Heap Overflow in Buffalo with Bearer Tokens

A heap overflow in the Buffalo web framework can occur when request handling logic performs unchecked copying into fixed-size buffers on the heap, for example while parsing headers such as Authorization. When Bearer Tokens are used, the Authorization header typically follows the format Authorization: Bearer <token>. If the server-side code uses a fixed-size character array or an unchecked buffer to extract the token substring after the Bearer prefix, an attacker can supply a very long token, causing a heap-based buffer overflow. This can corrupt adjacent heap metadata or overwrite function pointers, potentially leading to arbitrary code execution or denial of service.

The risk is compounded when the application relies on runtime parsing rather than schema validation. For instance, if the token is expected to conform to a known structure (e.g., JWT segments) but no length checks are applied, an oversized token can exploit the overflow to bypass intended access controls or leak memory contents. The combination of Bearer Tokens and improper memory handling in Buffalo routes increases the attack surface, especially when tokens are passed through multiple layers of middleware or logging.

During a black-box scan, middleBrick tests for input validation and authentication bypass by sending oversized or malformed Authorization headers. If the server responds with unexpected behavior (crashes, inconsistent responses, or information disclosure), a potential heap overflow may be flagged. This aligns with checks for Input Validation, Authentication, and Property Authorization in the 12 parallel security checks, highlighting how protocol-level assumptions can introduce memory safety issues.

Example of a vulnerable route in Buffalo that demonstrates the issue:

// Vulnerable: fixed-size buffer for Bearer token extraction
void handle_protected(bm::context* ctx) {
    const char* auth = ctx.Request.Header.Get("Authorization");
    if (auth != nullptr && strncmp(auth, "Bearer ", 7) == 0) {
        char token[256];
        strncpy(token, auth + 7, sizeof(token) - 1);
        token[sizeof(token) - 1] = '\0';
        // Use token…
    }
}

In this snippet, strncpy is used without verifying that the token length fits within the 256-byte buffer. A Bearer Token longer than 255 characters (plus null terminator) will overflow the destination buffer on the heap, since strncpy does not truncate when the source is longer than the destination. An attacker can craft a request with a long token to trigger the overflow, which may be detected by middleBrick during its runtime testing phase.

To mitigate heap overflow risks with Bearer Tokens in Buffalo, always validate token length before copying and use safe string handling APIs that respect buffer boundaries. Prefer dynamic allocation when the token size is unpredictable, and ensure that any substring extraction respects the full length of the header.

Remediation example using length checks and safe copying:

// Secure: validate length and use bounded copy
void handle_protected(bm::context* ctx) {
    const char* auth = ctx.Request.Header.Get("Authorization");
    if (auth != nullptr && strncmp(auth, "Bearer ", 7) == 0) {
        size_t token_len = strlen(auth + 7);
        if (token_len >= 255) {
            ctx.Error(400, "Invalid token length");
            return;
        }
        char token[256];
        strncpy(token, auth + 7, sizeof(token) - 1);
        token[sizeof(token) - 1] = '\0';
        // Use token…
    }
}

Alternatively, use dynamic allocation to avoid fixed-size buffers entirely:

// Secure: dynamic allocation based on actual token length
void handle_protected(bm::context* ctx) {
    const char* auth = ctx.Request.Header.Get("Authorization");
    if (auth != nullptr && strncmp(auth, "Bearer ", 7) == 0) {
        size_t token_len = strlen(auth + 7);
        if (token_len == 0) {
            ctx.Error(400, "Missing token");
            return;
        }
        char* token = (char*)malloc(token_len + 1);
        if (!token) {
            ctx.Error(500, "Internal error");
            return;
        }
        memcpy(token, auth + 7, token_len);
        token[token_len] = '\0';
        // Use token…
        free(token);
    }
}

These fixes ensure that the token length is verified before any copy operation, preventing heap overflow by either rejecting oversized tokens or allocating sufficient memory. They align with best practices for input validation and memory safety, reducing the likelihood of exploitable conditions that middleBrick would flag under Authentication and Input Validation checks.

When integrating with CI/CD, the Pro plan’s GitHub Action can be configured to fail builds if risk scores degrade due to such vulnerabilities. This helps maintain secure handling of Bearer Tokens across deployments without relying on manual code review alone.