My CI went red on a secret I'd never committed.
Not a close call, not a near-miss I'd half-forgotten about. gitleaks, the secret scanner, failed a merge request of mine on a private key that was not on my branch, was not in my change, and as far as I could tell had nothing to do with me at all. The job was adamant. I was baffled. Somewhere in between was a lesson about what a secret scanner actually scans.
Prove it isn't yours
First rule of being accused: don't get defensive, get evidence. The scanner pointed at a couple of commits carrying a test private key and a PEM block in a spec document. I genuinely didn't recognise them, but "I don't recognise it" is a feeling, not a fact, and feelings don't reopen a pipeline.
Git will tell you the truth if you ask it precisely. The question is: are these flagged commits actually part of my branch?
git merge-base --is-ancestor <flagged-sha> HEAD
That asks "is this commit an ancestor of where I am?". The answer came back no. The commits the scanner was choking on were not in my history. They weren't mine.
So whose were they? A bit of digging turned them up on a completely separate, still-unmerged branch, where someone (me, a few days earlier, on a different feature) had committed a throwaway test key and a PEM example in a spec, on purpose, as fixtures. Deliberate, harmless, and nowhere near the branch under review. And yet here they were, failing a merge request that had never touched them.
What gitleaks actually scans
Here's the bit I'd taken for granted. I assumed gitleaks detect scanned my change. It doesn't. With no further instruction it scans the whole history reachable in the checkout it's handed.
And the checkout it's handed is where the second half of the surprise lives. GitLab runners default to GIT_STRATEGY: fetch, which reuses the runner's working directory between jobs rather than cloning fresh every time. It's faster, and most of the time you never notice. But it means a shared runner accumulates refs from every branch it has ever built. My MR's job happened to run on a runner that had, at some point, built that other branch, so the fixtures were sitting right there in the local object store, fair game for a scanner walking the whole graph.
So gitleaks did exactly what I'd asked it to do, which was "scan everything", and "everything" turned out to be a great deal more than my change. It walked the lot and dutifully reported fixtures from a branch I wasn't even proposing to merge. The scanner wasn't wrong. My idea of what it was looking at was.
Unblock now, fix properly after
Two problems on two timescales. I needed the MR to merge today, and I needed this to never happen again. Those want different fixes.
The immediate one: tell gitleaks those specific fixtures are known and intended. They're test material, they're meant to be there, so they go in the allowlist:
paths = [
# ...
'''internal/cmd/keys/keys_test\.go''',
'''docs/development/specs/2026-06-08-keys-mint-command\.md''',
]
That unblocks the merge. It does nothing about the root cause, which is that the scan was looking at the wrong commits in the first place. Allowlisting individual false positives as they crop up is closing the stable door after the horse has bolted, one horse at a time, forever.
The real fix lives in the shared CI component, not in any one repo. Scope the scan to the commits the merge request actually introduces (cicd v0.10.3):
if [ -n "$CI_MERGE_REQUEST_DIFF_BASE_SHA" ]; then
gitleaks detect --source . --verbose --redact \
--log-opts="$CI_MERGE_REQUEST_DIFF_BASE_SHA..$CI_COMMIT_SHA"
else
gitleaks detect --source . --verbose --redact
fi
--log-opts is handed straight through to git log, so that range, base-of-the-MR to tip, is the exact set of commits the merge request adds and nothing else. On a merge request the scan now sees only what you're proposing to merge. Off a merge request (a plain branch or a tag pipeline) it falls back to the full scan, because there you genuinely do want the lot. The before-and-after in the job log tells the whole story: the entire accumulated history on one side, the handful of commits you actually wrote on the other.
Fixing the fix
There's a tax on touching CI shell, and I paid it. The change went into the go, rust and tofu security templates. Go and tofu went green. Rust failed.
The rust template had built its optional --config flag the clever way, inline:
... $([ -n "$CONFIG" ] && echo "--config $CONFIG")
As a command argument, that substitution is harmless: its exit status is thrown away, so nobody cares that the test inside returns false when there's no config. But when I rewrote the block and reached for the same pattern as an assignment, VAR=$(... && ...), it became a different animal. An assignment takes the exit status of the command substitution, and under set -e a non-zero status anywhere aborts the job. So on every run where the config was empty, which was most of them, the test returned false, the assignment inherited that false, and set -e killed the job stone dead. Same $(...), two completely different fates, decided entirely by whether it sat to the right of an = or got handed to a command as an argument. Go and tofu never used the assignment form, so only rust fell down the hole.
The fix was to stop being clever and write the boring if:
GITLEAKS_CONFIG=""
if [ -n "$CONFIG" ]; then
GITLEAKS_CONFIG="--config $CONFIG"
fi
Boring shell is good shell.
What a scanner is actually looking at
The whole mess came from one unspoken assumption: that a tool called to scan "my change" was scanning my change. It was scanning a checkout, and a checkout on a shared runner is a much bigger, messier thing than the diff in front of you. None of the pieces were broken. gitleaks did its job, GIT_STRATEGY: fetch did its job, my fixtures were exactly where I'd left them. They just added up to a red pipeline that had nothing to do with the code I was trying to ship. I'd spent a good chunk of the day proving my innocence to a scanner that was only ever doing as it was told... and the one thing I'd actually got wrong was being sure I already knew what it was looking at.
Originally published at phpboyscout.uk on 4 July 2026.
