CVE-2026-93872 in Cotonti
Summary
by MITRE • 09/18/2026
Cotonti 1.0.0 passes the base64-decoded cb parameter to unserialize() without allowed_classes restriction in the comments plugin EditAction. Registered users with comment write permissions can instantiate arbitrary PHP objects and potentially achieve file write or code execution through gadget chains.
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Analysis
by VulDB Data Team • 09/18/2026
The vulnerability identified in Cotonti version 1.0.0 represents a critical insecure deserialization flaw located within the comments plugin's EditAction functionality. This issue stems from the application's handling of user-supplied input, specifically the cb parameter, which is subjected to base64 decoding and subsequently passed directly to PHP’s unserialize function without any restrictions on allowed classes. In modern secure coding practices, it is imperative that deserialization operations either restrict the set of instantiable classes or avoid unserialization of external data entirely. By failing to implement an allowlist for class names during this process, the application exposes itself to remote code execution risks when interacting with authenticated users who possess comment write permissions.
From a technical perspective, the core flaw lies in the lack of validation and filtering applied to the decoded payload before it enters the object instantiation phase. When PHP's unserialize function processes data containing serialized objects, it attempts to reconstruct those objects by calling their constructors or magic methods if they exist within the loaded class definitions. Without an allowed_classes parameter set to a specific array of trusted classes, any class present in the application’s environment can be instantiated. This allows attackers who have access to comment editing capabilities to craft malicious payloads that leverage existing PHP libraries and framework components known as gadget chains. These chains are sequences of method calls within standard or third-party classes that result in unintended side effects when triggered during object reconstruction.
The operational impact of this vulnerability is severe, potentially leading to full system compromise through arbitrary file writes or remote code execution. An attacker can exploit the insecure deserialization by constructing a serialized payload that utilizes common PHP libraries such as Symfony Monolog, Guzzle HTTP client, or SwiftMailer, which are frequently included in web applications like Cotonti. By chaining methods within these libraries, an adversary can achieve outcomes ranging from writing arbitrary files to the server’s filesystem to executing system commands directly on the host machine. This capability effectively bypasses traditional input validation controls because the threat is not based on injecting malicious code strings but rather on manipulating the application's internal logic through legitimate object structures that have been twisted into harmful sequences of operations.
This vulnerability aligns with CWE-502, which describes Deserialization of Untrusted Data, a category of flaws where applications fail to validate or sanitize data before deserializing it, leading to potential remote code execution. Furthermore, the exploitation technique maps directly to MITRE ATT&CK techniques related to Command and Scripting Interpreter via System Commands Execution (T1059) and potentially File Write if gadget chains are used for persistence or lateral movement. The requirement for registered users with comment write permissions defines the attack vector as authenticated but low-privilege, which is common in many web applications where basic user roles can trigger complex backend processes without sufficient oversight.
To mitigate this vulnerability, immediate remediation steps should focus on restricting deserialization operations to a strict allowlist of trusted classes only. Developers must modify the EditAction function within the comments plugin to pass an array of allowed class names as the second argument to unserialize(), ensuring that no unexpected or dangerous classes can be instantiated from external input. Alternatively, if object serialization is not strictly necessary for this feature, replacing it with a safer data format such as JSON using json_decode() would eliminate the risk entirely since PHP does not automatically instantiate objects when decoding JSON strings. Additionally, implementing strict input validation on all parameters passed to deserialization functions and auditing other areas of the codebase for similar patterns will help prevent further exploitation vectors from being introduced or overlooked in future updates.