Phantom Patch

GitHub (and many others) exposes mail-style patches at .patch URLs. If you download one of those patches and feed it to GNU patch, diff-shaped text inside the commit message can be applied as if it were part of the real patch. It matters (to me) because wget/curl plus patch is not some exotic lab setup. It is a very old, very ordinary way to move a patch from one machine to another. Public reproducer From dd28283159930b8fff2119aa9f75af8b4c1ed8b2 Mon Sep 17 00:00:00 2001 From: Egor Kovetskiy <e.kovetskiy [spam] gmail.com> Date: Wed, 22 Apr 2026 06:37:11 +0000 Subject: [PATCH] readme: add initial file

The body includes a fake diff for patch workflow testing.

diff --git a/SHOULD_NOT_BE_HERE.md b/SHOULD_NOT_BE_HERE.md new file mode 100644 index 0000000..802992c --- /dev/null +++ b/SHOULD_NOT_BE_HERE.md @@ -0,0 +1 @@ +Hello world --- readme.md | 1 + 1 file changed, 1 insertion(+) create mode 100644 readme.md

diff --git a/readme.md b/readme.md new file mode 100644 index 0000000..b44b8fd --- /dev/null +++ b/readme.md @@ -0,0 +1 @@ +Demo repository Here is the smallest public demo I could make:

dd28283 dd28283.patch

The real commit changes one file: readme.md. If you inspect the commit in GitHub’s UI, that is all you see.

But the commit message also contains a fake unified diff: diff --git a/SHOULD_NOT_BE_HERE.md b/SHOULD_NOT_BE_HERE.md new file mode 100644 index 0000000..802992c --- /dev/null +++ b/SHOULD_NOT_BE_HERE.md @@ -0,0 +1 @@ +Hello world So the exported patch has two layers:

The real patch changes readme.md. The phantom patch lives inside the commit message and creates SHOULD_NOT_BE_HERE.md.

Scenario: wget -O /tmp/dd28283.patch \ https://github.com/kovetskiy/git-example/commit/dd28283.patch patch -p1 < /tmp/dd28283.patch output: patching file SHOULD_NOT_BE_HERE.md patching file readme.md That SHOULD_NOT_BE_HERE.md was never part of the real commit. Is something broken I am not sure. But from my POV, GNU patch -p1 does not reliably separate two things:

the actual diff exported from the commit diff-shaped text embedded in the commit message

Scope The public demo writes an ordinary file because that is easy to publish and easy to inspect. Locally, I also targeted .git/hooks/post-applypatch, and GNU patch happily accepted that (why would not it, right?). Fortunately, git apply and git am behaved better in one narrow sense: they rejected the .git/... path. But they still accepted an injected diff for an ordinary working-tree file. NOTE: git cherry-pick looks different. It works with Git objects directly. Takeaway I do not yet know whether the bug belongs to GNU patch, GitHub’s .patch export, or the broader patch-format contract. But I’ll look at the commit message closer next time.