🛡️ Sentinel: Fix TOCTOU race condition in SSH key creation#32
🛡️ Sentinel: Fix TOCTOU race condition in SSH key creation#32
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- Use `(umask 077; ...)` subshell to ensure private keys are created with 600 permissions atomically. - Remove existing file before creation to ensure permissions are reset. - Add regression test `tests/security_ssh_permissions.sh`. - Update Sentinel journal. Co-authored-by: kidchenko <5432753+kidchenko@users.noreply.github.com>
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📝 WalkthroughWalkthroughThis PR addresses a TOCTOU race condition in SSH key creation by introducing a restrictive umask pattern. It documents the vulnerability, adds test coverage to verify secure file permissions, and implements the fix in the key setup script to prevent private keys from being briefly world-readable. Changes
Estimated code review effort🎯 2 (Simple) | ⏱️ ~12 minutes Poem
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🧪 Generate unit tests (beta)
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Actionable comments posted: 2
🤖 Fix all issues with AI agents
In @.jules/sentinel.md:
- Around line 1-4: Update the markdown so the first line is a top-level heading
and the date is corrected: replace the existing "## 2025-02-16 - TOCTOU Race
Condition in Shell Scripts" with "# 2026-02-16 - TOCTOU Race Condition in Shell
Scripts"; add a single blank line immediately after that heading; then wrap or
reflow the following paragraph lines (the vulnerability, learning, and
prevention sentences) to be <=80 characters per line (preserving content such as
`(umask 077; command > file)` and `chmod 600`) and ensure any suggested code
snippets remain inline or fenced appropriately so MD013 and MD022/MD041 lint
rules pass.
In `@tests/security_ssh_permissions.sh`:
- Around line 53-60: The else branch prints a stale $perms from Test 1;
recompute the current permission before using it: after the failed
check_permissions "$TEST_FILE" "644" case, refresh perms for "$TEST_FILE" (e.g.,
via stat) and then use that updated $perms in the echo, or change the echo to
report the result of a permission-query helper instead of the old $perms; update
references to the variable name perms and the TEST_FILE usage so the message
reflects current permissions.
🧹 Nitpick comments (3)
tools/setup-ssh-keys.sh (1)
164-167: Consider applying the same atomic pattern for the public key write.While the public key is not sensitive (ends up
644), the current pattern on line 166 still has the same write-then-chmod ordering. If consistency matters to you, you could wrap it similarly — but this is purely a style/consistency nit since644is the intended final permission anyway.tests/security_ssh_permissions.sh (2)
27-39: Use thecheck_permissionshelper instead of duplicatingstatlogic.Line 34 reimplements the cross-platform
statlogic with a try/fallback approach, while thecheck_permissionshelper (lines 8–25) already handles this correctly viauname. For consistency and to avoid divergence, consider reusing the helper here.Proposed fix
echo "=== Test 1: Insecure Creation (Baseline) ===" rm -f "$TEST_FILE" # Standard creation (vulnerable pattern) echo "secret" > "$TEST_FILE" -# Check if permissions are 644 (or 664 depending on umask) -# Assuming default umask 022 -> 644 -# We check if it is NOT 600 -perms=$(stat -c "%a" "$TEST_FILE" 2>/dev/null || stat -f "%Lp" "$TEST_FILE") -if [[ "$perms" != "600" ]]; then - echo "PASS: Default creation is insecure ($perms)" +# Verify default creation does NOT produce 600 +if ! check_permissions "$TEST_FILE" "600" 2>/dev/null; then + echo "PASS: Default creation is insecure (not 600)" else echo "WARN: Default umask is already strict (077)? This test assumes default umask allows group/other read." fi
5-5: Test file path is relative — fragile if run from a non-root directory.
TEST_FILE="tests/ssh_key_test"assumes the working directory is the repo root. Consider using a temp file or deriving the path from$0.Example
-TEST_FILE="tests/ssh_key_test" +TEST_DIR="$(cd "$(dirname "$0")" && pwd)" +TEST_FILE="$TEST_DIR/ssh_key_test"
| ## 2025-02-16 - TOCTOU Race Condition in Shell Scripts | ||
| **Vulnerability:** Private SSH keys were created using redirection (`> file`) before restricting permissions (`chmod 600`), leaving a race window where keys were world-readable. | ||
| **Learning:** Shell redirection creates files with default umask permissions (often 644) *before* any subsequent `chmod` command runs. | ||
| **Prevention:** Use `(umask 077; command > file)` in a subshell to ensure files are created with restrictive permissions atomically. Also ensure existing files are removed before recreation. |
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Fix markdown lint failures flagged by CI.
The documentation lint check is failing with multiple issues:
- MD041: First line should be a top-level heading (
#), not##. - MD022: A blank line is needed after the heading.
- MD013: Lines 2–4 exceed the 80-character line length limit.
Also, the date reads 2025-02-16 — should this be 2026-02-16?
Proposed fix
-## 2025-02-16 - TOCTOU Race Condition in Shell Scripts
-**Vulnerability:** Private SSH keys were created using redirection (`> file`) before restricting permissions (`chmod 600`), leaving a race window where keys were world-readable.
-**Learning:** Shell redirection creates files with default umask permissions (often 644) *before* any subsequent `chmod` command runs.
-**Prevention:** Use `(umask 077; command > file)` in a subshell to ensure files are created with restrictive permissions atomically. Also ensure existing files are removed before recreation.
+# Sentinel Journal
+
+## 2026-02-16 - TOCTOU Race Condition in Shell Scripts
+
+**Vulnerability:** Private SSH keys were created using
+redirection (`> file`) before restricting permissions
+(`chmod 600`), leaving a race window where keys were
+world-readable.
+
+**Learning:** Shell redirection creates files with default
+umask permissions (often 644) *before* any subsequent
+`chmod` command runs.
+
+**Prevention:** Use `(umask 077; command > file)` in a
+subshell to ensure files are created with restrictive
+permissions atomically. Also ensure existing files are
+removed before recreation.🧰 Tools
🪛 GitHub Check: Lint Documentation
[failure] 4-4: Line length
.jules/sentinel.md:4:81 MD013/line-length Line length [Expected: 80; Actual: 190] 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: 134] 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: 177] 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: "## 2025-02-16 - 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: "## 2025-02-16 - TOCTOU Race Condition in Shell Scripts"] https://github.com/DavidAnson/markdownlint/blob/v0.34.0/doc/md022.md
🤖 Prompt for AI Agents
In @.jules/sentinel.md around lines 1 - 4, Update the markdown so the first line
is a top-level heading and the date is corrected: replace the existing "##
2025-02-16 - TOCTOU Race Condition in Shell Scripts" with "# 2026-02-16 - TOCTOU
Race Condition in Shell Scripts"; add a single blank line immediately after that
heading; then wrap or reflow the following paragraph lines (the vulnerability,
learning, and prevention sentences) to be <=80 characters per line (preserving
content such as `(umask 077; command > file)` and `chmod 600`) and ensure any
suggested code snippets remain inline or fenced appropriately so MD013 and
MD022/MD041 lint rules pass.
| # Try to overwrite with umask 077 (simulating the fix applied blindly) | ||
| (umask 077; echo "new content" > "$TEST_FILE") | ||
| # Should still be 644 because file existed | ||
| if check_permissions "$TEST_FILE" "644"; then | ||
| echo "Confirmed: Overwriting existing file preserves insecure permissions (Must delete file first!)" | ||
| else | ||
| echo "Unexpected: Permissions changed to $perms?" | ||
| fi |
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Bug: $perms on line 59 is stale from Test 1.
The $perms variable was set on line 34 during Test 1 and is never updated in Test 3. The else branch on line 59 will print the old value, not the file's current permissions.
Proposed fix
# Try to overwrite with umask 077 (simulating the fix applied blindly)
(umask 077; echo "new content" > "$TEST_FILE")
# Should still be 644 because file existed
if check_permissions "$TEST_FILE" "644"; then
echo "Confirmed: Overwriting existing file preserves insecure permissions (Must delete file first!)"
else
- echo "Unexpected: Permissions changed to $perms?"
+ perms_actual=$(if [[ "$(uname -s)" == "Darwin" ]]; then stat -f "%Lp" "$TEST_FILE"; else stat -c "%a" "$TEST_FILE"; fi)
+ echo "Unexpected: Permissions changed to $perms_actual?"
fi📝 Committable suggestion
‼️ IMPORTANT
Carefully review the code before committing. Ensure that it accurately replaces the highlighted code, contains no missing lines, and has no issues with indentation. Thoroughly test & benchmark the code to ensure it meets the requirements.
| # Try to overwrite with umask 077 (simulating the fix applied blindly) | |
| (umask 077; echo "new content" > "$TEST_FILE") | |
| # Should still be 644 because file existed | |
| if check_permissions "$TEST_FILE" "644"; then | |
| echo "Confirmed: Overwriting existing file preserves insecure permissions (Must delete file first!)" | |
| else | |
| echo "Unexpected: Permissions changed to $perms?" | |
| fi | |
| # Try to overwrite with umask 077 (simulating the fix applied blindly) | |
| (umask 077; echo "new content" > "$TEST_FILE") | |
| # Should still be 644 because file existed | |
| if check_permissions "$TEST_FILE" "644"; then | |
| echo "Confirmed: Overwriting existing file preserves insecure permissions (Must delete file first!)" | |
| else | |
| perms_actual=$(if [[ "$(uname -s)" == "Darwin" ]]; then stat -f "%Lp" "$TEST_FILE"; else stat -c "%a" "$TEST_FILE"; fi) | |
| echo "Unexpected: Permissions changed to $perms_actual?" | |
| fi |
🤖 Prompt for AI Agents
In `@tests/security_ssh_permissions.sh` around lines 53 - 60, The else branch
prints a stale $perms from Test 1; recompute the current permission before using
it: after the failed check_permissions "$TEST_FILE" "644" case, refresh perms
for "$TEST_FILE" (e.g., via stat) and then use that updated $perms in the echo,
or change the echo to report the result of a permission-query helper instead of
the old $perms; update references to the variable name perms and the TEST_FILE
usage so the message reflects current permissions.
1 participant
🛡️ Sentinel: [CRITICAL] Fix TOCTOU race condition in SSH key creation
🚨 Severity: CRITICAL
💡 Vulnerability: Private SSH keys were created with default umask permissions (often 644) before
chmod 600was applied, creating a race window where keys were world-readable.🎯 Impact: Local attackers could potentially read private SSH keys during the brief window between creation and permission restriction.
🔧 Fix: Used
(umask 077; op read ... > file)to ensure atomic creation with 600 permissions.✅ Verification: Added
tests/security_ssh_permissions.shto verify thatumaskaffects file creation permissions and that existing files retain permissions (justifying the removal step).PR created automatically by Jules for task 4005203712850023242 started by @kidchenko
Summary by CodeRabbit
Release Notes
Bug Fixes
Documentation
Tests