diff --git a/src/process/ProcessOwnership.runId.test.ts b/src/process/ProcessOwnership.runId.test.ts
index f9893d0e29..ff447ad17b 100644
--- a/src/process/ProcessOwnership.runId.test.ts
+++ b/src/process/ProcessOwnership.runId.test.ts
@@ -3,6 +3,22 @@ import { expect, it } from "vitest";
 import type { ProcessOwnershipHost } from "./ProcessOwnership.js";
 import { readProcessRunId } from "./ProcessOwnership.js";
 
+it.each([undefined, "selected-token", ""])(
+  "reads only the process environment's own run token %j",
+  async (selected) => {
+    const environment: Record<string, string> = Object.create({
+      REA_PROCESS_RUN_ID: "inherited-token",
+    });
+    if (selected !== undefined) environment.REA_PROCESS_RUN_ID = selected;
+    const host: ProcessOwnershipHost = {
+      listProcesses: async () => [],
+      environment: async () => environment,
+      signalGroup: () => undefined,
+    };
+    expect(await readProcessRunId(42, host)).toBe(selected);
+  },
+);
+
 it("passes cancellation through run-token reads", async () => {
   const controller = new AbortController();
   const cancellation = new Error("cancelled");
diff --git a/src/process/ProcessOwnership.test.ts b/src/process/ProcessOwnership.test.ts
index 424211e09c..d2e268e008 100644
--- a/src/process/ProcessOwnership.test.ts
+++ b/src/process/ProcessOwnership.test.ts
@@ -44,10 +44,18 @@ describe("owned process-group cleanup discovery", () => {
       ]),
     ).toEqual([100, 102]);
   });
-  it("drops nameless Linux environment entries", () => {
-    expect(
-      parseProcessEnvironment("=ignored\0REA_PROCESS_RUN_ID=owned\0EMPTY=\0"),
-    ).toEqual({ REA_PROCESS_RUN_ID: "owned", EMPTY: "" });
+  it("preserves only named source entries in the Linux environment", () => {
+    const environment = parseProcessEnvironment(
+      "=ignored\0REA_PROCESS_RUN_ID=earlier\0EMPTY=\0REA_PROCESS_RUN_ID=owned\0constructor=source\0__proto__=retained\0VALUE=a=b\0",
+    );
+    expect(environment).toEqual({
+      REA_PROCESS_RUN_ID: "owned",
+      EMPTY: "",
+      constructor: "source",
+      ["__proto__"]: "retained",
+      VALUE: "a=b",
+    });
+    expect(environment["toString"]).toBeUndefined();
   });
   it("matches macOS executable names after process-table normalization", () => {
     expect(
@@ -89,6 +97,19 @@ describe("owned process-group cleanup discovery", () => {
     });
     expect(signalGroup).toHaveBeenCalledWith(100, "SIGKILL");
   });
+  it("does not signal a group when a member only inherits its matching token", async () => {
+    const inherited: Record<string, string> = Object.create({
+      REA_PROCESS_RUN_ID: ownership.runId,
+    });
+    const { adapter, signalGroup } = host({
+      100: { REA_PROCESS_RUN_ID: ownership.runId },
+      101: inherited,
+    });
+    expect(await cleanupOwnedProcessGroup(ownership, adapter)).toMatchObject({
+      cleaned: false,
+    });
+    expect(signalGroup).not.toHaveBeenCalled();
+  });
 });
 
 describe("opaque process neighbors during cleanup", () => {
diff --git a/src/process/ProcessOwnership.ts b/src/process/ProcessOwnership.ts
index 2038c92971..027e5de518 100644
--- a/src/process/ProcessOwnership.ts
+++ b/src/process/ProcessOwnership.ts
@@ -12,7 +12,10 @@ import {
 } from "./ProcessOwnershipObservation.js";
 import { launcherIdentityFailure } from "./ProcessOwnershipIdentity.js";
 import { descendantsOf, liveProcesses } from "./ProcessOwnershipProcessTree.js";
-import { readProcessRunTokens } from "./ProcessRunTokenObservations.js";
+import {
+  processRunIdFromEnvironment,
+  readProcessRunTokens,
+} from "./ProcessRunTokenObservations.js";
 import { compareUnicodeCodePoints } from "../domain/unicodeCodePointOrder.js";
 import type { LinuxEnvironmentRestriction } from "./LinuxProcessStatus.js";
 import { ProcessOwnershipEvidenceOwner } from "./ProcessOwnershipEvidence.js";
@@ -310,7 +313,7 @@ export const readProcessRunId = async (
   host: ProcessOwnershipHost = systemHost,
   signal?: AbortSignal,
 ): Promise<string | undefined> =>
-  (await host.environment(pid, signal)).REA_PROCESS_RUN_ID;
+  processRunIdFromEnvironment(await host.environment(pid, signal));
 
 /** Merge duplicate uncertainty rows from discovery and final verification. */
 const mergeUnverifiedObservations = (
diff --git a/src/process/ProcessOwnershipObservation.ts b/src/process/ProcessOwnershipObservation.ts
index 057cacc855..55df0b1071 100644
--- a/src/process/ProcessOwnershipObservation.ts
+++ b/src/process/ProcessOwnershipObservation.ts
@@ -691,16 +691,15 @@ const unavailableLineage = (
 /** Parse the NUL-delimited Linux process environment without nameless keys. */
 export const parseProcessEnvironment = (
   value: string,
-): Readonly<Record<string, string>> =>
-  Object.fromEntries(
-    value
-      .split("\0")
-      .filter((entry) => entry.indexOf("=") > 0)
-      .map((entry) => {
-        const separator = entry.indexOf("=");
-        return [entry.slice(0, separator), entry.slice(separator + 1)];
-      }),
-  );
+): Readonly<Record<string, string>> => {
+  const environment: Record<string, string> = Object.create(null);
+  for (const entry of value.split("\0")) {
+    const separator = entry.indexOf("=");
+    if (separator > 0)
+      environment[entry.slice(0, separator)] = entry.slice(separator + 1);
+  }
+  return environment;
+};
 
 const identityFact = (
   observations: ReadonlyMap<number, ProcessIdentityObservation> | undefined,
@@ -1274,9 +1273,10 @@ export const createSystemProcessOwnershipHost = (
           throw new Error(
             observation?.reason ?? "process run token could not be read",
           );
-        return observation.runId === undefined
-          ? {}
-          : { REA_PROCESS_RUN_ID: observation.runId };
+        const environment: Record<string, string> = Object.create(null);
+        if (observation.runId !== undefined)
+          environment.REA_PROCESS_RUN_ID = observation.runId;
+        return environment;
       }
       const { stdout } = await execFileOutput(
         "ps",
@@ -1287,7 +1287,7 @@ export const createSystemProcessOwnershipHost = (
           ...(signal === undefined ? {} : { signal }),
         },
       );
-      const observedEnvironment: Record<string, string> = {};
+      const observedEnvironment: Record<string, string> = Object.create(null);
       for (const match of stdout.matchAll(
         /(?:^|\s)([A-Za-z_][A-Za-z0-9_]*)=([^\s]*)/gu,
       )) {
diff --git a/src/process/ProcessRunTokenObservations.ts b/src/process/ProcessRunTokenObservations.ts
index c0e3b5cdd1..076b6c936d 100644
--- a/src/process/ProcessRunTokenObservations.ts
+++ b/src/process/ProcessRunTokenObservations.ts
@@ -7,6 +7,14 @@ import type {
 const errorMessage = (cause: unknown): string =>
   cause instanceof Error ? cause.message : String(cause);
 
+/** Only a variable observed on this process supplies its run token. */
+export const processRunIdFromEnvironment = (
+  environment: Readonly<Record<string, string>>,
+): string | undefined =>
+  Object.hasOwn(environment, "REA_PROCESS_RUN_ID")
+    ? environment.REA_PROCESS_RUN_ID
+    : undefined;
+
 /** Read only ownership tokens, using per-process fallback for missing batch rows. */
 export async function* readProcessRunTokens(
   host: ProcessOwnershipHost,
@@ -42,7 +50,7 @@ export async function* readProcessRunTokens(
             : await host.environment(process.pid, signal);
         observation = {
           state: "readable",
-          runId: environment.REA_PROCESS_RUN_ID,
+          runId: processRunIdFromEnvironment(environment),
         };
       } catch (cause: unknown) {
         signal?.throwIfAborted();
diff --git a/tests/boundary/process/processRunTokenEnvironment.test.ts b/tests/boundary/process/processRunTokenEnvironment.test.ts
new file mode 100644
index 0000000000..3742a5374c
--- /dev/null
+++ b/tests/boundary/process/processRunTokenEnvironment.test.ts
@@ -0,0 +1,43 @@
+import { once } from "node:events";
+import { expect } from "vitest";
+
+import { readProcessRunId } from "../../../src/process/ProcessOwnership.js";
+import { createSystemProcessOwnershipHost } from "../../../src/process/ProcessOwnershipObservation.js";
+import { readProcessRunTokens } from "../../../src/process/ProcessRunTokenObservations.js";
+import { processTest } from "../../support/process/processFixture.js";
+
+processTest.skipIf(process.platform !== "linux").for([
+  { name: "absent", runId: undefined },
+  { name: "selected", runId: "source-process-token" },
+])(
+  "reads $name run tokens from the live process environment",
+  async ({ runId }, { processes }) => {
+    const environment = {
+      EMPTY: "",
+      VALUE: "a=b",
+      ["__proto__"]: "source-value",
+      ...(runId === undefined ? {} : { REA_PROCESS_RUN_ID: runId }),
+    };
+    const child = processes.spawn(
+      process.execPath,
+      ["-e", "process.stdin.resume()"],
+      { env: environment },
+    );
+    await once(child, "spawn");
+    const host = createSystemProcessOwnershipHost("linux", {});
+    const row = (await host.listProcesses()).find(
+      ({ pid }) => pid === child.pid,
+    );
+    if (row === undefined) throw new Error("Selected child was not observed");
+
+    expect(await host.environment(row.pid)).toEqual(environment);
+    expect(await readProcessRunId(row.pid, host)).toBe(runId);
+    const observations = [];
+    for await (const observation of readProcessRunTokens(host, [row]))
+      observations.push(observation);
+    expect(observations).toEqual([
+      { process: row, observation: { state: "readable", runId } },
+    ]);
+    child.stdin.end();
+  },
+);