Summary
A server-side request forgery vulnerability in Kiteworks Email Protection Gateway can cause the gateway to send attacker-influenced requests to internal or otherwise unintended network destinations. The risk arises when message content references external resources and the gateway renders those resources, potentially exposing internal information or triggering actions on reachable services.
Affected context
The vulnerability affects deployments of Kiteworks Email Protection Gateway where message content is rendered and external resource references are retrieved by the gateway. It becomes relevant when attacker-controlled or insufficiently trusted message content can influence outbound requests and the gateway has network reachability to internal services or other restricted destinations.
Exploit preconditions
- Attack surface: Email message rendering and external-resource retrieval
- Required access: Ability to deliver or influence a message processed by the gateway; authentication may not be required
- Trigger point: Rendering message content that references an external resource
- Network condition: The gateway must be able to reach the targeted internal or unintended destination
- Why exploitation is possible: Server-side request handling may permit attacker-influenced destinations without sufficiently restricting outbound network access
Why this happens
The underlying bug is insufficient control over server-side requests derived from untrusted message content. When a gateway fetches externally referenced resources without enforcing destination validation and network boundaries, an attacker may influence the gateway into contacting internal services, restricted interfaces, or unintended external endpoints. This behavior maps to CWE-918, Server-Side Request Forgery, because the server performs requests on behalf of an attacker and may expose its network position and privileges.
Potential impact
- An attacker may access sensitive information exposed by internal services.
- The gateway could reach administrative or metadata endpoints that are not publicly accessible.
- Internal systems could receive unintended requests or actions.
- Network segmentation and trust boundaries could be weakened.
- The vulnerability could support follow-on compromise if reachable services are themselves vulnerable.
General mitigation guidance
Mitigation generally focuses on reducing trust in resource references and
constraining all server-side outbound requests.
Common defensive patterns include:
- Allowing external resource retrieval only when it is necessary and explicitly permitted.
- Applying strict URL parsing, scheme restrictions, destination allowlists, and redirect validation.
- Blocking loopback, link-local, private, reserved, and otherwise restricted address ranges after DNS resolution.
- Enforcing outbound network egress controls so the gateway cannot freely reach internal services.
- Using isolated retrieval services with minimal privileges and limited network access.
- Avoiding transmission of sensitive headers, credentials, or ambient authentication context in fetched requests.
- Applying request timeouts, response-size limits, and logging for policy decisions and blocked destinations.
How teams detect issues like this
Organizations commonly detect SSRF risks through:
- Static analysis and code review checks for HTTP clients receiving URLs from message, template, or user-controlled data.
- Review signals such as unrestricted redirects, DNS resolution without post-resolution validation, and use of server credentials during resource retrieval.
- Dynamic testing with controlled endpoints that record request origin, redirect behavior, and destination filtering.
- Egress firewall and proxy policies that alert on gateway requests to private, loopback, link-local, metadata, or unexpected network ranges.
- Monitoring for unusual outbound requests generated during message rendering, including high-volume or anomalous destinations.
Automated security review tools may help identify request paths that lack destination validation or network policy enforcement.
