diff --git a/src/gateway/managed-image-attachments.test.ts b/src/gateway/managed-image-attachments.test.ts
index fcf6ef28991..b38e4ed4634 100644
--- a/src/gateway/managed-image-attachments.test.ts
+++ b/src/gateway/managed-image-attachments.test.ts
@@ -586,6 +586,59 @@ describe("createManagedOutgoingImageBlocks", () => {
     await expectPathMissing(path.join(stateDir, "media", "outgoing", "records"));
   });
 
+  it("rejects semicolon-heavy malformed data urls without backtracking", async () => {
+    const semicolons = ";".repeat(64);
+    const malformed = `data:image/png${semicolons}`;
+
+    await expect(
+      createManagedOutgoingImageBlocks({
+        sessionKey: "agent:main:main",
+        mediaUrls: [malformed],
+        stateDir,
+      }),
+    ).rejects.toThrow("Invalid image data URL");
+  });
+
+  it("rejects malformed data urls with a later ;base64, marker after a payload comma", async () => {
+    await expect(
+      createManagedOutgoingImageBlocks({
+        sessionKey: "agent:main:main",
+        mediaUrls: ["data:image/png;base64,garbage;base64,iVBORw0KGgo="],
+        stateDir,
+      }),
+    ).rejects.toThrow("Invalid image data URL");
+  });
+
+  it("requires the base64 marker to be adjacent to the payload comma", async () => {
+    await expect(
+      createManagedOutgoingImageBlocks({
+        sessionKey: "agent:main:main",
+        mediaUrls: [`data:image/png;base64\n,${TINY_PNG_BASE64}`],
+        stateDir,
+      }),
+    ).rejects.toThrow("Invalid image data URL");
+  });
+
+  it("preserves parameterized image data urls", async () => {
+    const blocks = await createManagedOutgoingImageBlocks({
+      sessionKey: "agent:main:main",
+      mediaUrls: [`data:image/png;charset=utf-8;base64,${TINY_PNG_BASE64}`],
+      stateDir,
+    });
+
+    expect(requireBlock(blocks).mimeType).toBe("image/png");
+  });
+
+  it("rejects data urls without a media type", async () => {
+    await expect(
+      createManagedOutgoingImageBlocks({
+        sessionKey: "agent:main:main",
+        mediaUrls: [`data:;base64,${TINY_PNG_BASE64}`],
+        stateDir,
+      }),
+    ).rejects.toThrow("Invalid image data URL");
+  });
+
   it("rewrites local image sources into managed display blocks without leaking the source path", async () => {
     const sourcePath = path.join(stateDir, "workspace", "fixtures", "dot.png");
     await fs.mkdir(path.dirname(sourcePath), { recursive: true });
diff --git a/src/gateway/managed-image-attachments.ts b/src/gateway/managed-image-attachments.ts
index 9f8b334e9ca..5a7b59c5586 100644
--- a/src/gateway/managed-image-attachments.ts
+++ b/src/gateway/managed-image-attachments.ts
@@ -279,22 +279,40 @@ function parseImageDataUrl(
   if (!trimmed.startsWith("data:")) {
     return { kind: "not-data-url" };
   }
-  const match = /^data:([^;,]+)(?:;[^,]*)*;base64,([A-Za-z0-9+/=\s]+)$/i.exec(trimmed);
-  if (!match) {
+
+  const afterPrefix = trimmed.slice("data:".length);
+  const commaIdx = afterPrefix.indexOf(",");
+  const mimeAndParams = commaIdx < 0 ? "" : afterPrefix.slice(0, commaIdx);
+  const base64Marker = ";base64";
+  if (mimeAndParams.slice(-base64Marker.length).toLowerCase() !== base64Marker) {
     throw new Error("Invalid image data URL");
   }
-  const contentType = match[1]?.trim().toLowerCase() ?? "";
+  const semicolonIdx = mimeAndParams.indexOf(";");
+  const contentType = (semicolonIdx < 0 ? mimeAndParams : mimeAndParams.slice(0, semicolonIdx))
+    .trim()
+    .toLowerCase();
+  if (!contentType) {
+    throw new Error("Invalid image data URL");
+  }
+
+  const base64Part = afterPrefix.slice(commaIdx + 1);
+  if (!/^[A-Za-z0-9+/=\s]+$/.test(base64Part)) {
+    throw new Error("Invalid image data URL");
+  }
+
   if (!contentType.startsWith("image/")) {
     return { kind: "non-image-data-url" };
   }
-  if (estimateBase64DecodedByteLength(match[2]) > limits.maxBytes) {
+
+  if (estimateBase64DecodedByteLength(base64Part) > limits.maxBytes) {
     throw createManagedImageAttachmentError(
       `Managed image attachment ${JSON.stringify(alt)} exceeds the ${formatLimitMiB(limits.maxBytes)} byte limit`,
     );
   }
+
   return {
     kind: "image-data-url",
-    buffer: Buffer.from(match[2].replace(/\s+/g, ""), "base64"),
+    buffer: Buffer.from(base64Part.replace(/\s+/g, ""), "base64"),
     contentType,
   };
 }