CVE-2026-25891

Description
A Path Traversal (CWE-22) vulnerability in Fiber allows a remote attacker to bypass the static middleware sanitizer and read arbitrary files on the server file system on Windows. This affects Fiber v3 through version 3.0.0. This has been patched in Fiber v3 version 3.1.0.

Details

The vulnerability resides in middleware/static/static.go within the sanitizePath function. This function attempts to sanitize the requested path by checking for backslashes, decoding the URL, and then cleaning the path.

The vulnerability stems from two combined issues:

• The check for backslash characters happens before the URL decoding loop. If an attacker sends a double-encoded backslash, the initial check sees %255C and passes. The loop then decodes this into a single backslash.
• The function uses path.Clean to clean the resulting string. path.Clean is designed for slash-separated paths and does not recognize backslashes as directory separators. Consequently, sequences like ..\..\ are treated as valid filenames.

When this sanitized path is later used, the backslashes are interpreted as valid separators, allowing the attacker to traverse up the directory tree.

// pkg/static/static.go
func sanitizePath(p []byte, filesystem fs.FS) ([]byte, error) {
    ...
    // this check happens BEFORE decoding
    if bytes.IndexByte(p, '\\') >= 0 {
        ...
    } 
    // This loop decodes %255C to %5C to \
    for strings.IndexByte(s, '%') >= 0 {
        us, err := url.PathUnescape(s)
        ...
        s = us
    }
    // path.Clean only understands forward slashes (/)
    s = pathpkg.Clean("/" + s)
    ...
    return utils.UnsafeBytes(s), nil
}

This impacts Fiber v3 prereleases through stable release version 3.0.0.

Successful exploitation requires the server to be using the static middleware on Windows, as this is the only OS where backslashes are treated as directory separators by the file system.

Exploitation allows directory traversal on the host server. An attacker can read arbitrary files within the scope of the application server context. Depending on permissions and deployment conditions, attackers may access sensitive files outside the web root, such as configuration files, source code, or system files. Leaking application secrets often leads to further compromise.

Input manipulates file paths to reach files outside the intended directory, such as configuration or credential files. Typical impact: unauthorized file read or write outside the intended directory.

Kodem intelligence

This has been patched in Fiber v3 version 3.1.0. Users are strongly encouraged to update to the latest available release.

1. What is CVE-2026-25891? CVE-2026-25891 is a high-severity path traversal vulnerability in github.com/gofiber/fiber/v3 (go), affecting versions <= 3.0.0. It is fixed in 3.1.0. Input manipulates file paths to reach files outside the intended directory, such as configuration or credential files.
2. Which versions of github.com/gofiber/fiber/v3 are affected by CVE-2026-25891? github.com/gofiber/fiber/v3 (go) versions <= 3.0.0 is affected.
3. Is there a fix for CVE-2026-25891? Yes. CVE-2026-25891 is fixed in 3.1.0. Upgrade to this version or later.
4. Is CVE-2026-25891 exploitable, and should I be worried? Whether CVE-2026-25891 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
5. What actually determines whether CVE-2026-25891 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
6. How do I fix CVE-2026-25891? Upgrade github.com/gofiber/fiber/v3 to 3.1.0 or later.