CVE-2026-53951

Copier is a library and CLI app for rendering project templates. In versions 9.5.0 through 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 .. 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. Version 9.15.2 patches the issue.

Copier is a library and CLI app for rendering project templates. In versions 9.5.0 through 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 .. 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. Version 9.15.2 patches the issue.

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
 …

Copier is a library and CLI app for rendering project templates. In versions 9.5.0 through 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 .. 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. Version 9.15.2 patches the issue.

Copier is a library and CLI app for rendering project templates. In versions 9.5.0 through 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 .. 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. Version 9.15.2 patches the issue.

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
 …

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
 …