CVE-2026-114101 in Coraza
Summary
by MITRE • 10/22/2026
Coraza before 3.8.1 extracts multipart upload filenames with Go's mime.ParseMediaType, which silently ignores RFC 5987 filename* parameters whose charset is not utf-8 or us-ascii. Unauthenticated attackers can send a decoy filename with filename*=iso-8859-1''shell.php, or filename* alone, so FILES-based rules never see the real name, letting dangerous uploads reach backends that honor filename*.
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Analysis
by VulDB Data Team • 10/22/2026
The vulnerability identified in Coraza versions prior to 3.8.1 stems from a critical flaw in how multipart form data filenames are parsed and processed by the underlying Go standard library function mime.ParseMediaType. This parsing mechanism adheres strictly to RFC 5987, which defines extended filename parameters using the filename* syntax with an optional charset specification. However, Coraza's implementation fails to handle cases where the specified character set is neither UTF-8 nor US-ASCII. When such a non-standard or unsupported encoding is detected in the filename parameter, the parser silently discards this specific attribute rather than raising an error or falling back to alternative parsing strategies. This silent failure creates a significant blind spot in the web application firewall's inspection capabilities, allowing attackers to bypass security controls that rely on file extension analysis for malicious content detection.
This technical flaw enables unauthenticated attackers to execute sophisticated upload attacks by exploiting the discrepancy between how the firewall parses headers and how backend servers interpret them. An attacker can craft a multipart request containing two filename parameters: one standard filename parameter with a benign name, such as image.jpg, which passes through Coraza's inspection rules without triggering any alerts, and a secondary filename parameter using an unsupported charset like ISO-8859-1 that contains the actual malicious payload, such as shell.php. Because Coraza ignores the filename parameter due to its encoding mismatch, it allows the request to pass based on the benign standard filename. However, many backend application servers are more lenient and will honor the RFC 5987 compliant filename* parameter when saving or processing the uploaded file. This divergence in behavior results in a dangerous PHP shell being written to disk despite appearing safe to the firewall's FILES-based rules.
The operational impact of this vulnerability is severe, as it effectively neutralizes one of the primary defense mechanisms against remote code execution via file uploads. By bypassing extension checks and content validation that are typically applied to uploaded files, attackers can gain unauthorized access to server resources, execute arbitrary commands, and potentially compromise the entire infrastructure behind the web application firewall. This type of attack is particularly dangerous because it does not require authentication or complex exploitation techniques beyond crafting a specific HTTP request with carefully encoded multipart boundaries. The silent nature of the parsing error means that no security alerts are generated during the initial inspection phase, leaving defenders unaware of the intrusion until malicious activity manifests on the backend systems.
To mitigate this vulnerability, organizations running Coraza versions earlier than 3.8.1 must upgrade to version 3.8.1 or later immediately. This update addresses the parsing logic to ensure that all filename parameters are handled consistently and securely, preventing attackers from exploiting charset discrepancies to hide malicious filenames. In addition to upgrading, administrators should implement defense-in-depth strategies by configuring backend servers to strictly validate file extensions against a whitelist of allowed types rather than relying solely on HTTP headers for identification. Furthermore, enabling strict MIME type verification at the application layer can provide an additional barrier against these attacks. Monitoring logs for unusual multipart request patterns and ensuring that web server configurations do not blindly trust filename parameters from user input are also recommended best practices to reduce exposure to similar bypass techniques in other contexts.