diff --git a/CHANGELOG.md b/CHANGELOG.md
index c081a84c1a..bbf15f386a 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,7 +6,9 @@
 
 ## [3.8.27] — TBD
 
-_Cycle open — entries are added here as PRs merge into `release/v3.8.27`._
+### 🔒 Security & Hardening
+
+- **fix(security): eliminate a polynomial ReDoS in the combo `<omniModel>` tag regex** — `comboAgentMiddleware`'s cache-tag pattern wrapped the tag in an unbounded newline run (`(?:\n|\r)*`), making `.test()` / `.replace()` run in O(n²) on inputs with many newlines (CodeQL `js/polynomial-redos`). The detection pattern now matches only the core `<omniModel>…</omniModel>` and the global strip pattern bounds the surrounding newline runs, keeping it linear; detection / extraction / multi-tag stripping behavior is unchanged. ([#3982](https://github.com/diegosouzapw/OmniRoute/pull/3982) — thanks @diegosouzapw)
 
 ---
 
diff --git a/open-sse/services/comboAgentMiddleware.ts b/open-sse/services/comboAgentMiddleware.ts
index 28b5de5f4f..f93de4d285 100644
--- a/open-sse/services/comboAgentMiddleware.ts
+++ b/open-sse/services/comboAgentMiddleware.ts
@@ -34,18 +34,25 @@ interface Message {
 
 // ── Context Caching Tag ─────────────────────────────────────────────────────
 
-// Handles both actual newlines (U+000A) and literal \n sequences injected
-// by combo.ts streaming around the <omniModel> tag (#531). Non-global so that
-// .exec() and .test() stay stateless (a global regex carries lastIndex between
+// Detection / extraction pattern. The newline runs combo.ts streaming wraps the
+// tag with (#531) are irrelevant to *finding* the tag or capturing the model id,
+// so they are intentionally NOT matched here: an unbounded `(?:\\n|\n|\r)*` prefix
+// on this unanchored regex made `.test()` / `.exec()` run in O(n²) on inputs with
+// many newlines (polynomial ReDoS — CodeQL js/polynomial-redos, #3870). Non-global
+// so `.exec()` / `.test()` stay stateless (a global regex carries lastIndex between
 // calls and would skip matches).
-const CACHE_TAG_PATTERN = /(?:\\n|\n|\r)*<omniModel>([^<]+)<\/omniModel>(?:\\n|\n|\r)*/;
+const CACHE_TAG_PATTERN = /<omniModel>([^<]+)<\/omniModel>/;
 
-// Global variant for .replace() callers that must strip EVERY tag. A non-global
-// regex only removes the first match, so a single message carrying more than one
-// <omniModel> tag (e.g. an Open WebUI follow-up/title request that inlines the
-// whole chat history) leaked the remaining tags to the provider — defeating the
-// cache-session protection stripModelTags exists to enforce (#454).
-const CACHE_TAG_PATTERN_GLOBAL = /(?:\\n|\n|\r)*<omniModel>([^<]+)<\/omniModel>(?:\\n|\n|\r)*/g;
+// Global variant for `.replace()` callers that must strip EVERY tag (a non-global
+// regex only removes the first match, so a message carrying more than one
+// <omniModel> tag — e.g. an Open WebUI follow-up/title request that inlines the
+// whole chat history — leaked the remaining tags to the provider, defeating the
+// cache-session protection stripModelTags enforces, #454). This variant still
+// consumes the newline run wrapping the tag (combo.ts streaming, #531) so removal
+// leaves no blank line, but the runs are BOUNDED ({0,16}) to keep the regex linear
+// (no polynomial backtracking, #3870); 16 is far beyond any real streaming wrap.
+const CACHE_TAG_PATTERN_GLOBAL =
+  /(?:\\n|\n|\r){0,16}<omniModel>([^<]+)<\/omniModel>(?:\\n|\n|\r){0,16}/g;
 
 /**
  * Inject the model tag into the last assistant message (or append a new one).
diff --git a/tests/unit/combo-omnimodel-tag-stripping.test.ts b/tests/unit/combo-omnimodel-tag-stripping.test.ts
index fc6d5fa3c0..fa30559fdb 100644
--- a/tests/unit/combo-omnimodel-tag-stripping.test.ts
+++ b/tests/unit/combo-omnimodel-tag-stripping.test.ts
@@ -3,6 +3,7 @@ import assert from "node:assert/strict";
 import {
   stripModelTags,
   injectModelTag,
+  extractPinnedModel,
 } from "../../open-sse/services/comboAgentMiddleware.ts";
 
 // Regression for the non-global CACHE_TAG_PATTERN bug: `.replace()` with a
@@ -56,3 +57,45 @@ describe("comboAgentMiddleware — <omniModel> tag stripping (non-global regex b
     );
   });
 });
+
+// Regression for CodeQL js/polynomial-redos (#3870): the unbounded `(?:\\n|\n|\r)*`
+// prefix/suffix on the unanchored CACHE_TAG_PATTERN made `.test()` / `.replace()`
+// run in O(n²) on inputs with many newlines. The fix drops the surrounding runs from
+// the detection pattern and bounds them ({0,16}) in the global strip pattern.
+describe("comboAgentMiddleware — ReDoS safety + newline-wrapped tags (#3870)", () => {
+  test("stripModelTags stays linear on a long run of newlines (no tag present)", () => {
+    const evil = "\n".repeat(50_000);
+    const start = process.hrtime.bigint();
+    const stripped = stripModelTags([{ role: "user", content: evil }]);
+    const elapsedMs = Number(process.hrtime.bigint() - start) / 1e6;
+    assert.equal(stripped[0].content, evil, "no tag → content returned unchanged");
+    assert.ok(elapsedMs < 1000, `regex must stay linear; took ${elapsedMs.toFixed(1)}ms`);
+  });
+
+  test("stripModelTags stays linear on many newlines before a never-closing tag", () => {
+    const evil = "\n".repeat(50_000) + "<omniModel" + "x".repeat(2000);
+    const start = process.hrtime.bigint();
+    stripModelTags([{ role: "user", content: evil }]);
+    const elapsedMs = Number(process.hrtime.bigint() - start) / 1e6;
+    assert.ok(elapsedMs < 1000, `regex must stay linear; took ${elapsedMs.toFixed(1)}ms`);
+  });
+
+  test("extractPinnedModel still finds a tag wrapped in newlines", () => {
+    const messages = [
+      {
+        role: "assistant",
+        content: "answer\n\n<omniModel>claude/claude-opus-4-8</omniModel>\n\n",
+      },
+    ];
+    assert.equal(extractPinnedModel(messages), "claude/claude-opus-4-8");
+  });
+
+  test("stripModelTags removes the newline run wrapping a tag (no blank line left)", () => {
+    const out = String(
+      stripModelTags([{ role: "user", content: "before\n\n<omniModel>a/b</omniModel>\n\nafter" }])[0]
+        .content
+    );
+    assert.ok(!out.includes("<omniModel>"), "tag removed");
+    assert.ok(!out.includes("\n\n\n"), "no triple newline left from stripping");
+  });
+});