# Cross-Site Scripting (XSS) Analysis Report

## 1. Executive Summary

- **Analysis Status:** Complete  
- **Key Outcome:** One high-confidence, externally exploitable XSS vulnerability was confirmed through live browser testing. A second vulnerability exists in the source code but is accidentally mitigated by PHP's internal behavior.
- **Purpose of this Document:** This report provides the strategic context, dominant patterns, and environmental intelligence necessary to effectively exploit the confirmed vulnerability.  

## 2. Dominant Vulnerability Patterns

**Pattern 1: Reflected XSS in Upload Success Message (EXPLOITABLE)**  
- **Description:** The file upload success message at line 41 reflects the user-controlled filename directly into an HTML `href` attribute using single quotes, without any output encoding. Single quotes are not URL-encoded by PHP's `$_FILES` handling, allowing attribute breakout.
- **Implication:** Any file upload request can inject malicious event handlers into the `<a>` tag. User interaction (clicking the "here" link) triggers JavaScript execution.  
- **Representative Finding:** XSS-VULN-02 (Reflected XSS via onclick injection).  
- **Attack Complexity:** Low - requires only a single file upload with a crafted filename.

**Pattern 2: Stored XSS in Image Display (CODE VULNERABLE, ACCIDENTALLY MITIGATED)**  
- **Description:** The main page displays the most recently uploaded filename in an `<img src>` attribute at line 15 without any output encoding. The filename is read from persistent storage (`uploaded.txt`), making this a Stored XSS vulnerability pattern.
- **Accidental Mitigation:** PHP's internal handling of `$_FILES["userfile"]["name"]` automatically URL-encodes double quotes as `%22`. Since the img tag uses double quotes, attribute breakout is prevented. However, this is NOT an intentional security control - the source code lacks proper output encoding.
- **Implication:** The code is vulnerable by design, but the exploit is prevented by PHP's incidental behavior. If PHP's behavior changes or if the quote style is modified, the vulnerability becomes exploitable.
- **Representative Finding:** XSS-VULN-01 (Stored XSS, not exploitable in current configuration).

## 3. Strategic Intelligence for Exploitation

**Content Security Policy (CSP) Analysis**  
- **Current CSP:** None - No Content-Security-Policy header observed  
- **Impact:** No CSP restrictions on script execution. All XSS vectors (inline scripts, event handlers, data: URIs) are permitted.  
- **Recommendation:** Exploitation can use simple inline event handlers like `onclick='alert(document.domain)'` without needing CSP bypass techniques.

**Cookie Security**  
- **Observation:** The application does not use any cookies or session management mechanisms.  
- **Impact:** No session cookies to steal. However, XSS can still be exploited for:
  - Defacement of the application
  - Phishing attacks (injecting fake forms)
  - Performing actions on behalf of users (uploading malicious files)
  - Browser-based exploits and pivoting
- **Recommendation:** Focus exploitation on actions rather than credential theft.

**Character Encoding Behavior**  
- **Critical Finding:** PHP's `$_FILES` superglobal automatically URL-encodes some special characters:
  - Double quotes (`"`) are encoded to `%22`
  - Single quotes (`'`) are NOT encoded
  - Angle brackets (`<`, `>`) are NOT encoded in certain contexts
- **Exploitation Impact:** The reflected XSS at line 41 uses single-quoted attributes, making it trivially exploitable since single quotes pass through without encoding.

**File Upload Validation**  
- **Extension Check:** Weak - uses `strpos()` to check if `.jpg` or `.jpeg` appears anywhere in the filename
- **Bypass:** Payloads like `malicious.jpg' onclick='alert(1)' x='.jpg` pass validation
- **Magic Number Check:** Only validates file content (JPEG signature), not the filename itself
- **Impact:** No restrictions on XSS payloads in filenames as long as they contain `.jpg` substring

## 4. Vectors Analyzed and Confirmed Secure

These input vectors were traced and confirmed to have robust, context-appropriate defenses (or accidental protections).

| Source (Parameter/Key) | Endpoint/File Location | Defense Mechanism Implemented | Render Context | Verdict |
|--------------------------|-------------------------|--------------------------------|----------------|---------|
| `$_FILES["userfile"]["name"]` at line 15 | `/index.php` line 15 | PHP's automatic URL encoding of double quotes (accidental protection) | HTML_ATTRIBUTE (img src with double quotes) | SAFE (but code is vulnerable) |

**Note:** Only one vector was found to be protected, and this protection is incidental rather than intentional. The lack of `htmlspecialchars()` means the code is still technically vulnerable.

## 5. Analysis Constraints and Blind Spots

- **PHP Internal Behavior:** The analysis revealed that PHP's internal handling of `$_FILES` applies inconsistent URL encoding. Double quotes are encoded, but single quotes are not. This behavior may vary across PHP versions or configurations, creating potential for version-specific exploits.

- **No Client-Side JavaScript:** The application has zero client-side JavaScript, eliminating DOM-based XSS vectors. All XSS vulnerabilities are server-side (Reflected or Stored).

- **Limited Attack Surface:** With only two XSS sinks identified and one confirmed exploitable, the attack surface is narrow but sufficient for achieving compromise.

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