fix: use original body from webhook payload for TOCTOU hardening

src/github/data/fetcher.ts

@@ -71,6 +71,33 @@ export function extractOriginalTitle(
   return undefined;
 }
 
+/**
+ * Extracts the original body from the GitHub webhook payload.
+ * This is the body as it existed when the trigger event occurred,
+ * preventing TOCTOU attacks where an attacker edits the body after
+ * the trigger but before the action reads it.
+ *
+ * @param context - Parsed GitHub context from webhook
+ * @returns The original body string, null (no body), or undefined (not available)
+ */
+export function extractOriginalBody(
+  context: ParsedGitHubContext,
+): string | null | undefined {
+  if (isIssueCommentEvent(context)) {
+    return context.payload.issue?.body;
+  } else if (isPullRequestEvent(context)) {
+    return context.payload.pull_request?.body;
+  } else if (isPullRequestReviewEvent(context)) {
+    return context.payload.pull_request?.body;
+  } else if (isPullRequestReviewCommentEvent(context)) {
+    return context.payload.pull_request?.body;
+  } else if (isIssuesEvent(context)) {
+    return context.payload.issue?.body;
+  }
+
+  return undefined;
+}
+
 /**
  * Filters comments to only include those that existed in their final state before the trigger time.
  * This prevents malicious actors from editing comments after the trigger to inject harmful content.
@@ -207,6 +234,7 @@ type FetchDataParams = {
   triggerUsername?: string;
   triggerTime?: string;
   originalTitle?: string;
+  originalBody?: string | null;
   includeCommentsByActor?: string;
   excludeCommentsByActor?: string;
 };
@@ -233,6 +261,7 @@ export async function fetchGitHubData({
   triggerUsername,
   triggerTime,
   originalTitle,
+  originalBody,
   includeCommentsByActor,
   excludeCommentsByActor,
 }: FetchDataParams): Promise<FetchDataResult> {
@@ -399,12 +428,21 @@ export async function fetchGitHubData({
       body: c.body,
     }));
 
-  // Add the main issue/PR body if it has content and wasn't edited after trigger
-  // This prevents a TOCTOU race condition where an attacker could edit the body
-  // between when an authorized user triggered Claude and when Claude processes the request
+  // Use the original body from the webhook payload if provided (TOCTOU protection).
+  // The webhook payload captures the body at event time, before any attacker edits.
+  if (originalBody !== undefined) {
+    contextData.body = originalBody ?? "";
+  }
+
+  // Add the main issue/PR body if it has content and wasn't edited after trigger.
+  // When originalBody is provided, the body is already safe (from webhook payload).
+  // Otherwise, fall back to timestamp-based validation.
   let mainBody: CommentWithImages[] = [];
   if (contextData.body) {
-    if (isBodySafeToUse(contextData, triggerTime)) {
+    if (
+      originalBody !== undefined ||
+      isBodySafeToUse(contextData, triggerTime)
+    ) {
       mainBody = [
         {
           ...(isPR

src/modes/tag/index.ts

@@ -12,6 +12,7 @@ import {
   fetchGitHubData,
   extractTriggerTimestamp,
   extractOriginalTitle,
+  extractOriginalBody,
 } from "../../github/data/fetcher";
 import { createPrompt, generateDefaultPrompt } from "../../create-prompt";
 import { isEntityContext } from "../../github/context";
@@ -79,6 +80,7 @@ export const tagMode: Mode = {
 
     const triggerTime = extractTriggerTimestamp(context);
     const originalTitle = extractOriginalTitle(context);
+    const originalBody = extractOriginalBody(context);
 
     const githubData = await fetchGitHubData({
       octokits: octokit,
@@ -88,6 +90,7 @@ export const tagMode: Mode = {
       triggerUsername: context.actor,
       triggerTime,
       originalTitle,
+      originalBody,
       includeCommentsByActor: context.inputs.includeCommentsByActor,
       excludeCommentsByActor: context.inputs.excludeCommentsByActor,
     });

test/data-fetcher.test.ts

@@ -2,6 +2,7 @@ import { describe, expect, it, jest, test } from "bun:test";
 import {
   extractTriggerTimestamp,
   extractOriginalTitle,
+  extractOriginalBody,
   fetchGitHubData,
   filterCommentsToTriggerTime,
   filterReviewsToTriggerTime,
@@ -106,6 +107,55 @@ describe("extractOriginalTitle", () => {
   });
 });
 
+describe("extractOriginalBody", () => {
+  it("should extract body from IssueCommentEvent on PR", () => {
+    const body = extractOriginalBody(mockPullRequestCommentContext);
+    expect(body).toBe("This PR fixes the memory leak issue reported in #788");
+  });
+
+  it("should extract body from PullRequestReviewEvent", () => {
+    const body = extractOriginalBody(mockPullRequestReviewContext);
+    expect(body).toBe(
+      "This PR improves error handling across all API endpoints",
+    );
+  });
+
+  it("should extract body from PullRequestReviewCommentEvent", () => {
+    const body = extractOriginalBody(mockPullRequestReviewCommentContext);
+    expect(body).toBe(
+      "This PR optimizes the search algorithm for better performance",
+    );
+  });
+
+  it("should extract body from pull_request event", () => {
+    const body = extractOriginalBody(mockPullRequestOpenedContext);
+    expect(body).toBe(
+      "## Summary\n\nThis PR adds JWT-based authentication to the API.\n\n## Changes\n\n- Added auth middleware\n- Added login endpoint\n- Added JWT token generation\n\n/claude please review the security aspects",
+    );
+  });
+
+  it("should extract body from issues event", () => {
+    const body = extractOriginalBody(mockIssueOpenedContext);
+    expect(body).toBe(
+      "## Description\n\nThe application crashes immediately after launching.\n\n## Steps to reproduce\n\n1. Install the app\n2. Launch it\n3. See crash\n\n/claude please help me fix this",
+    );
+  });
+
+  it("should return undefined for event without body", () => {
+    const context = createMockContext({
+      eventName: "issue_comment",
+      payload: {
+        comment: {
+          id: 123,
+          body: "test",
+        },
+      } as any,
+    });
+    const body = extractOriginalBody(context);
+    expect(body).toBeUndefined();
+  });
+});
+
 describe("filterCommentsToTriggerTime", () => {
   const createMockComment = (
     createdAt: string,
@@ -1099,6 +1149,186 @@ describe("fetchGitHubData integration with time filtering", () => {
       "Original Title (from webhook at trigger time)",
     );
   });
+
+  it("should use originalBody when provided instead of fetched body", async () => {
+    const mockOctokits = {
+      graphql: jest.fn().mockResolvedValue({
+        repository: {
+          pullRequest: {
+            number: 123,
+            title: "Test PR",
+            body: "Malicious body injected after trigger",
+            author: { login: "author" },
+            createdAt: "2024-01-15T10:00:00Z",
+            additions: 10,
+            deletions: 5,
+            state: "OPEN",
+            commits: { totalCount: 1, nodes: [] },
+            files: { nodes: [] },
+            comments: { nodes: [] },
+            reviews: { nodes: [] },
+          },
+        },
+        user: { login: "trigger-user" },
+      }),
+      rest: jest.fn() as any,
+    };
+
+    const result = await fetchGitHubData({
+      octokits: mockOctokits as any,
+      repository: "test-owner/test-repo",
+      prNumber: "123",
+      isPR: true,
+      triggerUsername: "trigger-user",
+      originalBody: "Original safe body from webhook",
+    });
+
+    expect(result.contextData.body).toBe("Original safe body from webhook");
+  });
+
+  it("should use fetched body when originalBody is not provided", async () => {
+    const mockOctokits = {
+      graphql: jest.fn().mockResolvedValue({
+        repository: {
+          pullRequest: {
+            number: 123,
+            title: "Test PR",
+            body: "Fetched body from GraphQL",
+            author: { login: "author" },
+            createdAt: "2024-01-15T10:00:00Z",
+            additions: 10,
+            deletions: 5,
+            state: "OPEN",
+            commits: { totalCount: 1, nodes: [] },
+            files: { nodes: [] },
+            comments: { nodes: [] },
+            reviews: { nodes: [] },
+          },
+        },
+        user: { login: "trigger-user" },
+      }),
+      rest: jest.fn() as any,
+    };
+
+    const result = await fetchGitHubData({
+      octokits: mockOctokits as any,
+      repository: "test-owner/test-repo",
+      prNumber: "123",
+      isPR: true,
+      triggerUsername: "trigger-user",
+    });
+
+    expect(result.contextData.body).toBe("Fetched body from GraphQL");
+  });
+
+  it("should use original body from webhook even if body was edited after trigger (TOCTOU prevention)", async () => {
+    const mockOctokits = {
+      graphql: jest.fn().mockResolvedValue({
+        repository: {
+          pullRequest: {
+            number: 123,
+            title: "Test PR",
+            body: "Malicious body (edited after trigger via GraphQL)",
+            author: { login: "author" },
+            createdAt: "2024-01-15T10:00:00Z",
+            lastEditedAt: "2024-01-15T12:30:00Z", // Edited after trigger
+            additions: 10,
+            deletions: 5,
+            state: "OPEN",
+            commits: { totalCount: 1, nodes: [] },
+            files: { nodes: [] },
+            comments: { nodes: [] },
+            reviews: { nodes: [] },
+          },
+        },
+        user: { login: "trigger-user" },
+      }),
+      rest: jest.fn() as any,
+    };
+
+    const result = await fetchGitHubData({
+      octokits: mockOctokits as any,
+      repository: "test-owner/test-repo",
+      prNumber: "123",
+      isPR: true,
+      triggerUsername: "trigger-user",
+      triggerTime: "2024-01-15T12:00:00Z",
+      originalBody: "Original safe body (from webhook at trigger time)",
+    });
+
+    // Body should be from webhook, not the malicious GraphQL-fetched version
+    expect(result.contextData.body).toBe(
+      "Original safe body (from webhook at trigger time)",
+    );
+  });
+
+  it("should handle null originalBody by setting body to empty string", async () => {
+    const mockOctokits = {
+      graphql: jest.fn().mockResolvedValue({
+        repository: {
+          issue: {
+            number: 123,
+            title: "Test Issue",
+            body: "Some body from GraphQL",
+            author: { login: "author" },
+            createdAt: "2024-01-15T10:00:00Z",
+            state: "OPEN",
+            labels: { nodes: [] },
+            comments: { nodes: [] },
+          },
+        },
+        user: { login: "trigger-user" },
+      }),
+      rest: jest.fn() as any,
+    };
+
+    const result = await fetchGitHubData({
+      octokits: mockOctokits as any,
+      repository: "test-owner/test-repo",
+      prNumber: "123",
+      isPR: false,
+      triggerUsername: "trigger-user",
+      originalBody: null,
+    });
+
+    // null originalBody means the issue had no body at trigger time
+    expect(result.contextData.body).toBe("");
+  });
+
+  it("should use null originalBody over malicious GraphQL body edited after trigger", async () => {
+    const mockOctokits = {
+      graphql: jest.fn().mockResolvedValue({
+        repository: {
+          issue: {
+            number: 123,
+            title: "Test Issue",
+            body: "Malicious body added after trigger",
+            author: { login: "author" },
+            createdAt: "2024-01-15T10:00:00Z",
+            lastEditedAt: "2024-01-15T12:30:00Z", // Edited after trigger
+            state: "OPEN",
+            labels: { nodes: [] },
+            comments: { nodes: [] },
+          },
+        },
+        user: { login: "trigger-user" },
+      }),
+      rest: jest.fn() as any,
+    };
+
+    const result = await fetchGitHubData({
+      octokits: mockOctokits as any,
+      repository: "test-owner/test-repo",
+      prNumber: "123",
+      isPR: false,
+      triggerUsername: "trigger-user",
+      triggerTime: "2024-01-15T12:00:00Z",
+      originalBody: null,
+    });
+
+    // Webhook says no body at trigger time — attacker-added GraphQL body must not be used
+    expect(result.contextData.body).toBe("");
+  });
 });
 
 describe("filterCommentsByActor", () => {