falco: add rules for git hook tampering and unexpected pack-service children

Written after the 2026-08-10/11 gitea internal-API log-poisoning attack
that planted a malicious uploadpack.packObjectsHook backdoor. Catches
both the planting (unexpected exec from a hooks/ path) and the firing
(git-upload-pack/git-receive-pack spawning anything but its own
pack-objects binary), independent of how the hook config got written.

falco/rules/unexpected-child-of-git.yaml

@@ -0,0 +1,28 @@
+# Custom rule: catch git-upload-pack/git-receive-pack forking anything other
+# than git's own pack-objects binary. Added after the 2026-08-10/11 gitea
+# log-poisoning attack, which planted a malicious `uploadpack.packObjectsHook`
+# global git config entry - a documented git RCE primitive where upload-pack
+# forks an attacker-chosen executable instead of git-pack-objects on every
+# fetch/clone. This is the moment the backdoor actually fires, independent of
+# how the hook config got planted, so it's a strong last-line catch even if
+# git-hook-tamper.yaml's file-write detection is bypassed some other way.
+
+- list: git_pack_children
+  items: [git-pack-objects, git, git-remote-https, git-remote-http]
+
+- rule: Unexpected child process of git pack service
+  desc: >
+    git-upload-pack or git-receive-pack spawned a process that isn't git's own
+    pack-objects binary. Normal fetch/push never does this - it's the exact
+    behavior of an abused uploadpack.packObjectsHook / pre-receive-style RCE.
+  condition: >
+    spawned_process
+    and container.name = "gitea"
+    and proc.pname in (git-upload-pack, git-receive-pack)
+    and not proc.name in (git_pack_children)
+  output: >
+    git pack service spawned unexpected child process
+    (user=%user.name command=%proc.cmdline parent=%proc.pname
+    container=%container.name pid=%proc.pid)
+  priority: CRITICAL
+  tags: [git, execution, mitre_execution]