Authentication vulnerabilities can create serious security risks for organisations, even without directly compromising the system. CVE-2026-51926 is a High-severity vulnerability in docuForm FSM Client v11.11c, classified as a User Enumeration flaw (CWE-203). The application’s login process reveals whether a username exists or not based on differences in its responses. This information can be used by an attacker to conduct brute-force attacks, credential stuffing, and phishing campaigns.
What is CVE-2026-51926
CVE-2026-51926 is a security vulnerability in docuForm FSM Client version 11.11c. It arises because the login.php file returns different responses depending on whether a submitted username is valid or not. This difference allows an attacker to determine whether an account exists without needing any valid credentials. Although this vulnerability does not grant direct system access, it provides information that can be used to launch more targeted attacks.
What is docuForm FSM Client
docuForm FSM Client is an enterprise software application developed by docuForm GmbH. It is used for document management, form management, and workflow automation in organisations. Because the software handles sensitive organisational data, securing its authentication system is essential. A weakness in the login process can expose stored data to unauthorised access.
Understanding User Enumeration (CWE-203)
User Enumeration (CWE-203: Observable Response Discrepancy) occurs when an application unintentionally reveals whether a username exists in the system. This happens when the login page responds differently to valid and invalid usernames. An attacker can exploit this inconsistency to build a list of valid accounts and then launch a targeted password attack against them.
How Does CVE-2026-51926 Work
The vulnerability exists in the login.php file of docuForm FSM Client version 11.11c. An attacker submits login attempts using multiple usernames with incorrect passwords. If the application responds differently for a valid username compared to an invalid one, the attacker can identify which accounts exist. Once valid usernames are identified, the attacker can conduct brute-force, password spraying, or credential stuffing attacks.
Affected docuForm FSM Client Versions
CVE-2026-51926 affects docuForm FSM Client version 11.11c. Systems running this version are vulnerable if the login page returns different responses for valid and invalid usernames. Organisations using this version should review the vendor’s security advisory and apply any available patches promptly.
CVSS Score and Severity of CVE-2026-51926
CVE-2026-51926 has a CVSS v3.1 score of 7.5, classified as High severity. While the vulnerability does not enable remote code execution, it exposes information about valid user accounts, significantly increasing the risk of brute-force, credential stuffing, and phishing attacks. Organisations running the affected version should apply the recommended security updates and authentication hardening measures without delay.
Impact of CVE-2026-51926
The primary impact of CVE-2026-51926 is that it allows an attacker to enumerate valid user accounts through observable differences in the login.php response. This information serves as a foundation for more targeted attacks against the affected system.
Observable Response Discrepancies
When checking login credentials, the server responds differently depending on whether a submitted username exists in the system or not. These variations can manifest as distinct error messages, different HTTP status codes, or measurable differences in server processing times.
Targeted Account Discovery
Because an attacker can reliably tell valid usernames apart from invalid ones, they can programmatically scan a list of potential names to map out active corporate accounts.
Gateway to Deeper Exploits
While user enumeration does not directly compromise the server, it acts as a highly effective reconnaissance step. Having a verified list of active usernames allows attackers to launch targeted brute-force, password-spraying, or credential-stuffing attacks with a much higher success rate.
How to Fix CVE-2026-51926
Since this vulnerability is tied to the design of the authentication scripts within the login.php file, resolving it requires updating the code’s handling of failed logins.
Apply Vendor Security Patches
Ensure your docuForm FSM Client installation is updated past version 11.11c. Check the official docuForm distribution channels or contact their technical support to obtain the patch that addresses authentication response consistency.
Unify Authentication Responses
Modify the backend code in login.php so that the server returns an identical, generic response (e.g., “Invalid username or password”) regardless of whether the username exists.
Implement Server-Side Rate Limiting
While patching, configure your web server (such as Nginx or Apache) or application firewall to rate-limit requests to the /login.php endpoint. Restricting the frequency of requests from a single IP address significantly hinders automated script scanning.
Best Practices to Prevent User Enumeration Attacks
User enumeration occurs when an application leaks state information through its login, password-reset, or registration flows. To prevent this class of information disclosure across your web servers:
- Use Generic Error Messages: Ensure that all login, password-reset, and sign-up interfaces return identical error text. Avoid exposing verbose prompts such as “Username does not exist” or “Incorrect password for this account.”
- Normalize Response Latency (Time-Constant Verification): Sometimes, a system verifies a password only after confirming the username exists. Because password hashing is computationally heavy, queries with valid usernames can take longer to process than invalid ones.
- Secure the Password Reset Flow: When a user requests a password reset link, return a neutral message on-screen, such as: “If this email address is registered, we have sent a password reset link.” Avoid indicating whether the email was successfully found in the database.
- Enforce CAPTCHA Challenges: Introduce a visual challenge (like reCAPTCHA or hCaptcha) on your login and recovery pages after a few failed attempts. This disrupts brute-force enumeration tools while keeping the user experience clean for legitimate logins.
Frequently Asked Questions (FAQs)
Conclusion
Reconnaissance is often one of the earliest stages of a cyber attack. CVE-2026-51926 demonstrates how differences in server responses can allow attackers to map out valid user accounts. Standardising login error messages, normalising response times, and updating docuForm FSM Client to a version beyond 11.11c are the most effective steps to close this exposure.
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