Buffer Overflow in Fiber with Jwt Tokens

A buffer overflow in a Fiber application that processes JWT tokens typically arises when untrusted token content is copied into fixed-size buffers without proper length checks. In C-based Fiber services or extensions (for example, when using a C HTTP parser or native addons), a JWT token that contains a long string in its payload or in header parameters such as kid or x5c can overflow a statically sized buffer. The overflow may corrupt adjacent memory, leading to unpredictable behavior or potentially allowing an attacker to redirect execution flow.

Because JWT tokens are often passed in HTTP headers (commonly the Authorization: Bearer <token> header), an attacker can supply a very large token crafted with oversized claims or deeply nested header fields. If the server-side code uses functions that do not validate input length before copying—such as strcpy, memcpy, or unsafe slicing in native bindings—this can trigger a classic stack-based buffer overflow. Even in safer contexts, such as when using a JSON parser that returns string views into a reused buffer, failing to bound the token length can lead to reading past allocated memory or writing beyond a fixed-size output buffer during token construction or validation.

Additionally, if the JWT token includes an x5c (X.509 certificate chain) claim containing large base64-encoded certificates, parsing and storing these values without size limits can expose the application to overflows in native verification code. In a Fiber service, this often manifests when integrating with C-based cryptography libraries that expect fixed-size inputs. The risk is compounded when the server does not enforce reasonable limits on incoming token size before handing the token to lower-level processing routines.

To detect and prevent these issues, scanning API endpoints including those using JWT tokens helps uncover risky input handling patterns. middleBrick scans unauthenticated attack surfaces and can surface findings related to input validation and unsafe consumption that are relevant when JWT tokens are processed in low-level code paths.

Remediation focuses on avoiding fixed-size buffers when handling JWT tokens and validating lengths before any copy operation. Prefer dynamic allocations or language-level strings that grow as needed, and enforce strict upper bounds on token size at the earliest point of ingestion.

Example 1: Safe handling in C with length checks before copying token data into a response buffer.

#define MAX_TOKEN_LENGTH 8192

int handle_request(const char *auth_header, char *out_buffer, size_t out_len) {
    if (!auth_header || strncmp(auth_header, "Bearer ", 7) != 0) {
        return -1;
    }
    const char *token = auth_header + 7;
    size_t token_len = strlen(token);
    if (token_len == 0 || token_len > MAX_TOKEN_LENGTH || token_len >= out_len) {
        return -1;
    }
    strncpy(out_buffer, token, out_len - 1);
    out_buffer[out_len - 1] = '\0';
    return 0;
}

Example 2: A safer approach in Go Fiber applications by limiting the size of the raw token before passing it to JWT parsing.

func VerifyToken(c *fiber.Ctx) error {
    auth := c.Get("Authorization")
    if !strings.HasPrefix(auth, "Bearer ") {
        return c.SendStatus(fiber.StatusUnauthorized)
    }
    token := strings.TrimPrefix(auth, "Bearer ")
    if len(token) > 8192 {
        return c.SendStatus(fiber.StatusRequestEntityTooLarge)
    }
    parsed, err := jwt.Parse(token, func(t *jwt.Token) (interface{}, error) {
        return []byte("your-secret"), nil
    })
    if err != nil || !parsed.Valid {
        return c.SendStatus(fiber.StatusUnauthorized)
    }
    return c.JSON(parsed.Claims)
}

Example 3: Using the middleware available in the official GitHub Action to enforce limits and fail builds when risk scores are too high. This CI/CD integration helps catch unsafe token handling before deployment.

# .github/workflows/api-security.yml
name: API Security Checks
on: [push]
jobs:
  scan:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - name: Run middleBrick scan
        uses: middlebv/middlebrick-action@v1
        with:
          url: ${{ secrets.TEST_API_URL }}
          threshold: C

Example 4: In languages with native bindings, always specify size-bounded copies and validate the JWT header kid and payload lengths before use. Never pass unbounded user-controlled strings directly to native verification functions.

size_t copy_escaped_token(const char *src, char *dest, size_t dest_size) {
    size_t len = strnlen(src, dest_size - 1);
    if (len == dest_size - 1) {
        // Token too long; handle error or truncate safely
        return (size_t)-1;
    }
    memcpy(dest, src, len);
    dest[len] = '\0';
    return len;
}