Out Of Bounds Read in Fiber with Api Keys

An Out Of Bounds Read in a Fiber application that uses API keys typically arises when key material is handled without strict length and boundary checks. In Go, slicing a byte slice beyond its capacity produces a slice that points to memory outside the intended allocation. If API keys are stored in byte slices or strings and manipulated with unsafe slicing, an attacker can influence the slice length and access memory that belongs to other variables, potentially leaking private data or causing a panic that aids further exploitation.

Consider a handler that receives an API key as a URL query parameter, validates its prefix, and then slices the key to extract a portion for internal use:

key := c.Param("key")
if len(key) < 10 {
    c.Status(http.StatusBadRequest)
    return
}
prefix := key[:5]
suffix := key[5:200]

Here, the prefix check ensures at least 10 bytes, but the suffix slice key[5:200] can exceed the underlying key’s length if the key is only 12 bytes. This creates an out-of-bounds read on the key’s backing array, reading memory beyond the intended data. If the key is stored in a larger byte buffer elsewhere, the out-of-bounds read may expose adjacent memory, including bytes that should remain private.

In a multi-tenant setup where API keys are stored in shared buffers or maps, an out-of-bounds read can cross boundaries between keys. For example, if keys are stored as fixed-size byte arrays in a pool and a handler incorrectly computes offsets, reading past the end of one key can return bytes belonging to another tenant’s key:

type keyPool [][32]byte
var pool [1024]keyPool
func getKey(id, offset int) []byte {
    return pool[id][offset:] // offset beyond 32 causes OOB read
}

When combined with API key usage patterns, such out-of-bounds reads can violate the isolation expected between authenticated requests. An attacker supplying crafted parameters or IDs can observe behavior changes or extract fragments of unrelated keys, which may assist in further attacks like privilege escalation or data exposure. The vulnerability is not about breaking authentication directly, but about undermining the integrity of key handling through unsafe memory access patterns.

Fiber’s routing and parameter parsing do not inherently introduce out-of-bounds reads; the risk comes from how application code processes key material. Even when using the official middlebrick scan to detect authentication issues and BOLA/IDOR findings, developers must still ensure bounds checks are correct before slicing or indexing key buffers. A scan can highlight risky patterns, but remediation requires careful Go-level boundary validation.

Remediation centers on eliminating unsafe slicing and enforcing strict length and bounds checks before any key manipulation. Always validate the length of the key against the expected size and use subslices only within verified limits. Prefer copying key material into fixed-size buffers when interacting with low-level operations to avoid accidental out-of-bounds access.

Instead of slicing with open-ended ranges, use explicit bounds and copy data into appropriately sized destinations:

const expectedKeyLen = 32
key := c.Param("key")
if len(key) != expectedKeyLen {
    c.Status(http.StatusBadRequest)
    c.JSON(http.StatusBadRequest, fiber.Map{"error": "invalid key length"})
    return
}
var safeKey [expectedKeyLen]byte
copy(safeKey[:], key)
// Use safeKey for cryptographic operations

When working with pools of fixed-size keys, ensure offset calculations never exceed the element size and use copy or manual bounds checks before any slice expression:

type keyPool [][32]byte
var pool [1024]keyPool
func getKeySafe(id, offset int) ([]byte, error) {
    if id < 0 || id >= len(pool) {
        return nil, errors.New("invalid pool id")
    }
    elem := pool[id][:]
    if offset < 0 || offset+32 > len(elem) {
        return nil, errors.New("offset out of element bounds")
    }
    dst := make([]byte, 32)
    copy(dst, elem[offset:offset+32])
    return dst, nil
}

For API key storage and comparison, avoid raw byte slices that can be sliced unsafely. Use constant-time comparison functions when checking keys, and keep key material in structures with explicit length fields. If integrating with middlebrick, follow the findings related to Authentication and BOLA/IDOR, and combine them with these code-level practices to reduce the risk of out-of-bounds reads in production.