CVE-2026-25891: Fiber Framework Path Traversal Flaw

CVE-2026-25891 Overview

A Path Traversal vulnerability (CWE-22) has been identified in Fiber, a popular Express-inspired web framework written in Go. This vulnerability allows remote attackers to bypass the static middleware sanitizer and read arbitrary files on the server file system when running on Windows systems. The flaw exists in how Fiber handles path sanitization, specifically failing to properly handle backslash characters which are valid path separators on Windows.

Critical Impact

Remote attackers can exploit this vulnerability to read sensitive files from the server, potentially exposing configuration files, credentials, source code, and other confidential data without authentication.

Affected Products

• Fiber v3 through version 3.0.0
• Applications using Fiber's static middleware on Windows servers
• Web applications serving static files through the vulnerable Fiber versions

Discovery Timeline

• 2026-02-24 - CVE-2026-25891 published to NVD
• 2026-02-24 - Last updated in NVD database

Technical Details for CVE-2026-25891

Vulnerability Analysis

This Path Traversal vulnerability stems from insufficient input validation in Fiber's static middleware component. The sanitizePath function failed to account for Windows-specific path separators (backslashes), allowing attackers to craft malicious requests that bypass the security controls designed to prevent directory traversal attacks. When the static middleware processes a request containing backslash-encoded path sequences, it fails to normalize these paths properly, enabling access to files outside the intended web root directory.

The vulnerability is particularly impactful because it requires no authentication and can be exploited remotely over the network with low complexity. An attacker can read any file that the web server process has permissions to access, potentially leading to exposure of sensitive configuration files, environment variables, database credentials, or application source code.

Root Cause

The root cause lies in the sanitizePath function within middleware/static/static.go which did not properly handle backslash characters (\) as path separators. On Windows systems, both forward slashes (/) and backslashes (\) are valid path separators. The original sanitization logic only accounted for forward slashes, allowing attackers to use backslash sequences like ..\ to traverse directories and escape the static file serving root.

Attack Vector

An attacker can exploit this vulnerability by sending specially crafted HTTP requests to a Fiber application serving static files. By including backslash-encoded directory traversal sequences in the URL path, the attacker can navigate outside the designated static file directory and access arbitrary files on the Windows file system.

For example, a request containing path components like ..\..\..\..\etc\passwd or ..\..\..\..\windows\system32\config\sam could potentially access sensitive system files, depending on the server's file permissions and configuration.

The security patch addresses this by adding proper handling of backslashes in the path sanitization logic:

 	"os"
 	pathpkg "path"
 	"path/filepath"
+	"slices"
 	"strconv"
 	"strings"
 	"sync"

Source: GitHub Commit

Additionally, the fix ensures proper return handling in the error path:

 		if catch := ctx.App().ErrorHandler(ctx, err); catch != nil {
 			_ = ctx.SendStatus(StatusInternalServerError) //nolint:errcheck // Always return nil
 		}
-		// TODO: Do we need to return here?
+		return
 	}
 }

Source: GitHub Commit

Detection Methods for CVE-2026-25891

Indicators of Compromise

• HTTP requests containing backslash sequences (\) or encoded backslashes (%5C) in URL paths targeting static file endpoints
• Access log entries showing unusual path patterns attempting to traverse parent directories using backslash notation
• Requests for sensitive files such as configuration files, .env files, or system files that should not be accessible
• Multiple failed or successful requests from a single source attempting various traversal patterns

Detection Strategies

• Implement Web Application Firewall (WAF) rules to detect and block requests containing path traversal patterns including backslash-encoded sequences
• Configure IDS/IPS signatures to alert on HTTP requests with ..\\, %2e%2e%5c, or similar encoded traversal attempts
• Review application access logs for suspicious patterns indicating directory traversal attempts against static file endpoints
• Deploy file integrity monitoring on sensitive configuration files to detect unauthorized access

Monitoring Recommendations

• Enable detailed access logging for Fiber applications, including full request paths and client IP addresses
• Set up alerts for anomalous file access patterns, particularly requests targeting files outside the static content directory
• Monitor for error responses (403, 404) that may indicate probing attempts by attackers testing traversal payloads
• Implement rate limiting on static file endpoints to slow down automated exploitation attempts

How to Mitigate CVE-2026-25891

Immediate Actions Required

• Upgrade Fiber v3 to version 3.1.0 or later immediately, as this version contains the security patch
• Review application logs for any historical evidence of exploitation attempts before the patch was applied
• Conduct a security assessment of files that may have been accessed if exploitation is suspected
• Temporarily disable static file serving or implement additional access controls while upgrading

Patch Information

The vulnerability has been patched in Fiber v3 version 3.1.0. The fix was implemented through Pull Request #4064, which updates the sanitizePath function to properly handle backslash characters on Windows systems. The security patch is available in commit 59133702301c2ab7b776dd123b474cbd995f2c86.

For complete details, refer to the GitHub Security Advisory GHSA-m3c2-496v-cw3v.

Workarounds

• Deploy a reverse proxy (nginx, Apache, or Caddy) in front of the Fiber application configured to normalize and validate all request paths before forwarding
• Implement custom middleware to sanitize incoming request paths by rejecting any requests containing backslash characters or encoded directory traversal sequences
• Restrict the static file middleware to serve only explicitly whitelisted file extensions and paths
• Run the Fiber application on Linux/Unix systems where backslash is not a valid path separator, reducing the attack surface

# Example: nginx configuration to block path traversal attempts
location / {
    # Block requests with backslashes or encoded traversal sequences
    if ($request_uri ~* "\\\\|\\.\\.|%2e%2e|%5c") {
        return 403;
    }
    proxy_pass http://fiber_backend;
}