CVE-2026-44583 Overview
CVE-2026-44583 is a Server-Side Request Forgery (SSRF) vulnerability [CWE-918] in Paymenter, a free and open-source webshop solution for managing hosting services. The flaw exists in the PayPal webhook endpoint /extensions/paypal/webhook, which processes the PAYPAL-CERT-URL HTTP header without validation. Remote unauthenticated attackers can coerce the server into issuing HTTP GET requests to arbitrary external or internal endpoints. The issue affects Paymenter versions prior to 1.5.0 and was resolved in version 1.5.0.
Critical Impact
Unauthenticated attackers can trigger blind SSRF requests from the Paymenter server to internal network services or attacker-controlled endpoints, enabling reconnaissance and potential data exfiltration through side channels.
Affected Products
- Paymenter versions prior to 1.5.0
- PayPal extension endpoint /extensions/paypal/webhook
- Self-hosted Paymenter deployments processing PayPal webhooks
Discovery Timeline
- 2026-07-20 - CVE-2026-44583 published to NVD
- 2026-07-23 - Last updated in NVD database
Technical Details for CVE-2026-44583
Vulnerability Analysis
The vulnerability resides in the PayPal webhook handler at /extensions/paypal/webhook. When a webhook request arrives, the application reads the PAYPAL-CERT-URL HTTP header and passes the value directly to file_get_contents for certificate retrieval. No allowlist, hostname validation, or signature verification is applied before the outbound request is issued.
This design gives any unauthenticated caller full control over the destination URL. The Paymenter server then performs the HTTP GET request from its own network position. Because the response body is not returned to the caller, exploitation produces blind SSRF rather than direct data disclosure. Attackers can still infer information through timing differences, error behavior, and out-of-band callbacks.
Root Cause
The root cause is missing input validation on an attacker-controlled HTTP header combined with unsafe use of file_get_contents as an HTTP client. PayPal's Instant Payment Notification flow expects the certificate URL to originate from PayPal's own domains, but the Paymenter implementation trusts the header value as supplied. Signature verification, which would normally reject forged webhook payloads, is also absent at this stage.
Attack Vector
An unauthenticated remote attacker sends a crafted POST request to /extensions/paypal/webhook and sets the PAYPAL-CERT-URL header to a URL of their choice. Suitable targets include internal metadata services such as http://169.254.169.254/, private network hosts like http://10.0.0.1/, and attacker-controlled collaborators used for out-of-band detection. The Paymenter host then issues a GET request to that URL, granting the attacker a probe from inside the trust boundary.
No verified public exploit code is available. See the GitHub Security Advisory GHSA-7wwh-xcc3-9fcg for maintainer details.
Detection Methods for CVE-2026-44583
Indicators of Compromise
- Inbound POST requests to /extensions/paypal/webhook where the PAYPAL-CERT-URL header does not resolve to a legitimate PayPal domain such as api.paypal.com or api.sandbox.paypal.com.
- Outbound HTTP GET requests from the Paymenter host to internal RFC1918 addresses, cloud metadata endpoints (169.254.169.254), or unfamiliar external domains shortly after webhook activity.
- DNS lookups from the Paymenter server for hosts referenced only in webhook headers.
Detection Strategies
- Inspect web server and application logs for the PAYPAL-CERT-URL header value and alert when the hostname is not on a PayPal allowlist.
- Correlate inbound webhook requests with outbound egress connections from the Paymenter host using network flow data.
- Deploy web application firewall rules that reject webhook requests whose PAYPAL-CERT-URL header points to private, loopback, or link-local address ranges.
Monitoring Recommendations
- Monitor egress traffic from the Paymenter server and baseline expected destinations, alerting on connections to internal subnets or cloud metadata IPs.
- Enable request logging for the /extensions/paypal/webhook endpoint and retain full header content for forensic review.
- Track PHP outbound stream wrapper usage (file_get_contents over HTTP) via application-level instrumentation where available.
How to Mitigate CVE-2026-44583
Immediate Actions Required
- Upgrade Paymenter to version 1.5.0 or later, which contains the official fix for this SSRF flaw.
- Restrict outbound network access from the Paymenter host to only the domains required by PayPal and other integrated payment processors.
- Review historical webhook logs for anomalous PAYPAL-CERT-URL header values that may indicate prior exploitation attempts.
Patch Information
The Paymenter maintainers addressed CVE-2026-44583 in version 1.5.0 by validating the PAYPAL-CERT-URL header before performing the outbound request. Administrators should upgrade using the standard Paymenter release process. Full remediation details are available in the Paymenter GitHub Security Advisory.
Workarounds
- Place a reverse proxy or WAF in front of Paymenter that rejects requests to /extensions/paypal/webhook when the PAYPAL-CERT-URL header does not match *.paypal.com.
- Apply egress firewall rules on the Paymenter host that block outbound connections to internal networks, loopback, and cloud metadata addresses.
- Temporarily disable the PayPal extension if the upgrade cannot be applied and PayPal processing is not required.
# Example egress restriction using iptables to block SSRF to internal ranges
iptables -A OUTPUT -m owner --uid-owner www-data -d 169.254.169.254 -j REJECT
iptables -A OUTPUT -m owner --uid-owner www-data -d 10.0.0.0/8 -j REJECT
iptables -A OUTPUT -m owner --uid-owner www-data -d 172.16.0.0/12 -j REJECT
iptables -A OUTPUT -m owner --uid-owner www-data -d 192.168.0.0/16 -j REJECT
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
