🛡️ Sentinel: Fix TOCTOU race condition in SSH key creation #22
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- Use `umask 077` in a subshell when creating `.ssh` directory and private key files. - Ensures files are created with restricted permissions from the start, closing the race condition window where they might be world-readable. - Add security journal entry in `.jules/sentinel.md`. Co-authored-by: kidchenko <5432753+kidchenko@users.noreply.github.com>
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📝 WalkthroughWalkthroughA new security documentation file is added describing a TOCTOU race condition vulnerability in SSH private key creation, while the SSH setup script is enhanced with secure directory initialization and umask-based file permission restrictions to prevent unauthorized access. Changes
Estimated code review effort🎯 3 (Moderate) | ⏱️ ~25 minutes Poem
🚥 Pre-merge checks | ✅ 3✅ Passed checks (3 passed)
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🧪 Generate unit tests (beta)
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Actionable comments posted: 1
🤖 Fix all issues with AI agents
In @.jules/sentinel.md:
- Around line 1-4: Change the first line to a top-level heading (use a single
leading '#') and ensure there is a blank line before and after that heading to
satisfy MD041 and MD022; break the long lines in the body (lines mentioning the
TOCTOU race, chmod, umask example) into sentences under 80 characters to fix
MD013; update the date string "2024-05-22" to "2026-02-06" and keep the heading
text "TOCTOU Race Condition in File Permissions" intact so reviewers can locate
the entry.
| ## 2024-05-22 - TOCTOU Race Condition in File Permissions | ||
| **Vulnerability:** SSH private keys were written to disk with default permissions and then restricted with `chmod` immediately after. This creates a Time-of-Check-Time-of-Use (TOCTOU) race condition window where the file is world-readable (depending on system umask). | ||
| **Learning:** `chmod` is not atomic with file creation. Relying on it for sensitive files leaves a security gap. | ||
| **Prevention:** Use `umask` in a subshell or the `install` command (if portable) to ensure files are created with restrictive permissions from the start. Example: `(umask 077; echo secret > file)` |
There was a problem hiding this comment.
Fix markdownlint violations flagged by the Lint Documentation check.
The linter reports several failures:
- MD041: First line should be a top-level heading (
#), not##. - MD022: Heading must be surrounded by blank lines.
- MD013: Lines 2–4 exceed the 80-character limit — wrap long lines or restructure into shorter sentences.
Also, the date 2024-05-22 looks incorrect given the PR was created on 2026-02-06.
Proposed structure (content abbreviated)
-## 2024-05-22 - TOCTOU Race Condition in File Permissions
-**Vulnerability:** SSH private keys were written to disk with default permissions and then restricted with `chmod` immediately after. This creates a Time-of-Check-Time-of-Use (TOCTOU) race condition window where the file is world-readable (depending on system umask).
-**Learning:** `chmod` is not atomic with file creation. Relying on it for sensitive files leaves a security gap.
-**Prevention:** Use `umask` in a subshell or the `install` command (if portable) to ensure files are created with restrictive permissions from the start. Example: `(umask 077; echo secret > file)`
+# Sentinel Security Journal
+
+## 2026-02-06 - TOCTOU Race Condition in File Permissions
+
+**Vulnerability:** SSH private keys were written to disk with default
+permissions and then restricted with `chmod` immediately after. This
+creates a TOCTOU race condition window where the file is
+world-readable (depending on system umask).
+
+**Learning:** `chmod` is not atomic with file creation. Relying on it
+for sensitive files leaves a security gap.
+
+**Prevention:** Use `umask` in a subshell or the `install` command
+(if portable) to ensure files are created with restrictive permissions
+from the start. Example: `(umask 077; echo secret > file)`📝 Committable suggestion
‼️ IMPORTANT
Carefully review the code before committing. Ensure that it accurately replaces the highlighted code, contains no missing lines, and has no issues with indentation. Thoroughly test & benchmark the code to ensure it meets the requirements.
| ## 2024-05-22 - TOCTOU Race Condition in File Permissions | |
| **Vulnerability:** SSH private keys were written to disk with default permissions and then restricted with `chmod` immediately after. This creates a Time-of-Check-Time-of-Use (TOCTOU) race condition window where the file is world-readable (depending on system umask). | |
| **Learning:** `chmod` is not atomic with file creation. Relying on it for sensitive files leaves a security gap. | |
| **Prevention:** Use `umask` in a subshell or the `install` command (if portable) to ensure files are created with restrictive permissions from the start. Example: `(umask 077; echo secret > file)` | |
| # Sentinel Security Journal | |
| ## 2026-02-06 - TOCTOU Race Condition in File Permissions | |
| **Vulnerability:** SSH private keys were written to disk with default | |
| permissions and then restricted with `chmod` immediately after. This | |
| creates a TOCTOU race condition window where the file is | |
| world-readable (depending on system umask). | |
| **Learning:** `chmod` is not atomic with file creation. Relying on it | |
| for sensitive files leaves a security gap. | |
| **Prevention:** Use `umask` in a subshell or the `install` command | |
| (if portable) to ensure files are created with restrictive permissions | |
| from the start. Example: `(umask 077; echo secret > file)` |
🧰 Tools
🪛 GitHub Check: Lint Documentation
[failure] 4-4: Line length
.jules/sentinel.md:4:81 MD013/line-length Line length [Expected: 80; Actual: 196] https://github.com/DavidAnson/markdownlint/blob/v0.34.0/doc/md013.md
[failure] 3-3: Line length
.jules/sentinel.md:3:81 MD013/line-length Line length [Expected: 80; Actual: 112] https://github.com/DavidAnson/markdownlint/blob/v0.34.0/doc/md013.md
[failure] 2-2: Line length
.jules/sentinel.md:2:81 MD013/line-length Line length [Expected: 80; Actual: 267] https://github.com/DavidAnson/markdownlint/blob/v0.34.0/doc/md013.md
[failure] 1-1: First line in a file should be a top-level heading
.jules/sentinel.md:1 MD041/first-line-heading/first-line-h1 First line in a file should be a top-level heading [Context: "## 2024-05-22 - TOCTOU Race Co..."] https://github.com/DavidAnson/markdownlint/blob/v0.34.0/doc/md041.md
[failure] 1-1: Headings should be surrounded by blank lines
.jules/sentinel.md:1 MD022/blanks-around-headings Headings should be surrounded by blank lines [Expected: 1; Actual: 0; Below] [Context: "## 2024-05-22 - TOCTOU Race Condition in File Permissions"] https://github.com/DavidAnson/markdownlint/blob/v0.34.0/doc/md022.md
🤖 Prompt for AI Agents
In @.jules/sentinel.md around lines 1 - 4, Change the first line to a top-level
heading (use a single leading '#') and ensure there is a blank line before and
after that heading to satisfy MD041 and MD022; break the long lines in the body
(lines mentioning the TOCTOU race, chmod, umask example) into sentences under 80
characters to fix MD013; update the date string "2024-05-22" to "2026-02-06" and
keep the heading text "TOCTOU Race Condition in File Permissions" intact so
reviewers can locate the entry.
1 participant
🛡️ Sentinel: [CRITICAL] Fix TOCTOU race condition in SSH key creation
🚨 Severity: CRITICAL
💡 Vulnerability: SSH private keys were written to disk with default permissions and then restricted with
chmodimmediately after. This creates a Time-of-Check-Time-of-Use (TOCTOU) race condition window where the file is world-readable.🎯 Impact: Sensitive private keys could be read by other users on the system if they monitor directory changes during the creation window.
🔧 Fix: Wrapped file creation in a subshell with
umask 077to ensure restrictive permissions are applied at creation time.✅ Verification: Verified script syntax with
bash -nand visual inspection of logic.PR created automatically by Jules for task 9668484018734191816 started by @kidchenko
Summary by CodeRabbit
Bug Fixes
Documentation