Summary
A server-side request forgery vulnerability in n8n's Slack Trigger webhook can bypass Slack signature verification when no signing secret is configured. An unauthenticated attacker who knows the webhook URL may cause the node to retrieve attacker-selected content and expose a workspace access token to an attacker-controlled host.
A server-side request forgery vulnerability in n8n's Slack Trigger webhook can bypass Slack signature verification when no signing secret is configured. An unauthenticated attacker who knows the webhook URL may cause the node to retrieve attacker-selected content and expose a workspace access token to an attacker-controlled host.
Affected context
The vulnerability affects n8n deployments using the Slack Trigger webhook without a configured Slack signing secret. It becomes risky when the webhook endpoint is reachable by unauthenticated clients and the node permits attacker-influenced file retrieval or outbound requests. The issue applies to affected n8n releases before the vendor-provided fixed release lines.
The vulnerability affects n8n deployments using the Slack Trigger webhook without a configured Slack signing secret. It becomes risky when the webhook endpoint is reachable by unauthenticated clients and the node permits attacker-influenced file retrieval or outbound requests. The issue applies to affected n8n releases before the vendor-provided fixed release lines.
Exploit preconditions
- Attack surface: The n8n Slack Trigger webhook endpoint
- Required access: No authentication is required if the attacker knows or can obtain the webhook URL
- Trigger point: Processing a request that causes the Slack Trigger node to retrieve attacker-selected content
- Why exploitation is possible: Signature verification is bypassed when no signing secret is configured, allowing unauthenticated input to influence an outbound request and expose the workspace access token
- Attack surface: The n8n Slack Trigger webhook endpoint
- Required access: No authentication is required if the attacker knows or can obtain the webhook URL
- Trigger point: Processing a request that causes the Slack Trigger node to retrieve attacker-selected content
- Why exploitation is possible: Signature verification is bypassed when no signing secret is configured, allowing unauthenticated input to influence an outbound request and expose the workspace access token
Why this happens
The vulnerability results from an incomplete authentication and request-validation path in the Slack Trigger webhook. When a signing secret is absent, the endpoint does not sufficiently verify that requests originate from Slack before processing attacker-controlled retrieval instructions. This allows server-side requests to be made on behalf of an unauthenticated requester and maps to CWE-918: Server-Side Request Forgery.
The vulnerability results from an incomplete authentication and request-validation path in the Slack Trigger webhook. When a signing secret is absent, the endpoint does not sufficiently verify that requests originate from Slack before processing attacker-controlled retrieval instructions. This allows server-side requests to be made on behalf of an unauthenticated requester and maps to CWE-918: Server-Side Request Forgery.
Potential impact
- An attacker may exfiltrate Slack workspace access tokens.
- An attacker could obtain unauthorized access to Slack resources permitted by the token.
- The n8n host may make outbound requests to attacker-controlled or internal destinations.
- Internal services or metadata endpoints could be exposed if network controls permit access.
- The compromised token could support broader workspace data theft or operational abuse.
- An attacker may exfiltrate Slack workspace access tokens.
- An attacker could obtain unauthorized access to Slack resources permitted by the token.
- The n8n host may make outbound requests to attacker-controlled or internal destinations.
- Internal services or metadata endpoints could be exposed if network controls permit access.
- The compromised token could support broader workspace data theft or operational abuse.
General mitigation guidance
Mitigation generally focuses on enforcing authenticated webhook requests and reducing trust in user-controlled destinations.
Common defensive patterns include:
- Require and validate Slack signing secrets for Slack Trigger webhooks.
- Reject requests when the required authentication or signature configuration is absent.
- Avoid allowing unauthenticated input to determine outbound request destinations or downloaded files.
- Apply outbound network allowlists, proxy controls, and restrictions on access to internal or metadata endpoints.
- Store workspace tokens securely and grant them only the minimum required permissions.
- Monitor and limit token use from unexpected hosts, networks, or workflows.
Mitigation generally focuses on enforcing authenticated webhook requests and reducing trust in user-controlled destinations.
Common defensive patterns include:
- Require and validate Slack signing secrets for Slack Trigger webhooks.
- Reject requests when the required authentication or signature configuration is absent.
- Avoid allowing unauthenticated input to determine outbound request destinations or downloaded files.
- Apply outbound network allowlists, proxy controls, and restrictions on access to internal or metadata endpoints.
- Store workspace tokens securely and grant them only the minimum required permissions.
- Monitor and limit token use from unexpected hosts, networks, or workflows.
How teams detect issues like this
Organizations can look for this issue through:
- Static analysis and configuration checks for webhook handlers that skip authentication when a secret is unset.
- Code review signals involving user-controlled URLs, file names, or download destinations passed to server-side request functions.
- Policy enforcement requiring signing-secret configuration for Slack integrations.
- Runtime monitoring for unusual outbound requests from n8n workers and unexpected Slack token usage.
- Automated security reviews that identify SSRF-prone request construction and missing authentication branches.
Organizations can look for this issue through:
- Static analysis and configuration checks for webhook handlers that skip authentication when a secret is unset.
- Code review signals involving user-controlled URLs, file names, or download destinations passed to server-side request functions.
- Policy enforcement requiring signing-secret configuration for Slack integrations.
- Runtime monitoring for unusual outbound requests from n8n workers and unexpected Slack token usage.
- Automated security reviews that identify SSRF-prone request construction and missing authentication branches.
