Out Of Bounds Write in Buffalo with Bearer Tokens

An Out Of Bounds Write occurs when a program writes data outside the intended memory region. In the Buffalo web framework for Go, this typically arises through unsafe handling of byte slices, buffers, or request bodies where length or capacity checks are missing or incorrectly applied. When Bearer Tokens are involved—often passed via the Authorization header as Authorization: Bearer <token>—the combination can expose or amplify the vulnerability if token values are copied into fixed-size buffers or processed without proper bounds checking.

Consider a Buffalo API endpoint that extracts a Bearer token from the Authorization header and uses it to index into a fixed-size lookup table or to pre-allocate a response buffer. If the token length is not validated against the destination buffer capacity, an attacker can supply an unusually long token, causing writes beyond the allocated memory. This may corrupt adjacent stack variables, overwrite return addresses, or affect other runtime state. Because Buffalo encourages clean routing and parameter binding, developers might mistakenly trust header-derived values without applying the same strict validation applied to path or query parameters.

In practice, this can manifest when using net/http helpers or custom middleware that copies the token string into a fixed-size byte array. For example, if a developer does something akin to the following—parsing the Authorization header with a simplistic split and then using copy into a fixed-length buffer—the risk of an out-of-bounds write exists if the token exceeds the destination length:

const tokenBufferSize = 256
tokenBuf := make([]byte, tokenBufferSize)
auth := r.Header.Get("Authorization")
if len(auth) > 7 && auth[:7] == "Bearer " {
    token := auth[7:]
    // Unsafe: token length not validated against tokenBufferSize
    copy(tokenBuf, token)
}

While Go’s copy function prevents writing beyond the destination slice’s capacity by limiting the copy to the minimum of lengths, if the developer later uses unsafe operations or casts the buffer to a fixed-size C-like buffer via cgo, or relies on assumptions about token length elsewhere in the request lifecycle, the boundary can be violated. The scanner checks in middleBrick’s 12 parallel security checks look for such unchecked header usage and unsafe memory patterns, correlating runtime behavior with the API specification to flag cases where Bearer Token inputs could lead to out-of-bounds conditions.

Additionally, if the API specification (OpenAPI 2.0/3.0/3.1) defines security schemes using Bearer tokens but does not enforce length constraints or content validation, and the runtime implementation does not align with those constraints, the mismatch becomes an observable finding. middleBrick’s OpenAPI/Swagger analysis resolves all $ref definitions and cross-references spec-defined security requirements with actual request tests, highlighting discrepancies that could enable insecure handling of authentication material.

Remediation focuses on validating and safely handling Bearer tokens before any buffer or memory operations. In Buffalo, you should treat the Authorization header as untrusted input, apply strict length checks, and avoid fixed-size buffers when possible. Use Go’s built-in slices safely and prefer higher-level abstractions that manage memory automatically.

First, validate the token length before any copying or processing. Define a reasonable maximum token length (for example, 4096 bytes for typical JWTs) and reject or truncate requests that exceed it:

const maxBearerTokenLength = 4096
auth := r.Header.Get("Authorization")
if len(auth) > 7 && auth[:7] == "Bearer " {
    token := auth[7:]
    if len(token) == 0 || len(token) > maxBearerTokenLength {
        http.Error(w, "invalid authorization header", http.StatusBadRequest)
        return
    }
    // Safe processing of token within known bounds
    _ = token // use token as needed
}

Second, avoid using fixed-size byte arrays for token storage. Instead, work with strings or dynamically sized byte slices. If you must copy token material into a buffer for cryptographic or transformation purposes, use a destination slice sized to the actual token length:

auth := r.Header.Get("Authorization")
if len(auth) > 7 && auth[:7] == "Bearer " {
    token := auth[7:]
    if len(token) == 0 {
        http.Error(w, "missing token", http.StatusBadRequest)
        return
    }
    tokenBuf := make([]byte, len(token))
    copy(tokenBuf, token)
    // tokenBuf now safely mirrors the token with exact length
    _ = tokenBuf
}

Third, if integrating with lower-level or cgo-based components, ensure bounds are explicitly checked before passing data. Prefer using Go’s memory-safe constructs and avoid manual pointer arithmetic when handling Bearer token values. middleBrick’s CLI tool can be used to scan your codebase and API definitions for such unsafe patterns; you can run scans from the terminal with middlebrick scan <url> to validate both runtime behavior and spec alignment.

Finally, leverage the GitHub Action to add API security checks to your CI/CD pipeline, failing builds if security scores drop below your defined threshold. This ensures that any future changes introducing unsafe Bearer Token handling are caught before deployment. For continuous monitoring across many endpoints, the Pro plan provides scheduled scans and alerts, while the MCP Server allows you to scan APIs directly from your AI coding assistant within the IDE.