diff --git a/src/openultrasast/cpg/backend.py b/src/openultrasast/cpg/backend.py
index c1bf50c..dffead0 100644
--- a/src/openultrasast/cpg/backend.py
+++ b/src/openultrasast/cpg/backend.py
@@ -24,6 +24,7 @@ import logging
 import os
 import re
 import shutil
+import signal
 import subprocess
 import tempfile
 from collections.abc import Callable, Mapping, Sequence
@@ -655,13 +656,56 @@ class JoernBackend:
         return f"-J-Xmx{configured if configured.isdigit() else CPG_HEAP_MB}m"
 
     def _run(self, command: list[str], *, timeout: int, cwd: Path | None = None) -> subprocess.CompletedProcess[str] | None:
+        """Run one Joern command, and on timeout kill the whole PROCESS GROUP rather than the child.
+
+        `joern` and `joern-parse` are shell scripts that launch a JVM as a separate process. Killing the
+        script -- which is all `subprocess.run(timeout=)` does -- leaves that JVM running, holding its heap,
+        for as long as the machine lasts. Every `query_failed: timeout` leaked about a gigabyte.
+
+        Measured the hard way: after a day of 300s and 2,400s taint timeouts, four orphaned JVMs three hours
+        old were still resident and the machine ran out of memory. On a CI runner doing several scans in a
+        row it would OOM rather than merely degrade -- and it would look like the SCAN needing more memory,
+        which is the wrong lesson entirely.
+        """
         run = self.runner if self.runner is not None else subprocess.run
+        if self.runner is not None:  # tests inject a fake runner and never spawn anything
+            try:
+                return run(command, capture_output=True, text=True, timeout=timeout, check=False, cwd=str(cwd) if cwd else None)
+            except (OSError, subprocess.SubprocessError):
+                return None
         try:
-            return run(command, capture_output=True, text=True, timeout=timeout, check=False, cwd=str(cwd) if cwd else None)
+            with subprocess.Popen(  # noqa: S603 -- the command is built here, never from user input
+                command,
+                stdout=subprocess.PIPE,
+                stderr=subprocess.PIPE,
+                text=True,
+                cwd=str(cwd) if cwd else None,
+                start_new_session=True,  # its own process group, so the JVM dies with the script
+            ) as process:
+                try:
+                    stdout, stderr = process.communicate(timeout=timeout)
+                except subprocess.TimeoutExpired:
+                    _terminate_group(process)
+                    return None
+                return subprocess.CompletedProcess(command, process.returncode, stdout, stderr)
         except (OSError, subprocess.SubprocessError):
             return None
 
 
+def _terminate_group(process: subprocess.Popen[str]) -> None:
+    """SIGTERM the process group, then SIGKILL what is left. Never raises: this is cleanup, not logic."""
+    for signal_number in (signal.SIGTERM, signal.SIGKILL):
+        try:
+            os.killpg(os.getpgid(process.pid), signal_number)
+        except (OSError, ProcessLookupError):
+            return
+        try:
+            process.wait(timeout=10)
+            return
+        except subprocess.TimeoutExpired:
+            continue
+
+
 # Every Joern frontend logs a dropped file the same way, at WARN, and then carries on:
 #
 #     WARN  AstCreationPass   Failed to process '/abs/path/to/file.php'
diff --git a/tests/test_cpg_backend.py b/tests/test_cpg_backend.py
index 0ccf22b..3b2c4d5 100644
--- a/tests/test_cpg_backend.py
+++ b/tests/test_cpg_backend.py
@@ -7,6 +7,7 @@ Nothing here imports a JVM binding, and every test runs with Joern absent.
 
 from __future__ import annotations
 
+import contextlib
 import json
 import subprocess
 from pathlib import Path
@@ -51,6 +52,7 @@ def test_the_seam_imports_nothing_outside_the_standard_library() -> None:
         "os",
         "re",
         "shutil",
+        "signal",
         "subprocess",
         "tempfile",
         "dataclasses",
@@ -684,3 +686,42 @@ def test_a_file_php2cpg_miscompiles_is_excluded_and_named(tmp_path: Path, monkey
     build = next(c for c in commands if "--script" not in c)
     assert "--exclude" in build and any("bad.php" in part for part in build), "and excluded from the build"
     assert not any("ok.php" in part for part in build if part != str(tmp_path)), "a global at function scope is fine"
+
+
+def test_a_timed_out_query_kills_the_jvm_not_just_the_script(tmp_path: Path) -> None:
+    """`joern` is a shell script that launches a JVM as a separate process.
+
+    `subprocess.run(timeout=)` kills the script and leaves the JVM resident, holding its heap. Measured the
+    hard way: after a day of 300s and 2,400s taint timeouts, four orphaned JVMs three hours old were still
+    using about a gigabyte each and the machine ran out of memory. On CI that is an OOM rather than a
+    degradation, and it would look like the scan needing more memory -- the wrong lesson entirely.
+
+    Uses a real shell script that backgrounds a child, because the whole bug is about grandchildren.
+    """
+    import os
+    import time
+
+    from openultrasast.cpg.backend import JoernBackend
+
+    marker = tmp_path / "child-alive"
+    script = tmp_path / "fake-joern.sh"
+    script.write_text(
+        f"#!/bin/sh\n( while true; do echo x > '{marker}'; sleep 0.2; done ) &\necho $! > " + str(tmp_path / "child.pid") + "\nsleep 60\n"
+    )
+    script.chmod(0o755)
+
+    started = time.monotonic()
+    assert JoernBackend()._run([str(script)], timeout=2) is None, "a timeout reports nothing, never a verdict"
+    assert time.monotonic() - started < 30, "and it does not wait for the script's own sleep"
+
+    child = int((tmp_path / "child.pid").read_text().strip())
+    time.sleep(0.5)
+    alive = True
+    try:
+        os.kill(child, 0)
+    except ProcessLookupError:
+        alive = False
+    if alive:  # cleanup, so a failure here does not leak the process it is complaining about
+        with contextlib.suppress(ProcessLookupError, PermissionError):
+            os.kill(child, 9)
+    assert not alive, "the grandchild JVM must die with the script, or every timeout leaks a gigabyte"