Out Of Bounds Read in Echo Go with Bearer Tokens

An Out Of Bounds Read occurs when a service reads memory beyond the intended buffer. In Go services built with the Echo framework, this often arises when request handling logic trusts unchecked indices derived from client-supplied data. When Bearer Tokens are involved—typically passed via the Authorization header as Bearer <token>—the risk is amplified if the token is used to index into slices, arrays, or strings without proper length validation.

Consider an endpoint that extracts a token from the Authorization header and uses it to look up user permissions stored in a slice. If the token is parsed incorrectly or an attacker provides a malformed token that decodes to a numeric value, that value can be used as a slice index. Because Go does not automatically enforce bounds on slice accesses in unchecked code paths, reading permissions[tokenIndex] where tokenIndex exceeds the slice length triggers an out of bounds read. This can expose adjacent memory, leading to information disclosure or process instability.

Echo middleware that parses tokens and maps them to internal data structures is particularly vulnerable when developers assume the token format is controlled. For example, a developer might split a token string by a delimiter and cast a segment to an integer to use as an index. If the token does not contain that segment, or if the segment represents a number larger than the slice length, the read goes out of bounds. Attackers can probe such endpoints with crafted Authorization headers to observe anomalous responses or data leakage, which middleBrick detects through its Input Validation and Data Exposure checks.

Moreover, when OpenAPI specifications are present, mismatches between documented parameters and runtime behavior can obscure the out of bounds path. A spec might define a Bearer token as a simple string, while the implementation interprets it as a composite key. This discrepancy means runtime scans—like those performed by middleBrick—can uncover the vulnerability even when the spec appears correct. The scanner’s parallel checks for Authentication and Input Validation highlight cases where token handling leads to unsafe memory access patterns.

In practice, an attacker might send a request such as Authorization: Bearer 999999 to an endpoint expecting a token that maps to a small integer index. If the service uses strconv.Atoi on the token segment and uses the result directly as a slice index, the read may traverse into uninitialized or sensitive memory. middleBrick’s LLM/AI Security and Property Authorization checks are designed to surface such logic flaws by correlating authentication schemes with data access patterns.

Remediation focuses on strict validation of any data derived from the Bearer token before using it as a slice index or in memory-sensitive operations. Never trust token contents; treat them as opaque strings unless explicitly validated.

Example vulnerable code

package main

import (
    "net/http"
    "strconv"
    "strings"

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

var permissions = []string{"read", "write", "admin"}

func getPermission(c echo.Context) error {
    auth := c.Request().Header.Get("Authorization")
    if len(auth) < 7 || auth[:7] != "Bearer " {
        return c.String(http.StatusUnauthorized, "invalid auth")
    }
    token := auth[7:]
    // Vulnerable: using token segment as index without bounds check
    parts := strings.Split(token, ".")
    idx, _ := strconv.Atoi(parts[1]) // potential out of bounds read
    return c.String(http.StatusOK, permissions[idx])
}

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

Secured code pattern

package main

import (
    "errors"
    "net/http"
    "strconv"
    "strings"

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

var permissions = []string{"read", "write", "admin"}

func getPermission(c echo.Context) error {
    auth := c.Request().Header.Get("Authorization")
    if len(auth) < 7 || auth[:7] != "Bearer " {
        return c.String(http.StatusUnauthorized, "invalid auth")
    }
    token := auth[7:]
    parts := strings.Split(token, ".")
    if len(parts) < 2 {
        return c.String(http.StatusBadRequest, "malformed token")
    }
    idx, err := strconv.Atoi(parts[1])
    if err != nil || idx < 0 || idx >= len(permissions) {
        return c.String(http.StatusForbidden, "insufficient scope")
    }
    return c.String(http.StatusOK, permissions[idx])
}

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

Key remediation steps:

• Validate the Authorization header format strictly before parsing the token.
• Ensure any numeric extraction from the token includes error checking and bounds comparison against the length of the target slice.
• Avoid using raw token segments as indices; map tokens to identifiers via a controlled lookup map if needed, and verify the identifier exists before access.
• Use structured tokens (e.g., JWT) with verified claims instead of custom delimited strings, and validate claims server-side.

Tools like middleBrick’s CLI can help verify fixes by scanning the service after changes. Use middlebrick scan <url> to re-evaluate the authentication and input validation checks. The GitHub Action can enforce that no regression introduces unsafe token handling, and the MCP Server allows you to scan APIs directly from your IDE during development.