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Fencer Intel
high 8.8

CVE-2026-53951 json

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

published 4mo ago modified 17d ago also EUVD-2026-42309 , GHSA-9gmc-jqmh-3rvm , PYSEC-2026-3818

cvss
8.8
version 4.0, CNA
cisa kev
Not listed
not in CISA's catalog
epss
0.3%
EPSS 0.3% chance of exploitation in 30 days, percentile 18%
public exploits
0
none known

Summary

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.

Patch / fix

  • pypi copier 9.15.2
  • copier-org copier 9.15.2

Is CVE-2026-53951 in your dependencies? Fencer shows which of your repositories are affected as soon as an advisory lands, and opens the fix as a pull request.

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CISA SSVC

Exploitation
Proof of concept is public
Automatable
no
Technical impact
total
Assessed
4mo ago

Exploit prediction

0.26% chance of exploitation in the next 30 days, higher than 18% of all scored CVEs (FIRST EPSS, 2026-11-11).

Exploits and checks

  • github.com https://github.com/copier-org/copier/security/advisories/GHSA-9gmc-jqmh-3rvm referenced PoC, tagged "exploit" by a source, unverified

CVSS scores

assigned by version score vector
CNA 4.0 8.8 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
CNA 4.0 8.8 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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

Affected packages

ecosystem package affected fixed in
pypi copier
>=9.5.0, <9.15.2
9.15.2

Affected products

  • copier-org copier

Classification

Weakness
  • CWE-22 Improper Limitation of a Pathname to a Restricted Directory
  • CWE-94 Improper Control of Generation of Code

References

Sources

Fencer Intel keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.

CPEs and fixed versions use Anchore's correction of NVD's data: Added CPE configurations because not yet analyzed by NVD.