diff --git a/packages/backend/src/checks/index.ts b/packages/backend/src/checks/index.ts
index 1c28c38..ae25238 100644
--- a/packages/backend/src/checks/index.ts
+++ b/packages/backend/src/checks/index.ts
@@ -21,6 +21,7 @@ import hashDisclosureScan from "./hash-disclosure";
 import jsonHtmlResponseScan from "./json-html-response";
 import missingContentTypeScan from "./missing-content-type";
 import openRedirectScan from "./open-redirect";
+import openApiDefinitionScan from "./openapi-definition";
 import pathTraversalScan from "./path-traversal";
 import phpinfoScan from "./phpinfo";
 import privateIpDisclosureScan from "./private-ip-disclosure";
@@ -59,6 +60,7 @@ export const Checks = {
   HASH_DISCLOSURE: "hash-disclosure",
   JSON_HTML_RESPONSE: "json-html-response",
   MISSING_CONTENT_TYPE: "missing-content-type",
+  OPENAPI_DEFINITION_FOUND: "openapi-definition-found",
   OPEN_REDIRECT: "open-redirect",
   PATH_TRAVERSAL: "path-traversal",
   PHPINFO: "phpinfo",
@@ -100,6 +102,7 @@ export const checks = [
   jsonHtmlResponseScan,
   missingContentTypeScan,
   openRedirectScan,
+  openApiDefinitionScan,
   pathTraversalScan,
   phpinfoScan,
   privateIpDisclosureScan,
diff --git a/packages/backend/src/checks/openapi-definition/index.spec.ts b/packages/backend/src/checks/openapi-definition/index.spec.ts
new file mode 100644
index 0000000..e6ac6f2
--- /dev/null
+++ b/packages/backend/src/checks/openapi-definition/index.spec.ts
@@ -0,0 +1,68 @@
+import { createMockRequest, createMockResponse, runCheck } from "engine";
+import { describe, expect, it } from "vitest";
+
+import openApiCheck from "./index";
+
+const runOpenApiCheck = async (body: string): Promise<unknown[]> => {
+  const request = createMockRequest({
+    id: "req-openapi",
+    host: "example.com",
+    method: "GET",
+    path: "/openapi.json",
+    headers: { Host: ["example.com"] },
+  });
+
+  const response = createMockResponse({
+    id: "res-openapi",
+    code: 200,
+    headers: { "content-type": ["application/json"] },
+    body,
+  });
+
+  const execution = await runCheck(openApiCheck, [{ request, response }]);
+  return execution[0]?.steps[execution[0].steps.length - 1]?.findings ?? [];
+};
+
+describe("OpenAPI definition check", () => {
+  it("detects JSON OpenAPI documents", async () => {
+    const findings = await runOpenApiCheck(
+      JSON.stringify({
+        openapi: "3.0.1",
+        info: { title: "API", version: "1.0" },
+        paths: {},
+      }),
+    );
+
+    expect(findings).toHaveLength(1);
+    expect(findings[0]).toMatchObject({
+      name: "OpenAPI definition exposed",
+      severity: "medium",
+    });
+  });
+
+  it("detects YAML OpenAPI documents", async () => {
+    const findings = await runOpenApiCheck(
+      [
+        "openapi: 3.0.2",
+        "info:",
+        "  title: API",
+        "paths:",
+        "  /users:",
+        "    get:",
+        "      responses:",
+        "        '200':",
+        "          description: OK",
+      ].join("\n"),
+    );
+
+    expect(findings).toHaveLength(1);
+  });
+
+  it("does not flag unrelated JSON", async () => {
+    const findings = await runOpenApiCheck(
+      JSON.stringify({ message: "hello" }),
+    );
+
+    expect(findings).toHaveLength(0);
+  });
+});
diff --git a/packages/backend/src/checks/openapi-definition/index.ts b/packages/backend/src/checks/openapi-definition/index.ts
new file mode 100644
index 0000000..cee12c4
--- /dev/null
+++ b/packages/backend/src/checks/openapi-definition/index.ts
@@ -0,0 +1,131 @@
+import { defineCheck, done, Severity } from "engine";
+
+import { Tags } from "../../types";
+import { keyStrategy } from "../../utils";
+
+type FindingData = {
+  format: "json" | "yaml";
+  version?: string;
+};
+
+const YAML_PATTERN = /\b(openapi|swagger)\s*:\s*["']?([\d.]+)["']?/i;
+
+const parseOpenApiJson = (body: string): FindingData | undefined => {
+  try {
+    const json = JSON.parse(body) as Record<string, unknown>;
+
+    if (json === null || typeof json !== "object") {
+      return undefined;
+    }
+
+    const version =
+      typeof json.openapi === "string"
+        ? json.openapi
+        : typeof json.swagger === "string"
+          ? json.swagger
+          : undefined;
+
+    if (version === undefined) {
+      return undefined;
+    }
+
+    if (
+      json.paths !== undefined &&
+      typeof json.paths === "object" &&
+      json.info !== undefined
+    ) {
+      return { format: "json", version };
+    }
+  } catch {
+    // Not JSON, ignore
+  }
+
+  return undefined;
+};
+
+const parseOpenApiYaml = (body: string): FindingData | undefined => {
+  const match = body.match(YAML_PATTERN);
+  if (match === null) {
+    return undefined;
+  }
+
+  const version = match[2];
+  if (version === undefined) {
+    return undefined;
+  }
+
+  if (/\bpaths\s*:\s*/i.test(body) && /\binfo\s*:\s*/i.test(body)) {
+    return { format: "yaml", version };
+  }
+
+  return undefined;
+};
+
+const buildDescription = (data: FindingData): string => {
+  const formatText = data.format === "json" ? "JSON" : "YAML";
+  const versionText =
+    data.version !== undefined ? ` (version ${data.version})` : "";
+
+  return [
+    `An OpenAPI definition${versionText} was returned in the response (${formatText}).`,
+    "",
+    "OpenAPI/Swagger documentation often exposes full API surface area and can assist attackers in discovering sensitive endpoints or understanding authentication flows.",
+    "",
+    "Restrict access to API documentation in production or ensure it does not contain sensitive endpoints.",
+  ].join("\n");
+};
+
+export default defineCheck<Record<never, never>>(({ step }) => {
+  step("detectOpenApiDefinition", (state, context) => {
+    const { response } = context.target;
+
+    if (response === undefined) {
+      return done({ state });
+    }
+
+    const body = response.getBody()?.toText();
+    if (body === undefined || body.length === 0) {
+      return done({ state });
+    }
+
+    const findingData = parseOpenApiJson(body) ?? parseOpenApiYaml(body);
+
+    if (findingData === undefined) {
+      return done({ state });
+    }
+
+    return done({
+      state,
+      findings: [
+        {
+          name: "OpenAPI definition exposed",
+          description: buildDescription(findingData),
+          severity: Severity.MEDIUM,
+          correlation: {
+            requestID: context.target.request.getId(),
+            locations: [],
+          },
+        },
+      ],
+    });
+  });
+
+  return {
+    metadata: {
+      id: "openapi-definition-found",
+      name: "OpenAPI definition exposed",
+      description:
+        "Detects responses that expose OpenAPI/Swagger service definitions.",
+      type: "passive",
+      tags: [Tags.INFORMATION_DISCLOSURE],
+      severities: [Severity.MEDIUM],
+      aggressivity: {
+        minRequests: 0,
+        maxRequests: 0,
+      },
+    },
+    initState: () => ({}),
+    dedupeKey: keyStrategy().withHost().withPath().build(),
+    when: (target) => target.response !== undefined,
+  };
+});
diff --git a/packages/backend/src/stores/config.ts b/packages/backend/src/stores/config.ts
index 6b716b7..1dd60d5 100644
--- a/packages/backend/src/stores/config.ts
+++ b/packages/backend/src/stores/config.ts
@@ -188,6 +188,10 @@ export class ConfigStore {
               checkID: Checks.MISSING_CONTENT_TYPE,
               enabled: true,
             },
+            {
+              checkID: Checks.OPENAPI_DEFINITION_FOUND,
+              enabled: true,
+            },
           ],
         },
         {