CVE-2026-67429
Summary
image.download fetches a URL and writes the response to disk. It does not use the central path guard (validate_path_with_env_config, which confines writes to FLYTO_SANDBOX_DIR); instead it confines the output to output_dir, but output_dir is itself a caller parameter. Since the attacker sets both the target and the base it is checked against, the check is meaningless, and attacker-controlled bytes (the HTTP response) land at any absolute path the process can write.
Affected code
src/core/modules/atomic/image/download.py:
output_path = params.get('output_path')
output_dir = params.get('output_dir', '/tmp') # caller-controlled base
...
base_real = os.path.realpath(output_dir)
target_real = os.path.realpath(output_path)
if os.path.commonpath([base_real, target_real]) != base_real:
raise Exception('Invalid file path') # base is attacker-chosen, so always passes
...
content = await response.read() # attacker-hosted bytes
with open(target_real, 'wb') as f:
f.write(content)
commonpath is used correctly, but the base is caller-supplied, so setting output_dir='/' passes any target. file.write, by contrast, uses validate_path_with_env_config() and stays inside FLYTO_SANDBOX_DIR.
This is not isolated to image.download. Most other file-writing modules write to a caller output_path with no path check at all: image.convert, image.resize, image.crop, image.compress, image.rotate, image.watermark, image.qrcode_generate, document.excel_write, document.pdf_fill_form, document.word_to_pdf, document.pdf_to_word and browser.pagination. Their content is format-constrained (a valid PNG/XLSX/SVG/PDF) but the path is fully attacker-chosen; image.download is the strongest because the bytes are arbitrary.
Reproduction
Save as filewrite_poc.py, run with PYTHONPATH=src/src python filewrite_poc.py. It sets FLYTO_SANDBOX_DIR to a sandbox dir and writes to a sibling directory outside it.
#!/usr/bin/env python3
import asyncio
import os
import tempfile
import threading
from http.ser
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Exploitation outlook
- Low exploitation risk0.49% 30-day exploitation probability — currently an unlikely target, but scores change as exploit code circulates. Riskier than 40% of all EPSS-scored CVEs.
Advisory coverage (2)
External references
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