users. The vulnerability is particularly dangerous because the "id" and "cid" parameters are typically sequential integers, making them easily guessable compared to complex usernames. This predictable parameter structure significantly reduces the attack surface complexity from brute-forcing random user identifiers to simply incrementing through known sequential values.
The operational impact of this vulnerability extends beyond simple unauthorized access to encompass serious cryptographic security implications. An attacker with access to a PGP private key can decrypt all communications encrypted for that user, potentially compromising sensitive business data, personal information, and confidential correspondence. The vulnerability affects both internal users and external guests who utilize the system's external mail reader functionality, broadening the attack vector significantly. This weakness particularly impacts organizations relying on Open-Xchange Guard for email encryption, as the compromise of a single user's private key can potentially expose communications across multiple users within the same context or organization.
Security practitioners should recognize this vulnerability as a manifestation of CWE-285: Improper Authorization and CWE-310: Cryptographic Issues, specifically relating to weak authentication mechanisms and insufficient access control validation. The attack pattern aligns with ATT&CK techniques such as T1213.002: Access to Cloud Storage, T1566.002: Phishing: Spearphishing Attachment, and T1078.004: Valid Accounts: Cloud Accounts, demonstrating how weak authentication can lead to unauthorized data access. The vulnerability represents a critical failure in the principle of least privilege, where the system grants access to cryptographic keys based solely on parameter validation rather than proper authentication and authorization checks. Organizations should implement immediate mitigations including updating to Open-Xchange Guard 2.2.0-rev8 or later, implementing rate limiting on API calls, enforcing stronger password policies, and establishing proper access control validation for all cryptographic key retrieval operations.
The root cause of this vulnerability lies in the system's failure to properly validate user permissions against the requested resource. The authentication mechanism relies entirely on the hashed password verification without cross-checking the user context, creating a fundamental security gap where the same password validation can be used to access multiple user accounts. This design flaw enables what security researchers categorize as a privilege escalation attack, where an attacker can leverage a valid authentication token to access resources belonging to different users. The vulnerability's exploitability is further amplified by the widespread use of predictable and weak passwords, which can be systematically tested against the API endpoint to identify matching accounts. This makes the attack surface particularly vulnerable to automated exploitation and increases the potential for mass compromise of user data across the system.
Organizations should address this vulnerability through multiple layers of defense including immediate software patching, implementation of robust access control policies, and enhanced monitoring of API access patterns. The system configuration should enforce strict validation of user context parameters, implement proper session management, and establish audit trails for all cryptographic key access attempts. Security teams should also consider implementing additional authentication factors beyond password hashing, such as multi-factor authentication, to prevent unauthorized access even if the primary authentication mechanism is compromised. The vulnerability serves as a reminder of the critical importance of proper authorization validation in cryptographic systems, where a single flaw can undermine the entire security posture of an organization's encrypted communications infrastructure.