Summary
A server-side request forgery vulnerability in Kiteworks Email Protection Gateway may allow a remote, unauthenticated attacker to cause the gateway to send requests to internal or otherwise unintended network destinations. The risk arises during online certificate status checks for inbound messages and could expose sensitive information or affect reachable services.
Affected context
The vulnerability occurs in deployments where the Email Protection Gateway performs online certificate status checks for inbound messages and processes attacker-influenced certificate or message-related data. If the gateway can reach internal services or restricted network destinations, crafted input may cause it to issue requests beyond the intended certificate-status infrastructure.
Exploit preconditions
- Attack surface: Internet-facing inbound email processing and certificate status checking
- Required access: No authentication may be required when the gateway accepts attacker-controlled inbound messages
- Trigger point: Online certificate status validation performed for an inbound message
- Why exploitation is possible: The gateway may insufficiently restrict destination selection for server-side requests, allowing attacker-influenced processing to reach unintended network locations
Why this happens
The underlying issue is insufficient destination validation in a server-side request workflow. Instead of limiting outbound requests to approved certificate-status services and safely handling redirects or resolved addresses, the gateway may allow attacker-controlled data to influence where it connects.
This behavior maps to CWE-918: Server-Side Request Forgery, in which a server makes requests to destinations selected or influenced by an external party without adequate network or protocol restrictions.
Potential impact
- An attacker may obtain sensitive information from reachable internal services
- The gateway could expose service metadata, credentials, or configuration details
- Internal network boundaries may be bypassed from the gateway's network position
- Requests could disrupt or overload unintended services
- Further compromise may be possible if reachable services contain additional weaknesses
General mitigation guidance
Mitigation generally focuses on restricting server-initiated requests and reducing trust in externally influenced certificate or message data.
Common defensive patterns include:
- Allowing outbound requests only to an explicit, minimal set of approved certificate-status destinations
- Validating destination schemes, hostnames, ports, DNS resolution results, and redirect targets
- Blocking loopback, link-local, private, reserved, and metadata-service address ranges
- Rechecking resolved addresses to reduce DNS rebinding and redirect-based bypasses
- Applying egress network policies and proxy controls at the deployment boundary
- Running the gateway with least-privilege network access
- Using short timeouts, bounded response sizes, and controlled retry behavior
- Monitoring and reviewing outbound connections generated during message processing
How teams detect issues like this
Detection approaches can include:
- Static analysis and code review for outbound HTTP clients receiving URLs or destinations from message, certificate, or header data
- Review signals such as unrestricted redirects, missing private-address checks, URL parsing inconsistencies, and DNS resolution performed without post-resolution validation
- Egress firewall, proxy, and network-flow monitoring for gateway connections to private, loopback, link-local, cloud metadata, or unexpected external destinations
- Alerts for certificate-status requests to destinations outside approved allowlists
- Dynamic testing with controlled internal endpoints to verify destination and redirect enforcement
- Automated security review tools and dependency scanners that identify SSRF-prone request flows
