diff --git a/library/helpers/extractPathStringsFromUserInputCached.ts b/library/helpers/extractPathStringsFromUserInputCached.ts
index 8136f1cad..94eb58cc3 100644
--- a/library/helpers/extractPathStringsFromUserInputCached.ts
+++ b/library/helpers/extractPathStringsFromUserInputCached.ts
@@ -1,7 +1,6 @@
 import { Context } from "../agent/Context";
 import { SOURCES } from "../agent/Source";
 import { extractStringsFromUserInput } from "./extractStringsFromUserInput";
-import { sep } from "node:path";
 
 type ReturnValue = ReturnType<typeof extractStringsFromUserInput>;
 
@@ -23,7 +22,9 @@ export function extractPathStringsFromUserInputCached(
       // Performance optimization: only keep strings that contain a path separator
       // as only those can be used for path traversal
       // keeps the set smaller and speeds up `fs` and `path` operations
-      if (item.includes(sep)) {
+      // Check both separators: attackers may send backslash-only payloads on POSIX
+      // that the application normalizes to forward slashes before reaching the sink.
+      if (item.includes("/") || item.includes("\\")) {
         userStrings.add(item);
       }
     }
diff --git a/library/tsconfig.json b/library/tsconfig.json
index 0b0a799c2..ddc0502c5 100644
--- a/library/tsconfig.json
+++ b/library/tsconfig.json
@@ -1,7 +1,7 @@
 {
   "compilerOptions": {
-    "target": "ES2019",
-    "lib": ["ES2019", "DOM"],
+    "target": "ES2021",
+    "lib": ["ES2021", "DOM"],
     "module": "commonjs",
     "moduleResolution": "node",
     "declaration": true,
diff --git a/library/vulnerabilities/path-traversal/checkContextForPathTraversal.test.ts b/library/vulnerabilities/path-traversal/checkContextForPathTraversal.test.ts
index b6c1a9357..a2cc3311b 100644
--- a/library/vulnerabilities/path-traversal/checkContextForPathTraversal.test.ts
+++ b/library/vulnerabilities/path-traversal/checkContextForPathTraversal.test.ts
@@ -20,6 +20,42 @@ const unsafeContext = {
   },
 };
 
+t.test(
+  "it detects path traversal when attacker uses backslashes (POSIX bypass)",
+  async () => {
+    t.same(
+      checkContextForPathTraversal({
+        filename: "../../../../etc/hosts",
+        operation: "path.resolve",
+        context: {
+          cookies: {},
+          headers: {},
+          remoteAddress: "ip",
+          method: "GET",
+          url: "url",
+          query: {
+            file: "..\\..\\..\\..\\etc\\hosts",
+          },
+          body: {},
+          routeParams: {},
+          source: "express",
+          route: undefined,
+        },
+      }),
+      {
+        operation: "path.resolve",
+        kind: "path_traversal",
+        source: "query",
+        pathsToPayload: [".file"],
+        metadata: {
+          filename: "../../../../etc/hosts",
+        },
+        payload: "..\\..\\..\\..\\etc\\hosts",
+      }
+    );
+  }
+);
+
 t.test("it detects path traversal from route parameter", async () => {
   t.same(checkContextForPathTraversal(unsafeContext), {
     operation: "operation",
diff --git a/library/vulnerabilities/path-traversal/detectPathTraversal.ts b/library/vulnerabilities/path-traversal/detectPathTraversal.ts
index f26ca40a4..550427b4e 100644
--- a/library/vulnerabilities/path-traversal/detectPathTraversal.ts
+++ b/library/vulnerabilities/path-traversal/detectPathTraversal.ts
@@ -29,13 +29,27 @@ export function detectPathTraversal(
     }
   }
 
-  if (userInput.length > filePath.length) {
+  // Normalize backslashes to forward slashes on both sides for cross-platform comparison.
+  // Attackers may send backslash payloads that the application normalizes to forward
+  // slashes before reaching the sink (filePath has '/', userInput still has '\').
+  const normalizedFilePath = filePath.includes("\\")
+    ? filePath.replaceAll("\\", "/")
+    : filePath;
+  const normalizedUserInput = userInput.includes("\\")
+    ? userInput.replaceAll("\\", "/")
+    : userInput;
+
+  if (normalizedUserInput.length > normalizedFilePath.length) {
     // We ignore cases where the user input is longer than the file path.
     // Because the user input can't be part of the file path.
     return false;
   }
 
-  if (!filePath.toLowerCase().includes(userInput.toLowerCase())) {
+  if (
+    !normalizedFilePath
+      .toLowerCase()
+      .includes(normalizedUserInput.toLowerCase())
+  ) {
     // We ignore cases where the user input is not part of the file path.
     return false;
   }