GHSA-9GMC-JQMH-3RVM

Vulnerability from github – Published: 2026-08-19 19:16 – Updated: 2026-08-19 19:16
VLAI
Summary
Copier has a trust-prefix bypass via path traversal that runs tasks unprompted
Details

Copier: trust-prefix bypass via path traversal runs tasks unprompted

Summary

In copier >= 9.5.0, <= 9.15.1, the trust setting's prefix match (copier/_settings.py) compares the template URL against a trusted prefix with a raw str.startswith and no path normalization, while the URL is normalized when the template is actually fetched (Path.resolve() for local paths; libcurl dot-segment removal for https). A template reference that textually starts with a trusted prefix but contains .. (e.g. https://github.com/trusted-org/../attacker-org/repo.git) is therefore granted trust yet resolves to a different, attacker-controlled template, whose tasks / migrations / jinja_extensions then run without the --trust prompt — arbitrary command execution. Likely CWE-22 (Improper Limitation of a Pathname) in the trust check leading to CWE-94 (code execution).

Details

trust lets users mark template locations as trusted so copier skips the unsafe-feature gate. A trailing / makes an entry a prefix match (docs/settings.md: "Locations ending with / will be matched as prefixes, trusting all templates from that location").

copier/_settings.py:141-146 (tag v9.15.1):

    return any(
        repository.startswith(_normalize(t))
        if t.endswith("/")
        else repository == _normalize(t)
        for t in trust
    )

_normalize only expands ~; it does not touch .. or collapse segments — copier/_settings.py:149-152 (tag v9.15.1):

def _normalize(url: str) -> str:
    if url.startswith("~"):  # Only expand on str to avoid messing with URLs
        url = expanduser(url)  # noqa: PTH111
    return url

This decision gates code execution — copier/_main.py:293 (tag v9.15.1):

        if self.unsafe or is_trusted_repository(self.settings.trust, self.template.url):
            return  # skip the unsafe-feature check entirely

The chain: the trust comparison sees the raw URL, so "https://github.com/safeorg/../evilorg/t.git".startswith("https://github.com/safeorg/") is True; but the value copier hands to git/pathlib is normalized, so the template actually loaded is evilorg/t (a different, attacker-owned org). Trust is granted to a location the user never trusted, and _check_unsafe returns early, so the malicious template's tasks execute with no prompt.

This is most acute on copier update, which reads _src_path from the project's .copier-answers.yml (copier/_subproject.py) — i.e. an attacker who hands you a project controls the URL that the trust check is applied to.

In-repo asymmetry that confirms the omission: copier consistently resolves paths everywhere else it makes a security decision — Path.resolve() plus is_relative_to(...) guards in _render_template, template_copy_root, and _external_data — but not in the trust comparison.

PoC

Self-contained standalone script; runs against a clean, pinned PyPI install via the real copier CLI only. Static by default (copier copy --pretend reaches the trust decision but does not execute tasks); --prove-exec is an opt-in supplementary run that fires an inert marker (echo + touch). The full poc.py accompanies this report.

Build and run:

python -m venv venv && . venv/bin/activate
pip install "copier==9.15.1"
python poc.py                # static proof (default)
python poc.py --prove-exec   # also fire the inert marker

Observed output (copier 9.15.1):

== version proof ==
  copier == 9.15.1
  module : .../site-packages/copier/__init__.py

== inputs ==
  trusted prefix (settings.yml): /tmp/copier_trust_poc_XXXX/trusted_templates/
  control src (canonical)      : /tmp/copier_trust_poc_XXXX/attacker/evil_template
  exploit src (traversal)      : /tmp/copier_trust_poc_XXXX/trusted_templates/../attacker/evil_template
  both resolve to the SAME dir : True
  exploit startswith trusted/  : True
  minimal delta                : exploit = '/tmp/copier_trust_poc_XXXX/trusted_templates/..' + '/attacker/evil_template'

== static proof (copier copy --pretend; payload NOT executed) ==
  control (canonical, untrusted): exit=4  -> BLOCKED (UnsafeTemplateError)
  exploit (trusted-prefix /..)  : exit=0  -> TRUSTED, task reached
  marker on disk after --pretend: False (expected False: --pretend does not run tasks)

  --- copier's own output for the exploit (note the task it WOULD run) ---
  | Copying from template version None
  |     create  hello.txt
  |  > Running task 1 of 1: echo COPIER-TRUST-BYPASS-RCE-MARKER && touch COPIER_RCE_PROOF

== VERDICT ==
  BYPASS CONFIRMED: identical template is refused by canonical path
  (exit 4) yet granted trust via '<trusted>/..' traversal (exit 0),
  so its tasks run with no --trust prompt.

== --prove-exec: running the exploit for real (inert marker) ==
  | COPIER-TRUST-BYPASS-RCE-MARKER
  |  > Running task 1 of 1: echo COPIER-TRUST-BYPASS-RCE-MARKER && touch COPIER_RCE_PROOF
  exit=0  marker file 'COPIER_RCE_PROOF' created: True
  -> ARBITRARY COMMAND EXECUTED via a 'trusted' template, no --trust

The exploit is the same template as the control plus the minimal delta <trusted_prefix>/... Deterministic: same input → same result. The PoC uses a local trusted prefix for a self-contained, network-free run; the https case is identical because git normalizes .. before the request — e.g. git ls-remote "https://github.com/copier-org/../pallets/flask.git" emits warning: redirecting to https://github.com/pallets/flask.git/ and returns pallets/flask's refs, a different org than the trusted copier-org/.

Impact

A user who has configured a trusted prefix (a trailing-/ entry in trust, a documented feature) no longer gets the unsafe-feature prompt for a template that merely appears to live under that prefix. Any party who can influence the template URL — most realistically the author of a project the victim runs copier update on, since _src_path comes from that project's .copier-answers.yml — can host the real template under a different org/location reached via .. and have its tasks/migrations/ jinja_extensions execute arbitrary commands with no prompt. It fires on a default, modern git for both local paths and https.

Proposed severity: High, comparable to the project's prior unsafe-template advisory (GHSA-3xw7-v6cj-5q8h). Proposed CVSS v4 vector (maintainer to finalize; AT:P reflects the required trusted-prefix configuration): CVSS:4.0/AV:L/AC:L/AT:P/PR:N/UI:P/VC:H/VI:H/VA:H/SC:H/SI:H/SA:H. Conservative variant if you scope impact to the user account only (no host escape claim): drop SC/SI/SA to N.

Recommended fix

Normalize both sides before comparing, instead of raw startswith. For local entries, compare resolved absolute paths (Path(t).resolve() vs Path(repository).resolve()) using segment containment / is_relative_to, the pattern already used in _render_template and template_copy_root. For URL entries, parse the URL and reject or collapse .././empty path segments before the prefix test. As defense-in-depth, reject any _src_path read from an answers file that contains .. segments after the scheme/host, since legitimate template URLs never need them.

References

  • CWE-22 — https://cwe.mitre.org/data/definitions/22.html
  • CWE-94 — https://cwe.mitre.org/data/definitions/94.html
  • Affected source (tag v9.15.1): copier/_settings.py:141-146 (prefix match), copier/_settings.py:149-152 (_normalize), copier/_main.py:293 (trust gate).
  • Documented prefix behavior: docs/settings.md ("Locations ending with / will be matched as prefixes").
  • https .. normalization: libcurl removes dot segments by default (CURLOPT_PATH_AS_IS defaults to off) — https://curl.se/libcurl/c/CURLOPT_PATH_AS_IS.html
  • Novelty: distinct from copier's published advisories, which concern filesystem read/write traversal in rendered output; this is an authorization bypass in the trust setting's URL matching. The flawed match is identical between released v9.15.1 and current master HEAD, and unchanged since the trust-prefix feature was introduced in v9.5.0 (originally copier/settings.py, commit 71358ed; renamed to copier/_settings.py in the v9.12.0 refactor).
Severity
8.8 (High)
CVSS:4.0/AV:L/AC:L/AT:P/PR:N/UI:P/VC:H/VI:H/VA:H/SC:H/SI:H/SA:H
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{
  "affected": [
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 9.15.1"
      },
      "package": {
        "ecosystem": "PyPI",
        "name": "copier"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "9.5.0"
            },
            {
              "fixed": "9.15.2"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-53951"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-22",
      "CWE-94"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-08-19T19:16:54Z",
    "nvd_published_at": "2026-07-08T16:16:30Z",
    "severity": "HIGH"
  },
  "details": "# Copier: trust-prefix bypass via path traversal runs tasks unprompted\n\n### Summary\n\nIn copier `\u003e= 9.5.0, \u003c= 9.15.1`, the `trust` setting\u0027s prefix match\n(`copier/_settings.py`) compares the template URL against a trusted prefix with\na raw `str.startswith` and **no path normalization**, while the URL *is*\nnormalized when the template is actually fetched (`Path.resolve()` for local\npaths; libcurl dot-segment removal for `https`). A template reference that\ntextually starts with a trusted prefix but contains `..`\n(e.g. `https://github.com/trusted-org/../attacker-org/repo.git`) is therefore\ngranted trust yet resolves to a different, attacker-controlled template, whose\n`tasks` / `migrations` / `jinja_extensions` then run **without the `--trust`\nprompt** \u2014 arbitrary command execution. Likely **CWE-22 (Improper Limitation of\na Pathname)** in the trust check leading to **CWE-94 (code execution)**.\n\n### Details\n\n`trust` lets users mark template locations as trusted so copier skips the\nunsafe-feature gate. A trailing `/` makes an entry a **prefix** match\n(`docs/settings.md`: *\"Locations ending with `/` will be matched as prefixes,\ntrusting all templates from that location\"*).\n\n`copier/_settings.py:141-146` (tag `v9.15.1`):\n\n```python\n    return any(\n        repository.startswith(_normalize(t))\n        if t.endswith(\"/\")\n        else repository == _normalize(t)\n        for t in trust\n    )\n```\n\n`_normalize` only expands `~`; it does **not** touch `..` or collapse segments \u2014\n`copier/_settings.py:149-152` (tag `v9.15.1`):\n\n```python\ndef _normalize(url: str) -\u003e str:\n    if url.startswith(\"~\"):  # Only expand on str to avoid messing with URLs\n        url = expanduser(url)  # noqa: PTH111\n    return url\n```\n\nThis decision gates code execution \u2014 `copier/_main.py:293` (tag `v9.15.1`):\n\n```python\n        if self.unsafe or is_trusted_repository(self.settings.trust, self.template.url):\n            return  # skip the unsafe-feature check entirely\n```\n\nThe chain: the trust comparison sees the **raw** URL, so\n`\"https://github.com/safeorg/../evilorg/t.git\".startswith(\"https://github.com/safeorg/\")`\nis `True`; but the value copier hands to `git`/`pathlib` is **normalized**, so\nthe template actually loaded is `evilorg/t` (a different, attacker-owned org).\nTrust is granted to a location the user never trusted, and `_check_unsafe`\nreturns early, so the malicious template\u0027s tasks execute with no prompt.\n\nThis is most acute on `copier update`, which reads `_src_path` from the\nproject\u0027s `.copier-answers.yml` (`copier/_subproject.py`) \u2014 i.e. an attacker who\nhands you a project controls the URL that the trust check is applied to.\n\nIn-repo asymmetry that confirms the omission: copier consistently resolves\npaths *everywhere else* it makes a security decision \u2014 `Path.resolve()` plus\n`is_relative_to(...)` guards in `_render_template`, `template_copy_root`, and\n`_external_data` \u2014 but not in the trust comparison.\n\n### PoC\n\nSelf-contained standalone script; runs against a clean, pinned PyPI install via\nthe real `copier` CLI only. **Static by default** (`copier copy --pretend`\nreaches the trust decision but does not execute tasks); `--prove-exec` is an\nopt-in supplementary run that fires an inert marker (`echo` + `touch`). The full\n`poc.py` accompanies this report.\n\nBuild and run:\n\n```bash\npython -m venv venv \u0026\u0026 . venv/bin/activate\npip install \"copier==9.15.1\"\npython poc.py                # static proof (default)\npython poc.py --prove-exec   # also fire the inert marker\n```\n\nObserved output (`copier 9.15.1`):\n\n```\n== version proof ==\n  copier == 9.15.1\n  module : .../site-packages/copier/__init__.py\n\n== inputs ==\n  trusted prefix (settings.yml): /tmp/copier_trust_poc_XXXX/trusted_templates/\n  control src (canonical)      : /tmp/copier_trust_poc_XXXX/attacker/evil_template\n  exploit src (traversal)      : /tmp/copier_trust_poc_XXXX/trusted_templates/../attacker/evil_template\n  both resolve to the SAME dir : True\n  exploit startswith trusted/  : True\n  minimal delta                : exploit = \u0027/tmp/copier_trust_poc_XXXX/trusted_templates/..\u0027 + \u0027/attacker/evil_template\u0027\n\n== static proof (copier copy --pretend; payload NOT executed) ==\n  control (canonical, untrusted): exit=4  -\u003e BLOCKED (UnsafeTemplateError)\n  exploit (trusted-prefix /..)  : exit=0  -\u003e TRUSTED, task reached\n  marker on disk after --pretend: False (expected False: --pretend does not run tasks)\n\n  --- copier\u0027s own output for the exploit (note the task it WOULD run) ---\n  | Copying from template version None\n  |     create  hello.txt\n  |  \u003e Running task 1 of 1: echo COPIER-TRUST-BYPASS-RCE-MARKER \u0026\u0026 touch COPIER_RCE_PROOF\n\n== VERDICT ==\n  BYPASS CONFIRMED: identical template is refused by canonical path\n  (exit 4) yet granted trust via \u0027\u003ctrusted\u003e/..\u0027 traversal (exit 0),\n  so its tasks run with no --trust prompt.\n\n== --prove-exec: running the exploit for real (inert marker) ==\n  | COPIER-TRUST-BYPASS-RCE-MARKER\n  |  \u003e Running task 1 of 1: echo COPIER-TRUST-BYPASS-RCE-MARKER \u0026\u0026 touch COPIER_RCE_PROOF\n  exit=0  marker file \u0027COPIER_RCE_PROOF\u0027 created: True\n  -\u003e ARBITRARY COMMAND EXECUTED via a \u0027trusted\u0027 template, no --trust\n```\n\nThe exploit is the same template as the control plus the minimal delta\n`\u003ctrusted_prefix\u003e/..`. Deterministic: same input \u2192 same result. The PoC uses a\nlocal trusted prefix for a self-contained, network-free run; the `https` case is\nidentical because git normalizes `..` before the request \u2014 e.g.\n`git ls-remote \"https://github.com/copier-org/../pallets/flask.git\"` emits\n`warning: redirecting to https://github.com/pallets/flask.git/` and returns\n`pallets/flask`\u0027s refs, a different org than the trusted `copier-org/`.\n\n### Impact\n\nA user who has configured a trusted **prefix** (a trailing-`/` entry in\n`trust`, a documented feature) no longer gets the unsafe-feature prompt for a\ntemplate that merely *appears* to live under that prefix. Any party who can\ninfluence the template URL \u2014 most realistically the author of a project the\nvictim runs `copier update` on, since `_src_path` comes from that project\u0027s\n`.copier-answers.yml` \u2014 can host the real template under a different\norg/location reached via `..` and have its `tasks`/`migrations`/\n`jinja_extensions` execute arbitrary commands with no prompt. It fires on a\ndefault, modern git for both local paths and `https`.\n\nProposed severity: **High**, comparable to the project\u0027s prior unsafe-template\nadvisory (GHSA-3xw7-v6cj-5q8h). Proposed CVSS v4 vector (maintainer to finalize;\n`AT:P` reflects the required trusted-prefix configuration):\n`CVSS:4.0/AV:L/AC:L/AT:P/PR:N/UI:P/VC:H/VI:H/VA:H/SC:H/SI:H/SA:H`. Conservative\nvariant if you scope impact to the user account only (no host escape claim):\ndrop `SC/SI/SA` to `N`.\n\n### Recommended fix\n\nNormalize **both** sides before comparing, instead of raw `startswith`. For\nlocal entries, compare resolved absolute paths (`Path(t).resolve()` vs\n`Path(repository).resolve()`) using segment containment / `is_relative_to`, the\npattern already used in `_render_template` and `template_copy_root`. For URL\nentries, parse the URL and reject or collapse `..`/`.`/empty path segments\nbefore the prefix test. As defense-in-depth, reject any `_src_path` read from an\nanswers file that contains `..` segments after the scheme/host, since\nlegitimate template URLs never need them.\n\n### References\n\n- CWE-22 \u2014 https://cwe.mitre.org/data/definitions/22.html\n- CWE-94 \u2014 https://cwe.mitre.org/data/definitions/94.html\n- Affected source (tag `v9.15.1`): `copier/_settings.py:141-146` (prefix match),\n  `copier/_settings.py:149-152` (`_normalize`), `copier/_main.py:293` (trust gate).\n- Documented prefix behavior: `docs/settings.md` (\"Locations ending with `/`\n  will be matched as prefixes\").\n- `https` `..` normalization: libcurl removes dot segments by default\n  (`CURLOPT_PATH_AS_IS` defaults to off) \u2014 https://curl.se/libcurl/c/CURLOPT_PATH_AS_IS.html\n- Novelty: distinct from copier\u0027s published advisories, which concern filesystem\n  read/write traversal in rendered output; this is an authorization bypass in\n  the `trust` setting\u0027s URL matching. The flawed match is identical between\n  released `v9.15.1` and current `master` HEAD, and unchanged since the\n  trust-prefix feature was introduced in `v9.5.0` (originally `copier/settings.py`,\n  commit `71358ed`; renamed to `copier/_settings.py` in the v9.12.0 refactor).",
  "id": "GHSA-9gmc-jqmh-3rvm",
  "modified": "2026-08-19T19:16:54Z",
  "published": "2026-08-19T19:16:54Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/copier-org/copier/security/advisories/GHSA-9gmc-jqmh-3rvm"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-53951"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/copier-org/copier"
    },
    {
      "type": "WEB",
      "url": "https://github.com/copier-org/copier/releases/tag/v9.15.2"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:4.0/AV:L/AC:L/AT:P/PR:N/UI:P/VC:H/VI:H/VA:H/SC:H/SI:H/SA:H",
      "type": "CVSS_V4"
    }
  ],
  "summary": "Copier has a trust-prefix bypass via path traversal that runs tasks unprompted"
}



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