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Fix: Fix DNS Rebinding/TOCTOU Vulernability #5653
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Part of advisory https://github.com/FlowiseAI/Flowise/security/advisories/GHSA-2x8m-83vc-6wv4 Ensures that the validated host/ip is the same one used when resolving later on. Refactoring methods secureFetch and secureAxiosRequest to use resolver helper methods
Summary of ChangesHello @christopherholland-workday, I'm Gemini Code Assist1! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed! This pull request addresses critical Server-Side Request Forgery (SSRF) bypass vulnerabilities by implementing robust security measures for outbound HTTP requests. It focuses on eliminating DNS rebinding (TOCTOU) issues, enforcing deny-list validation at the point of connection, and securing redirect handling. The changes ensure that all requests utilize DNS-pinned connections that match validated IP addresses, thereby removing insecure default behaviors and significantly strengthening the application's defense against network-based attacks. Highlights
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Code Review
This pull request effectively addresses a critical DNS Rebinding (TOCTOU) vulnerability in secureAxiosRequest and secureFetch by implementing DNS pinning. The changes ensure that outbound requests connect to the same IP that was validated, even across redirects. I also appreciate the move to a secure-by-default configuration that requires HTTP_DENY_LIST to be set. The code is much cleaner and more secure. I have a few minor suggestions to improve code consistency and clarity.
Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>
Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>
Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>
…/Flowise into feature/fix-toctou-bug
packages/components/src/httpSecurity.ts
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| const resolved = await resolveAndValidate(currentUrl) | ||
| const agent = createPinnedAgent(resolved) | ||
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| const response = await fetch(currentUrl, { ...currentInit, agent: () => agent }) |
Check failure
Code scanning / CodeQL
Server-side request forgery Critical
URL
user-provided value
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AI 1 day ago
In general, to fix this kind of SSRF issue you ensure that user-controlled input cannot directly choose an arbitrary request target. Instead, you validate or transform the input into a safe value: enforce allowed schemes (typically http/https only), disallow raw IPs if appropriate, and restrict hostnames to an allow-list or at least block link-local/metadata endpoints and private networks regardless of environment configuration. You should also ensure these checks are applied consistently wherever user-controlled URLs are used.
For this codebase, the best minimal fix without changing existing functionality is to strengthen checkDenyList in packages/components/src/httpSecurity.ts so it always enforces baseline scheme and host/IP restrictions, even if HTTP_DENY_LIST is not set. All user-controlled URLs in this flow go through checkDenyList already (fetch-links service calls it explicitly; xmlScrape calls secureFetch, which uses resolveAndValidate internally), so augmenting checkDenyList to also reject non-HTTP(S) schemes and clearly dangerous hostnames/IPs will ensure secureFetch cannot be used for SSRF against internal/metadata endpoints. Concretely:
- Parse the URL and immediately reject:
- Non-
http/httpsschemes. - Hostnames like
localhost,127.0.0.1,::1, and well-known cloud metadata hostnames (169.254.169.254,metadata.google.internal, etc.).
- Non-
- Maintain the existing deny-list behavior, but apply it in addition to the new static rules.
- To avoid code duplication and future errors, introduce a small helper (e.g.
isDeniedHostname) at the top ofcheckDenyListto encapsulate these checks before doing DNS resolution.
This keeps the public API the same (checkDenyList signature unchanged) and requires no changes to the callers. All edits are confined to the checkDenyList function in packages/components/src/httpSecurity.ts; no new imports are needed because we reuse URL and existing ipaddr/dns imports.
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| /** | ||
| * Checks if a URL is allowed based on HTTP_DENY_LIST environment variable | ||
| * and built-in SSRF protections (scheme and hostname/IP restrictions). | ||
| * @param url - URL to check | ||
| * @throws Error if URL hostname resolves to a denied IP | ||
| * @throws Error if URL hostname or resolved IP is denied | ||
| */ | ||
| export async function checkDenyList(url: string): Promise<void> { | ||
| const urlObj = new URL(url) | ||
| const hostname = urlObj.hostname | ||
| const protocol = urlObj.protocol.toLowerCase() | ||
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| // Enforce allowed protocols | ||
| if (protocol !== 'http:' && protocol !== 'https:') { | ||
| throw new Error(`Access to this URL is denied by policy: unsupported protocol "${protocol}"`) | ||
| } | ||
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| // Block obvious local/metadata hosts regardless of HTTP_DENY_LIST | ||
| const loweredHost = hostname.toLowerCase() | ||
| const blockedHostnames = new Set<string>([ | ||
| 'localhost', | ||
| '127.0.0.1', | ||
| '::1', | ||
| '169.254.169.254', // AWS/GCP metadata IP | ||
| 'metadata.google.internal', // GCP metadata hostname | ||
| '169.254.169.250', | ||
| '169.254.170.2' // AWS ECS task metadata | ||
| ]) | ||
| if (blockedHostnames.has(loweredHost)) { | ||
| throw new Error('Access to this host is denied by policy.') | ||
| } | ||
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| const httpDenyListString: string | undefined = process.env.HTTP_DENY_LIST | ||
| if (!httpDenyListString) return | ||
| if (!httpDenyListString) { | ||
| // Even if no explicit deny list is configured, still prevent direct IP access | ||
| if (ipaddr.isValid(hostname)) { | ||
| throw new Error('Access to this host is denied by policy.') | ||
| } | ||
| return | ||
| } | ||
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| const httpDenyList = httpDenyListString.split(',').map((ip) => ip.trim()) | ||
| const urlObj = new URL(url) | ||
| const hostname = urlObj.hostname | ||
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| if (ipaddr.isValid(hostname)) { | ||
| isDeniedIP(hostname, httpDenyList) |
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This is by design/an artifact from a previous commit: https://github.com/FlowiseAI/Flowise/blob/main/packages/components/src/httpSecurity.ts#L178
Will check with the team if there are other actions required to fix this
| } | ||
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| async function resolveAndValidate(url: string): Promise<ResolvedTarget> { | ||
| const denyListString = process.env.HTTP_DENY_LIST |
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By forcing every users to specify HTTP_DENY_LIST, its not user/begineer friendly.
The decision we took is to allow users to have the option to specify if needed, otherwise its not blocking by default. Its not secured by default. We have also documented here
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Got it, I will revert this
Overview
Part of advisory https://github.com/FlowiseAI/Flowise/security/advisories/GHSA-2x8m-83vc-6wv4
This PR fixes multiple SSRF bypass issues in the HTTP security wrappers by eliminating DNS rebinding (TOCTOU) vulnerabilities and enforcing deny-list validation at request time.
The changes ensure that every outbound HTTP request uses a DNS-pinned connection that matches the validated IP, including across redirects, and that insecure default behavior is removed.
Solution
1. Default insecure configuration
Previously, if
HTTP_DENY_LISTwas unset, requests were allowed without restriction, including access tolocalhostand private IP ranges.Fix:
Requests now fail fast when
HTTP_DENY_LISTis not defined, ensuring SSRF protections are always enforced.DNS rebinding (TOCTOU) vulnerability
The previous implementation validated hostnames using dns.lookup, but the HTTP client would perform a second DNS resolution when opening the socket. This allowed attackers to return a safe IP during validation and switch to a blocked IP during connection. This seemed to be the case for both the
secureFetchandsecureAxiosRequestmethods.Callers of these methods, like
utils#crawlshould be protected via strengthening of these methodsFix:
DNS resolution and validation now occur immediately before each request, and the resolved IP is pinned into the socket using a custom http.Agent / https.Agent. This guarantees the request connects to the same IP that was validated.
Redirect safety
Redirects were previously followed automatically by the HTTP client, allowing bypass of validation on redirected URLs.
Fix:
Automatic redirects are disabled. Each redirect target is:
Redirect method rewriting (e.g. POST → GET on 303) follows HTTP specifications.
node-fetch agent handling
When using [email protected], passing an Agent instance directly may not guarantee it is used, potentially allowing DNS re-resolution.
Fix:
The agent is now passed as a function (agent: () => agent), which node-fetch v2 reliably invokes, ensuring the pinned agent is always used.