fix: rate-limit visitors by real IP regardless of Cloudflare proxy hops (#750)

Since the night of 2026-07-16 production keyed the rate limiter on
Cloudflare edge IPs instead of visitor IPs: every visitor routed through
the same edge shared one 175-req/15min bucket, causing widespread 429s
(confirmed by probing: sequential requests alternated 200/429 across
fresh and exhausted buckets).

Root cause: 'trust proxy' used a fixed hop count (TRUST_PROXY=1), which
silently breaks whenever the proxy chain gains or loses an
X-Forwarded-For entry (e.g. a Cloudflare-side change).

- TRUST_PROXY now accepts a comma-separated subnet list; the new default
  'loopback,uniquelocal,cloudflare' expands Cloudflare's published IP
  ranges so Express skips trusted proxies no matter how many entries
  they add. Plain numbers keep the legacy hop-count behavior.
- If resolution still stops at a Cloudflare address (visitor missing
  from X-Forwarded-For entirely), the limiter key falls back to
  cf-connecting-ip — safe because request.ip can only be a Cloudflare
  address when the whole chain to it is trusted.
- CIDR matching uses Node's built-in net.BlockList (no new dependency).

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

test/config.test.js

@@ -38,7 +38,7 @@ describe("Config environment variable parsing", function () {
     MAX_REPO_SIZE: 60000,
     ENABLE_DOWNLOAD: true,
     RATE_LIMIT: 350,
-    TRUST_PROXY: 1,
+    TRUST_PROXY: "loopback,uniquelocal,cloudflare",
     SESSION_SECRET: "SESSION_SECRET",
     CLIENT_ID: "CLIENT_ID",
     APP_HOSTNAME: "anonymous.4open.science",
@@ -80,14 +80,14 @@ describe("Config environment variable parsing", function () {
     });
 
     it("handles zero correctly", function () {
-      const config = parseConfigFromEnv(defaults, { TRUST_PROXY: "0" });
-      expect(config.TRUST_PROXY).to.equal(0);
-      expect(config.TRUST_PROXY).to.be.a("number");
+      const config = parseConfigFromEnv(defaults, { RATE_LIMIT: "0" });
+      expect(config.RATE_LIMIT).to.equal(0);
+      expect(config.RATE_LIMIT).to.be.a("number");
     });
 
     it("handles negative numbers", function () {
-      const config = parseConfigFromEnv(defaults, { TRUST_PROXY: "-1" });
-      expect(config.TRUST_PROXY).to.equal(-1);
+      const config = parseConfigFromEnv(defaults, { RATE_LIMIT: "-1" });
+      expect(config.RATE_LIMIT).to.equal(-1);
     });
 
     it("correctly compares parsed numbers (no string comparison bug)", function () {
@@ -163,6 +163,13 @@ describe("Config environment variable parsing", function () {
       const config = parseConfigFromEnv(defaults, { STORAGE: "s3" });
       expect(config.STORAGE).to.equal("s3");
     });
+
+    it("keeps TRUST_PROXY as a string (legacy hop counts included)", function () {
+      // resolveTrustProxy() interprets the string later; a numeric env value
+      // like "2" must survive as-is.
+      const config = parseConfigFromEnv(defaults, { TRUST_PROXY: "2" });
+      expect(config.TRUST_PROXY).to.equal("2");
+    });
   });
 
   // ---------------------------------------------------------------

test/trust-proxy.test.js

@@ -0,0 +1,142 @@
+const { expect } = require("chai");
+require("ts-node/register/transpile-only");
+
+const {
+  resolveTrustProxy,
+  isCloudflareIP,
+  CLOUDFLARE_IP_RANGES,
+} = require("../src/server/trustProxy");
+
+describe("trustProxy", function () {
+  describe("resolveTrustProxy", function () {
+    it("parses a plain integer as a legacy hop count", function () {
+      expect(resolveTrustProxy("1")).to.equal(1);
+      expect(resolveTrustProxy("2")).to.equal(2);
+      expect(resolveTrustProxy(" 3 ")).to.equal(3);
+    });
+
+    it("parses zero as a number", function () {
+      expect(resolveTrustProxy("0")).to.equal(0);
+    });
+
+    it("expands the cloudflare keyword to the published ranges", function () {
+      const result = resolveTrustProxy("cloudflare");
+      expect(result).to.be.an("array");
+      expect(result).to.have.members([...CLOUDFLARE_IP_RANGES]);
+    });
+
+    it("keeps named subnets and mixes them with cloudflare ranges", function () {
+      const result = resolveTrustProxy("loopback,uniquelocal,cloudflare");
+      expect(result).to.include("loopback");
+      expect(result).to.include("uniquelocal");
+      expect(result).to.include("173.245.48.0/20");
+      expect(result).to.include("2400:cb00::/32");
+      expect(result).to.have.length(2 + CLOUDFLARE_IP_RANGES.length);
+    });
+
+    it("accepts literal CIDRs and trims whitespace and empty tokens", function () {
+      const result = resolveTrustProxy(" 10.0.0.0/8 , , 192.168.1.1 ");
+      expect(result).to.deep.equal(["10.0.0.0/8", "192.168.1.1"]);
+    });
+
+    it("is case-insensitive for the cloudflare keyword", function () {
+      const result = resolveTrustProxy("Cloudflare");
+      expect(result).to.have.length(CLOUDFLARE_IP_RANGES.length);
+    });
+  });
+
+  describe("isCloudflareIP", function () {
+    it("matches IPv4 addresses inside Cloudflare ranges", function () {
+      expect(isCloudflareIP("104.16.132.229")).to.be.true; // 104.16.0.0/13
+      expect(isCloudflareIP("172.64.0.1")).to.be.true; // 172.64.0.0/13
+      expect(isCloudflareIP("173.245.48.10")).to.be.true; // 173.245.48.0/20
+    });
+
+    it("matches IPv6 addresses inside Cloudflare ranges", function () {
+      expect(isCloudflareIP("2400:cb00::1")).to.be.true;
+      expect(isCloudflareIP("2606:4700:4700::1111")).to.be.true;
+    });
+
+    it("matches IPv4-mapped IPv6 representations", function () {
+      expect(isCloudflareIP("::ffff:104.16.0.1")).to.be.true;
+    });
+
+    it("rejects addresses outside Cloudflare ranges", function () {
+      expect(isCloudflareIP("8.8.8.8")).to.be.false;
+      expect(isCloudflareIP("127.0.0.1")).to.be.false;
+      expect(isCloudflareIP("192.168.1.1")).to.be.false;
+      expect(isCloudflareIP("2001:4860:4860::8888")).to.be.false;
+      expect(isCloudflareIP("::ffff:8.8.8.8")).to.be.false;
+    });
+
+    it("rejects garbage input without throwing", function () {
+      expect(isCloudflareIP("")).to.be.false;
+      expect(isCloudflareIP("not-an-ip")).to.be.false;
+      expect(isCloudflareIP("104.16.0")).to.be.false;
+    });
+  });
+
+  describe("integration with Express trust proxy resolution", function () {
+    const express = require("express");
+
+    function requestIp(trustProxy, headers) {
+      const app = express();
+      app.set("trust proxy", trustProxy);
+      let captured = null;
+      app.get("/", (req, res) => {
+        captured = req.ip;
+        res.end();
+      });
+      return new Promise((resolve, reject) => {
+        const server = app.listen(0, "127.0.0.1", () => {
+          const { port } = server.address();
+          const http = require("http");
+          const req = http.get(
+            { host: "127.0.0.1", port, path: "/", headers },
+            (res) => {
+              res.resume();
+              res.on("end", () => {
+                server.close();
+                resolve(captured);
+              });
+            }
+          );
+          req.on("error", (err) => {
+            server.close();
+            reject(err);
+          });
+        });
+      });
+    }
+
+    it("resolves the visitor IP through Cloudflare hops regardless of count", async function () {
+      const trust = resolveTrustProxy("loopback,uniquelocal,cloudflare");
+      // Simulate: visitor -> Cloudflare edge -> local nginx -> app, where
+      // Cloudflare appended an extra internal hop to X-Forwarded-For.
+      const ip = await requestIp(trust, {
+        "x-forwarded-for": "203.0.113.7, 104.16.0.9, 172.64.0.3",
+      });
+      expect(ip).to.equal("203.0.113.7");
+    });
+
+    it("does not honour X-Forwarded-For from an untrusted public peer entry", async function () {
+      const trust = resolveTrustProxy("loopback,uniquelocal,cloudflare");
+      // The last entry is a non-Cloudflare public address: resolution stops
+      // there, so a forged visitor entry further left is ignored.
+      const ip = await requestIp(trust, {
+        "x-forwarded-for": "1.2.3.4, 8.8.8.8",
+      });
+      expect(ip).to.equal("8.8.8.8");
+    });
+
+    it("legacy hop count still works", async function () {
+      const ip = await requestIp(resolveTrustProxy("1"), {
+        "x-forwarded-for": "203.0.113.7, 104.16.0.9",
+      });
+      // hop count 1 trusts only the direct peer, so req.ip is the last
+      // X-Forwarded-For entry — the fragile behavior this change moves
+      // away from.
+      expect(ip).to.equal("104.16.0.9");
+    });
+  });
+});