DNN Vulnerability Exposes NTLM Credentials via Unicode Normalization Bypass

Security researchers have discovered a critical vulnerability in DNN (formerly DotNetNuke), one of the oldest open-source content management systems, that allows attackers to steal NTLM credentials through a sophisticated Unicode normalization bypass technique.

The vulnerability, tracked as CVE-2025-52488, affects the widely-used enterprise CMS platform and demonstrates how defensive coding measures can be circumvented through carefully crafted exploits that abuse Windows and .NET framework quirks.

CVE-2025-52488: Technical Details and Impact

CVE-2025-52488 represents a significant security flaw that enables attackers to make SMB calls to arbitrary hosts without authentication.

The vulnerability stems from a pre-authentication file upload endpoint in DNN’s HTML editor provider, specifically located in FileUploader.ashx.cs.

Despite multiple security boundaries implemented by developers to prevent malicious file path manipulation, the vulnerability exploits Unicode normalization that occurs after these protective measures.

The attack vector leverages the dangerous behavior of C#’s Path.Combine function, which ignores previous path components when the second argument contains an absolute path.

When combined with Windows file system operations like File.Exists, this creates an opportunity for attackers to trigger SMB connections to attacker-controlled servers, potentially exposing NTLM credentials.

The vulnerability exploits DNN’s ConvertUnicodeChars function, which normalizes Unicode characters to ASCII after security validation.

Researchers discovered that specific Unicode characters—U+FF0E (fullwidth full stop) and U+FF3C (fullwidth reverse solidus)—bypass initial security filters but normalize to standard periods and backslashes during ASCII conversion.

The exploitation process involves uploading a file with a specially crafted filename containing these Unicode characters.

The filename %EF%BC%BC%EF%BC%BCoqi3o3fv9cpyquhbd6h8bx19a0gs4nsc%EF%BC%8Eoastify%EF%BC%8Ecom%EF%BC%BC%EF%BC%BCc$%EF%BC%BC%EF%BC%BCan.jpg demonstrates how attackers can construct UNC paths that survive initial validation but become malicious after normalization.

Security Implications and Mitigation

This vulnerability highlights the persistent threat of NTLM credential theft in Windows environments, particularly on older systems hosting legacy applications like DNN.

The attack succeeds because the Unicode normalization occurs after security boundaries are checked, creating a classic time-of-check-time-of-use vulnerability.

Organizations running DNN should immediately update to patched versions and implement network-level mitigations to prevent SMB connections to external hosts.

 The vulnerability also affects authenticated users through a similar endpoint in Browser.aspx.cs, though this vector requires authentication.

The discovery underscores the importance of proper Unicode handling in security-critical applications and the need for comprehensive security testing that considers character encoding transformations in the context of file system operations.

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