Description

Fiber Crashes in BodyParser Due to Unvalidated Large Slice Index in Decoder

Description

When using Fiber's Ctx.BodyParser to parse form data containing a large numeric key that represents a slice index (e.g., test.18446744073704), the application crashes due to an out-of-bounds slice allocation in the underlying schema decoder.

The root cause is that the decoder attempts to allocate a slice of length idx + 1 without validating whether the index is within a safe or reasonable range. If idx is excessively large, this leads to an integer overflow or memory exhaustion, causing a panic or crash.

Steps to Reproduce

Create a POST request handler that accepts x-www-form-urlencoded data

package main

import (
	"fmt"
	"net/http"

	"github.com/gofiber/fiber/v2"
)...
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Run the server and send a POST request with a large numeric key in form data, such as:

curl -v -X POST localhost:3000 --data-raw 'test.18446744073704' \
  -H 'Content-Type: application/x-www-form-urlencoded'

Relevant Code Snippet

Within the decoder's decode method:

idx := parts[0].index
if v.IsNil() || v.Len() < idx+1 {
    value := reflect.MakeSlice(t, idx+1, idx+1)  // <-- Panic/crash occurs here when idx is huge
    if v.Len() < idx+1 {
        reflect.Copy(value, v)
    }
    v.Set(value)
}...
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The idx is not validated before use, leading to unsafe slice allocation for extremely large values.

Impact

Application panic or crash on malicious or malformed input.

Potential denial of service (DoS) via memory exhaustion or server crash.

Lack of defensive checks in the parsing code causes instability.

Mitigation

Aliases

References