CVE-2026-25766: Echo Go Framework Path Traversal Flaw

CVE-2026-25766 Overview

CVE-2026-25766 is a Path Traversal vulnerability affecting the Echo Go web framework. In versions 5.0.0 through 5.0.2 on Windows, Echo's middleware.Static using the default filesystem allows path traversal via backslashes, enabling unauthenticated remote file read outside the static root. The vulnerability stems from inconsistent handling of path separators between URL path cleaning and the Windows operating system.

Critical Impact

Unauthenticated attackers can remotely read arbitrary files outside the intended static file root directory on Windows systems running vulnerable versions of the Echo framework.

Affected Products

• Echo Go Web Framework versions 5.0.0 through 5.0.2
• Windows deployments using middleware.Static with default filesystem

Discovery Timeline

• 2026-02-19 - CVE CVE-2026-25766 published to NVD
• 2026-02-19 - Last updated in NVD database

Technical Details for CVE-2026-25766

Vulnerability Analysis

This vulnerability is classified as CWE-22 (Improper Limitation of a Pathname to a Restricted Directory), commonly known as Path Traversal or Directory Traversal. The flaw enables unauthenticated remote attackers to read files outside the designated static content directory on Windows systems.

The core issue lies in the path normalization process within middleware/static.go. When a request is received, the path is unescaped and normalized using path.Clean, which follows URL semantics. However, path.Clean does not recognize backslashes (\) as path separators—it treats them as regular characters. This means sequences like ..\ remain intact after cleaning.

When the cleaned path is passed to currentFS.Open(...) and the filesystem is set to the default (nil), Echo uses defaultFS which ultimately calls os.Open. On Windows, os.Open interprets backslashes as path separators and resolves ..\ sequences, allowing traversal outside the static root directory.

Root Cause

The root cause is the semantic mismatch between path.Clean (which uses forward-slash URL semantics) and Windows os.Open (which interprets backslashes as directory separators). The vulnerable code path allowed unfiltered backslash sequences to bypass path normalization and be processed by the operating system's file open function.

Attack Vector

The attack is network-accessible without requiring authentication or user interaction. An attacker crafts HTTP requests containing backslash-encoded path traversal sequences (e.g., ..\\..\\) targeting the static middleware endpoint. On Windows systems, these sequences bypass the path.Clean sanitization and are interpreted by os.Open, resulting in arbitrary file reads.

The following code shows the security patch applied in echo.go:

	dir, _ := os.Getwd()
	return &defaultFS{
		prefix: dir,
-		fs:     nil,
+		fs:     os.DirFS(dir),
	}
}

func (fs defaultFS) Open(name string) (fs.File, error) {
-	if fs.fs == nil {
-		return os.Open(name) // #nosec G304
-	}
	return fs.fs.Open(name)
}

Source: GitHub Commit Update

The fix replaces the direct os.Open call with os.DirFS(dir), which provides proper filesystem isolation and prevents traversal outside the designated directory root.

Detection Methods for CVE-2026-25766

Indicators of Compromise

• HTTP request logs containing backslash-encoded path traversal sequences such as ..\\ or %5C..%5C
• Requests to static endpoints with unusual path patterns attempting to access system files (e.g., ..\\..\\Windows\\System32\\)
• Web server error logs showing file access attempts outside the static root directory
• Anomalous file read operations from the web application process targeting sensitive system locations

Detection Strategies

• Implement web application firewall (WAF) rules to detect and block requests containing backslash path traversal sequences
• Monitor HTTP access logs for patterns matching ..\\ or URL-encoded variants (%5C, %2e%2e%5c)
• Deploy endpoint detection rules to alert on web server processes attempting to read files outside expected directories
• Enable verbose logging on static file middleware to track all file access requests

Monitoring Recommendations

• Configure log aggregation to correlate path traversal attempts across multiple endpoints
• Set up alerts for high-frequency requests containing path traversal indicators from single source IPs
• Monitor for access patterns targeting sensitive Windows system files such as win.ini, system.ini, or registry hives
• Review Echo framework version in deployed applications and flag any instances running 5.0.0 through 5.0.2

How to Mitigate CVE-2026-25766

Immediate Actions Required

• Upgrade Echo framework to version 5.0.3 or later immediately on all Windows deployments
• Audit application logs for any evidence of exploitation attempts using backslash traversal patterns
• Implement WAF rules to block requests containing ..\\ or encoded variants as a temporary measure
• Review any applications using middleware.Static with default filesystem configuration

Patch Information

The vulnerability is fixed in Echo version 5.0.3. The patch modifies the default filesystem implementation to use os.DirFS(dir) instead of directly calling os.Open, ensuring proper filesystem boundary enforcement. For detailed information, see the GitHub Security Advisory GHSA-pgvm-wxw2-hrv9 and the GitHub Pull Request #2891.

Workarounds

• Replace the default filesystem with a custom fs.FS implementation that properly sanitizes paths before file operations
• Implement reverse proxy filtering to reject requests containing backslash characters before they reach the Echo application
• Deploy network-level access controls to limit which clients can access static file endpoints
• Consider disabling static file middleware on Windows systems until the patch can be applied

# Upgrade Echo framework to patched version
go get github.com/labstack/echo/v5@v5.0.3
go mod tidy