CVE-2026-9491 Overview
CVE-2026-9491 is a Server-Side Request Forgery (SSRF) vulnerability in the webhook component of Synology Chat Server. The flaw affects versions before 2.4.5-22148 and allows remote authenticated users to coerce the server into issuing outbound requests that reveal non-sensitive information. The weakness is categorized under CWE-918: Server-Side Request Forgery. Synology addressed the issue in a security update tracked as advisory SA-26-10.
Critical Impact
Authenticated attackers can abuse the webhook feature to make the Chat Server issue arbitrary outbound HTTP requests, enabling limited information disclosure about reachable network resources.
Affected Products
- Synology Chat Server prior to version 2.4.5-22148
- Deployments exposing webhook functionality to authenticated users
- Synology DiskStation Manager (DSM) hosts running the vulnerable Chat Server package
Discovery Timeline
- 2026-08-28 - CVE-2026-9491 published to the National Vulnerability Database
- 2026-09-01 - Last updated in NVD database
Technical Details for CVE-2026-9491
Vulnerability Analysis
The vulnerability resides in the webhook handler of Synology Chat Server. Webhooks are designed to let authorized users register callback URLs so the server can push events to external endpoints. When the destination URL is not properly validated, an authenticated attacker can supply URLs that point to internal or restricted network resources. The Chat Server then issues those requests on the attacker's behalf, returning limited response data to the caller.
Exploitation requires valid credentials on the Chat Server, which raises the attack barrier. The impact is limited to confidentiality of non-sensitive information, with no direct effect on integrity or availability. See the Synology Security Advisory SA-26-10 for vendor guidance.
Root Cause
The root cause is insufficient validation of user-supplied URLs passed to the webhook subsystem. The application does not enforce an allowlist of destinations or block requests to internal address space, loopback interfaces, or link-local metadata endpoints before dispatching the outbound HTTP request.
Attack Vector
An authenticated user submits a crafted webhook configuration or invocation pointing at an internal resource. The Chat Server initiates an HTTP request to the specified target from its own network position. Response metadata, status codes, or body fragments returned to the user can reveal whether internal hosts and services exist. The vector is network-based and requires low attack complexity, but privileges are required and user interaction is not.
No verified public exploit code is available. Refer to the vendor advisory for technical details.
Detection Methods for CVE-2026-9491
Indicators of Compromise
- Outbound HTTP requests originating from the Chat Server host to internal RFC1918 addresses, 127.0.0.1, or cloud metadata endpoints such as 169.254.169.254
- Webhook configuration entries containing non-public hostnames, IP literals, or unusual ports
- Repeated webhook invocations from a single authenticated account targeting sequential internal addresses
Detection Strategies
- Inspect Chat Server webhook audit logs for destination URLs pointing to private network ranges or loopback addresses
- Correlate authenticated user sessions with unexpected outbound HTTP connections from the Synology appliance
- Baseline normal webhook destinations and alert on deviations, especially port scanning patterns
Monitoring Recommendations
- Forward DSM and Chat Server logs to a centralized SIEM for retention and correlation
- Monitor egress traffic from the Chat Server host and restrict it via network policy where possible
- Alert on webhook creation events issued by low-privilege accounts or newly provisioned users
How to Mitigate CVE-2026-9491
Immediate Actions Required
- Upgrade Synology Chat Server to version 2.4.5-22148 or later as documented in advisory SA-26-10
- Audit existing webhook configurations and remove any pointing to internal or unexpected destinations
- Review Chat Server user accounts and revoke access for stale or unnecessary identities
Patch Information
Synology released a fixed build in Chat Server 2.4.5-22148. Administrators should apply the update through Package Center on DSM or download it directly from the Synology download portal. Consult the Synology Security Advisory SA-26-10 for full remediation guidance and package availability by platform.
Workarounds
- Restrict outbound network access from the Chat Server host to only required external destinations
- Limit webhook creation permissions to trusted administrators until the patch is deployed
- Place the Chat Server behind an egress proxy that enforces a destination allowlist
# Example egress restriction using iptables to block SSRF to internal ranges
iptables -A OUTPUT -m owner --uid-owner synochat -d 127.0.0.0/8 -j REJECT
iptables -A OUTPUT -m owner --uid-owner synochat -d 10.0.0.0/8 -j REJECT
iptables -A OUTPUT -m owner --uid-owner synochat -d 172.16.0.0/12 -j REJECT
iptables -A OUTPUT -m owner --uid-owner synochat -d 192.168.0.0/16 -j REJECT
iptables -A OUTPUT -m owner --uid-owner synochat -d 169.254.0.0/16 -j REJECT
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
