Symlink Attack in Echo Go with Bearer Tokens

A symlink attack in an Echo Go service using Bearer tokens can occur when file system operations intersect with token validation logic in ways that bypass intended access controls. In this scenario, an API endpoint accepts a filename or path parameter, resolves it on disk, and uses a Bearer token to decide whether to allow the request. If the endpoint resolves paths before validating the token, or uses path traversal inputs to locate files, an attacker can place a symbolic link that redirects the resolution to a sensitive file. Because the Bearer token is checked after or independently of path resolution, the request appears authorized even though the resolved file is outside the permitted directory.

For example, an endpoint designed to serve user-uploaded documents might join a base directory with a user-supplied filename. An attacker can craft a request with a path like ../../../etc/shadow or a carefully crafted relative path that traverses outside the intended directory. If a symlink exists or can be created at an intermediate location, the resolved path points to a sensitive system file. The Echo Go handler validates the Bearer token successfully, but the file served is not the one the developer intended to expose. This combination of path manipulation and token-based authorization creates a bypass where the token’s presence prevents outright rejection, yet the actual data access is uncontrolled.

Another variant involves storing temporary files or caches keyed by token identifiers without ensuring directory isolation. If token values or associated metadata are used to build filesystem paths without normalization, an attacker who can control a symlink in a shared temporary location can redirect reads or writes. Because the Echo route uses Bearer tokens for authorization but relies on unchecked path concatenation, the token becomes a misleading proof of identity rather than a gatekeeper for safe file access. This pattern is common when developers assume that token validation alone is sufficient to protect filesystem operations, without canonicalizing paths or confining them to a strict root.

These issues are not theoretical; they map to common web vulnerabilities such as path traversal and insecure direct object references, which appear in the OWASP API Top 10. The presence of Bearer tokens does not inherently prevent filesystem abuse; it only provides identity context. Without strict path controls, the token becomes an incidental component rather than a security boundary. The scanner categories in middleBrick, such as BOLA/IDOR and Input Validation, are designed to detect conditions where path resolution and authorization are misaligned, including scenarios involving symlinks and token misuse.

Remediation centers on ensuring that Bearer token validation and filesystem path resolution are tightly coupled and that paths are canonicalized and confined before any file operation. In Echo Go, implement a handler that first validates the token, derives an allowed base path from the token’s claims, and then resolves the requested filename against that base using functions that eliminate traversal sequences.

Use filepath.Clean and filepath.Abs to canonicalize paths, and ensure the final path remains within the intended directory by checking it starts with the base prefix. Do not rely on the token alone to prevent access to arbitrary files; enforce directory boundaries in the resolution step itself.

package main

import (
    "fmt"
    "net/http"
    "path/filepath"
    "strings"

    "github.com/labstack/echo/v4"
)

// deriveBaseFromToken maps a Bearer token to an allowed base directory.
// In practice, validate the token and extract claims to determine the base.
func deriveBaseFromToken(token string) (string, bool) {
    // Example mapping; use proper token validation and claims extraction.
    switch token {
    case "valid-token-abc":
        return "/safe/base/user1", true
    case "valid-token-def":
        return "/safe/base/user2", true
    default:
        return "", false
    }
}

func safeFileHandler(c echo.Context) error {
    auth := c.Request().Header.Get("Authorization")
    if auth == "" {
        return echo.NewHTTPError(http.StatusUnauthorized, "missing authorization")
    }
    const bearerPrefix = "Bearer "
    if !strings.HasPrefix(auth, bearerPrefix) {
        return echo.NewHTTPError(http.StatusUnauthorized, "invalid authorization scheme")
    }
    token := strings.TrimPrefix(auth, bearerPrefix)
    base, ok := deriveBaseFromToken(token)
    if !ok {
        return echo.NewHTTPError(http.StatusForbidden, "invalid token")
    }

    requested := c.QueryParam("file")
    if requested == "" {
        return echo.NewHTTPError(http.StatusBadRequest, "file parameter required")
    }
    // Canonicalize and confine to base.
    clean := filepath.Clean("/" + requested)
    if strings.HasPrefix(clean, "//") {
        clean = clean[1:]
    }
    full := filepath.Join(base, clean)
    absBase, _ := filepath.Abs(base)
    absFull, err := filepath.Abs(full)
    if err != nil {
        return echo.NewHTTPError(http.StatusInternalServerError, "server error")
    }
    if !strings.HasPrefix(absFull+string(filepath.Separator), absBase+string(filepath.Separator)) && absFull != absBase {
        return echo.NewHTTPError(http.StatusForbidden, "path traversal detected")
    }

    // At this point, token validated and path confined; safe to operate.
    c.Response().Header().Set("X-Token-User", token)
    return c.File(absFull)
}

func main() {
    e := echo.New()
    e.GET("/files", safeFileHandler)
    e.Start(":8080")
}

In this example, the Bearer token is validated first, and the base directory is derived from the token’s identity. The requested file path is cleaned and joined under the base, and an absolute prefix check ensures it cannot escape the intended directory. This approach prevents symlink-based bypasses because the resolved path is verified against the token-derived root before any file system access. middleBrick scans can verify that such controls are present by checking for proper authorization and input validation across the API surface.
Additionally, avoid using token identifiers directly as filenames or in temporary file paths without namespace isolation. If you must store files associated with tokens, incorporate a one-way mapping (e.g., hash of token) and enforce strict directory permissions. These practices reduce the risk that a symlink placed in a shared location affects token-bound resources. The combination of token-bound base paths, canonicalization, and containment checks aligns with remediation guidance reflected in middleBrick’s findings and supports compliance mappings to frameworks such as OWASP API Top 10 and SOC2.