CVE-2026-97155 in Folio Client
Summary
by MITRE • 09/24/2026
Fabasoft Folio Client before 2026, a locally installed component that communicates with the Fabasoft browser extension via web messaging, does not restrict which web origins may invoke its functions by default. The registry value VALIDDOMAINS, which limits permitted origins, was optional and empty by default, resulting in all domains being trusted. As a consequence, any website visited by a user with the Folio Client and browser extension installed could invoke client functions, e.g., related to downloading documents, opening documents, and synchronizing files. The first fixed builds are Fabasoft Folio Client 2026 (Build 26.0.0.10) and Fabasoft Folio Client 2026 April Release (Build 26.4.0.76). This client is, for example, shipped with Fabasoft eGov-Suite.
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Analysis
by VulDB Data Team • 09/24/2026
The vulnerability in question resides within the local installation of the Fabasoft Folio Client prior to version 2026, specifically affecting its communication mechanism with the associated browser extension via web messaging protocols. This component is designed to facilitate interactions between web-based applications and local file system operations, such as downloading, opening, or synchronizing documents. However, a critical architectural flaw exists in how origin validation is handled during these inter-process communications. By default, the client does not enforce strict restrictions on which web origins are permitted to invoke its exposed functions. The security control intended to mitigate this risk relies on a registry value named VALIDDOMAINS, which serves as an allowlist of trusted domains. In standard installations, this configuration parameter is optional and remains empty by default, effectively disabling origin validation and resulting in the client trusting all incoming requests regardless of their source domain.
This lack of input validation allows any website visited by a user who has both the Fabasoft Folio Client and its corresponding browser extension installed to interact with the local application without authorization. An attacker can craft a malicious webpage that exploits this trust relationship to invoke sensitive functions within the client software. These functions include initiating document downloads, opening files on the local machine, or triggering file synchronization processes. Because the vulnerability relies on user interaction in the form of visiting a compromised or malicious website, it falls under the category of cross-origin vulnerabilities where the browser's same-origin policy is bypassed due to improper validation by the native application host. This scenario aligns with CWE-942, which describes Permissive Cross-domain Whitelisting, as the application accepts requests from any domain rather than a predefined set of trusted sources.
The operational impact of this vulnerability is significant for users relying on Fabasoft eGov-Suite and related products that ship with the vulnerable client version. An attacker could potentially force the download of malicious files to the user's local system, leading to malware execution if the downloaded content is subsequently opened. Furthermore, the ability to open arbitrary documents or synchronize files could lead to data leakage or unauthorized modification of sensitive information stored locally. The attack vector requires no special privileges beyond those of the current user and typically involves social engineering tactics such as phishing links that direct victims to a malicious site hosting the exploit code. This aligns with MITRE ATT&CK technique T1059, Command and Scripting Interpreter, specifically through browser-based execution contexts, and potentially T1048, Exfiltration Over Alternative Protocol, if data is exfiltrated via synchronized files or downloaded payloads that communicate back to an attacker-controlled server.
Mitigation strategies primarily involve upgrading the Fabasoft Folio Client to a patched version where this origin validation logic has been corrected. The first fixed builds are identified as Fabasoft Folio Client 2026 with Build number 26.0.0.10 and the April Release of Fabasoft Folio Client 2026 with Build number 26.4.0.76. Until an upgrade can be performed, administrators should manually configure the VALIDDOMAINS registry value to include only the specific domains that are required for legitimate business operations. This explicit allowlisting approach ensures that even if a user visits a malicious website, the client will reject requests originating from untrusted origins. Additionally, organizations should enforce strict browser security policies and monitor for unusual file system activity associated with web-based interactions to detect potential exploitation attempts in environments where patching may be delayed.