CVE-2026-103521 in Thelia
Summary
by MITRE • 10/12/2026
In Thelia v2.6.0, the document upload endpoint properly blocks the upload of executable file types, but the document update endpoint does not perform the same validation. An attacker with administrative Thelia permissions can exploit this vulnerability to upload arbitrary executable files, such as malicious PHP scripts, to achieve remote code execution on the underlying server.
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Analysis
by VulDB Data Team • 10/12/2026
The vulnerability in Thelia version 2.6.0 represents a critical inconsistency in input validation logic between two distinct API endpoints within the application's document management subsystem. While the initial upload endpoint correctly implements security controls by blocking executable file types, such as PHP scripts or other server-side interpretable files, the subsequent update endpoint fails to replicate these same restrictions. This discrepancy creates an asymmetric defense posture where the primary ingestion point is secured against direct malicious uploads, but a secondary modification pathway remains wide open for exploitation. The root cause lies in the lack of centralized validation logic; instead of inheriting or calling a shared security function that checks file extensions and content types during any write operation to the document storage area, the update endpoint operates with insufficient scrutiny regarding the nature of the files being modified.
From an attacker's perspective, this flaw allows for remote code execution if they possess administrative privileges within the Thelia application. Although requiring admin access raises the bar for initial exploitation compared to unauthenticated attacks, it does not eliminate the severity of the risk, particularly in environments where privilege escalation or insider threats are concerns. An adversary with valid credentials can navigate to the document update interface and replace a legitimate file with one containing malicious code, such as a PHP web shell. Because the server processes these files according to their extension rather than strictly validating them against a whitelist during the update process, the uploaded executable is stored in a publicly accessible directory or processed by the backend interpreter, leading directly to arbitrary command execution on the host system.
The operational impact of this vulnerability extends beyond simple file upload issues. Successful exploitation enables an attacker to gain full control over the underlying web server infrastructure. This can lead to data exfiltration, modification of application logic, installation of backdoors for persistent access, and lateral movement within the internal network. In e-commerce platforms like Thelia, such compromise could result in significant financial loss, reputational damage, and violation of regulatory compliance standards regarding customer data protection. The ability to execute arbitrary code effectively bypasses all other security controls that rely on application-level authentication, rendering them ineffective once this foothold is established.
This vulnerability aligns with CWE-434, which describes the unrestricted upload of files with dangerous types, a common flaw in web applications where developers fail to enforce consistent validation across all file handling operations. Furthermore, it maps to MITRE ATT&CK technique T1505.003, Server Component Binary Injection, as well as T1105, Ingress Tool Transfer, depending on the specific method used to deliver the payload. The attack vector involves manipulating server-side processes through trusted administrative interfaces, highlighting a failure in defense-in-depth principles where multiple layers of validation should be applied regardless of the entry point.
To mitigate this risk, developers must refactor the document handling logic to ensure that all write operations, including updates and replacements, utilize the same rigorous validation checks as the initial upload process. This involves implementing a centralized file verification service that inspects both the file extension and content type against an allowlist of safe formats before any data is persisted to disk. Additionally, deploying Web Application Firewalls with specific rules for detecting malicious payloads in update requests can provide an additional layer of defense. Regular security audits focusing on API endpoint consistency are essential to identify similar asymmetries in other parts of the application architecture.