🛡️ Sentinel: [HIGH] Fix TOCTOU vulnerability in SSH key creation#31
🛡️ Sentinel: [HIGH] Fix TOCTOU vulnerability in SSH key creation#31
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Wrapped private key creation in a subshell with `umask 077` to ensure the file is created with 0600 permissions from the start, preventing a brief window where it might be world-readable. Added verification script `tests/reproduce_issue.sh`. Co-authored-by: kidchenko <5432753+kidchenko@users.noreply.github.com>
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📝 WalkthroughWalkthroughThis PR addresses a TOCTOU vulnerability in SSH private key creation. A documentation file explains the security issue, a test script verifies the fix, and the setup script is modified to create SSH keys with restrictive umask permissions to prevent temporary world-readable exposure. Changes
Estimated code review effort🎯 2 (Simple) | ⏱️ ~12 minutes Poem
🚥 Pre-merge checks | ✅ 4✅ Passed checks (4 passed)
✏️ Tip: You can configure your own custom pre-merge checks in the settings. ✨ Finishing touches
🧪 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: Change the heading to a top-level heading and add the
required blank line after it, and wrap the long lines to <=80 characters to
satisfy MD041, MD022 and MD013; specifically update the heading "2024-05-22 -
SSH Key TOCTOU Vulnerability" to use a leading '#' and insert a blank line below
it, then reflow the following three lines ("Vulnerability: ...", "Learning:
...", "Prevention: ...") so each line is within 80 characters (or split into
multiple short lines) while preserving the original wording (e.g., break long
sentences into separate lines or list items).
In `@tests/reproduce_issue.sh`:
- Around line 46-51: The test currently only verifies existence of
"$HOME/.ssh/id_ed25519"; add a permission assertion immediately after that check
to confirm the private key's mode is 0600. Use a portable permission check (for
example, call stat -c %a "$HOME/.ssh/id_ed25519" and compare the result to "600"
or use another reliable method on the platform) and if the mode is not 600,
print a clear error like "Incorrect permissions on id_ed25519" and exit 1;
update the success message to reflect both existence and correct permissions.
| ## 2024-05-22 - SSH Key TOCTOU Vulnerability | ||
| **Vulnerability:** SSH private keys were created with default permissions (likely 644/664) and then chmod'ed to 600, creating a Time-of-Check Time-of-Use (TOCTOU) race condition where the key was briefly world-readable. | ||
| **Learning:** Shell redirection (`>`) creates files with default umask permissions before any subsequent `chmod` command can run. | ||
| **Prevention:** Use `(umask 077 && command > file)` in a subshell to ensure the file is created with restrictive permissions (600) from the very beginning. |
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Fix markdown lint violations flagged by CI.
The Lint Documentation check is failing with multiple issues:
- MD041: First line should be a top-level heading (
#instead of##). - MD022: Heading on line 1 needs a blank line below it.
- MD013: Lines 2–4 exceed the 80-character line length limit.
Proposed fix
-## 2024-05-22 - SSH Key TOCTOU Vulnerability
-**Vulnerability:** SSH private keys were created with default permissions (likely 644/664) and then chmod'ed to 600, creating a Time-of-Check Time-of-Use (TOCTOU) race condition where the key was briefly world-readable.
-**Learning:** Shell redirection (`>`) creates files with default umask permissions before any subsequent `chmod` command can run.
-**Prevention:** Use `(umask 077 && command > file)` in a subshell to ensure the file is created with restrictive permissions (600) from the very beginning.
+# Sentinel Notes
+
+## 2024-05-22 - SSH Key TOCTOU Vulnerability
+
+**Vulnerability:** SSH private keys were created with default
+permissions (likely 644/664) and then chmod'ed to 600, creating a
+TOCTOU race condition where the key was briefly world-readable.
+
+**Learning:** Shell redirection (`>`) creates files with default
+umask permissions before any subsequent `chmod` command can run.
+
+**Prevention:** Use `(umask 077 && command > file)` in a subshell
+to ensure the file is created with restrictive permissions (600)
+from the very beginning.📝 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 - SSH Key TOCTOU Vulnerability | |
| **Vulnerability:** SSH private keys were created with default permissions (likely 644/664) and then chmod'ed to 600, creating a Time-of-Check Time-of-Use (TOCTOU) race condition where the key was briefly world-readable. | |
| **Learning:** Shell redirection (`>`) creates files with default umask permissions before any subsequent `chmod` command can run. | |
| **Prevention:** Use `(umask 077 && command > file)` in a subshell to ensure the file is created with restrictive permissions (600) from the very beginning. | |
| # Sentinel Notes | |
| ## 2024-05-22 - SSH Key TOCTOU Vulnerability | |
| **Vulnerability:** SSH private keys were created with default | |
| permissions (likely 644/664) and then chmod'ed to 600, creating a | |
| TOCTOU race condition where the key was briefly world-readable. | |
| **Learning:** Shell redirection (`>`) creates files with default | |
| umask permissions before any subsequent `chmod` command can run. | |
| **Prevention:** Use `(umask 077 && command > file)` in a subshell | |
| to ensure the file is created with restrictive permissions (600) | |
| from the very beginning. |
🧰 Tools
🪛 GitHub Check: Lint Documentation
[failure] 4-4: Line length
.jules/sentinel.md:4:81 MD013/line-length Line length [Expected: 80; Actual: 155] 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: 129] 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: 219] 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 - SSH Key TOCTOU..."] 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 - SSH Key TOCTOU Vulnerability"] 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 heading to a top-level
heading and add the required blank line after it, and wrap the long lines to
<=80 characters to satisfy MD041, MD022 and MD013; specifically update the
heading "2024-05-22 - SSH Key TOCTOU Vulnerability" to use a leading '#' and
insert a blank line below it, then reflow the following three lines
("Vulnerability: ...", "Learning: ...", "Prevention: ...") so each line is
within 80 characters (or split into multiple short lines) while preserving the
original wording (e.g., break long sentences into separate lines or list items).
| if [[ -f "$HOME/.ssh/id_ed25519" ]]; then | ||
| echo "Key restored successfully to $HOME/.ssh/id_ed25519" | ||
| else | ||
| echo "Key restore failed" | ||
| exit 1 | ||
| fi |
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Test doesn't verify file permissions — the core assertion for this fix is missing.
The entire PR is about ensuring the private key is created with 0600 permissions, but the test only checks file existence. Add a permission check to make this test meaningful for the TOCTOU fix.
Proposed fix
# Verify file exists in the fake home
if [[ -f "$HOME/.ssh/id_ed25519" ]]; then
echo "Key restored successfully to $HOME/.ssh/id_ed25519"
+ # Verify permissions are 600 (the whole point of the TOCTOU fix)
+ PERMS=$(stat -c '%a' "$HOME/.ssh/id_ed25519" 2>/dev/null || stat -f '%Lp' "$HOME/.ssh/id_ed25519")
+ if [[ "$PERMS" != "600" ]]; then
+ echo "FAIL: Expected permissions 600, got $PERMS"
+ exit 1
+ fi
+ echo "Permissions verified: $PERMS"
else
echo "Key restore failed"
exit 1
fi🤖 Prompt for AI Agents
In `@tests/reproduce_issue.sh` around lines 46 - 51, The test currently only
verifies existence of "$HOME/.ssh/id_ed25519"; add a permission assertion
immediately after that check to confirm the private key's mode is 0600. Use a
portable permission check (for example, call stat -c %a "$HOME/.ssh/id_ed25519"
and compare the result to "600" or use another reliable method on the platform)
and if the mode is not 600, print a clear error like "Incorrect permissions on
id_ed25519" and exit 1; update the success message to reflect both existence and
correct permissions.
1 participant
🚨 Severity: HIGH
💡 Vulnerability: SSH private keys were created with default permissions (644/664) and then chmod'ed to 600, creating a Time-of-Check Time-of-Use (TOCTOU) race condition where the key was briefly world-readable.
🎯 Impact: An attacker could potentially read the private key during the brief window between file creation and chmod.
🔧 Fix: Wrapped the file creation command in a subshell with
umask 077to ensure the file is created with 600 permissions from the start.✅ Verification: Added
tests/reproduce_issue.shto verify the key creation process works correctly (permissions verified by code inspection as the race is hard to test deterministically).PR created automatically by Jules for task 10259571738023374831 started by @kidchenko
Summary by CodeRabbit
Release Notes