Vulnerability in py7zr (CVE-2026-55206)
Summary
Vulnerability in py7zr (CVE-2026-55206). Risk of unauthorized operations or information disclosure. Mitigation: upgrade to 1.1.3 or later.
Original description
The description as published by the source.
Summary
PackInfo._read() uses an O(n^2) cumulative sum pattern where
numstreams is read directly from the archive header. A crafted .7z
archive with a large numstreams value causes excessive CPU consumption
during SevenZipFile.init() — no extraction is needed. A 50 KB
archive takes ~7 seconds of CPU time.
Details
The vulnerable code is in PackInfo._read() (archiveinfo.py):
self.packpositions = [sum(self.packsizes[:i]) for i in
range(self.numstreams + 1)]
numstreams is parsed from the archive header via read_uint64() and is
attacker-controlled. Each sum(self.packsizes[:i]) re-sums from the
beginning, producing O(n^2) total work. This runs during header
parsing in SevenZipFile.init(), before any extraction.
Suggested fix — replace with O(n) cumulative sum:
from itertools import accumulate
self.packpositions = [0] + list(accumulate(self.packsizes))
PoC
import py7zr
from py7zr.archiveinfo import write_uint64, PROPERTY
MAGIC = b'\x37\x7a\xbc\xaf\x27\x1c'
def encode_uint64(v):
buf = io.BytesIO()
write_uint64(buf, v)
return buf.getvalue()
def build_7z_with_streams(numstreams):
header = io.BytesIO()
header.write(PROPERTY.HEADER)
header.write(PROPERTY.MAIN_STREAMS_INFO)
header.write(PROPERTY.PACK_INFO)
header.write(encode_uint64(0))
header.write(encode_uint64(numstreams))
header.write(PROPERTY.SIZE)
for _ in range(…
Source: GitHub Security Advisories (Global) NVD GitHub Advisory
Auto-generated summary
Assembled by SecPulse from the source data (CVSS, CWE, affected versions) using templates — no AI involved. Check the original description and references below for exact details.
A vulnerability tracked as CVE-2026-55206 has been found in py7zr.
Risk of unauthorized operations or information disclosure.
What to do: upgrade py7zr to 1.1.3 or later.
If unsure, ask your IT team or search "py7zr CVE-2026-55206" on the vendor's site.
CVE-2026-55206 (py7zr) — CWE-407
Patched: 1.1.3 — apply immediately
Plan: 1) Audit SBOM/dependencies, 2) Stage→prod upgrade, 3) Add WAF/proxy monitoring on affected endpoints, 4) Hunt IOCs in logs.
Refs: see the GHSA / vendor advisory / patched release linked on this page.
A vulnerability (CWE-407) exists in py7zr.
Check the vendor advisory for the affected versions of py7zr.
Severity: ?. Risk of unauthorized operations or information disclosure.
Update to 1.1.3.
Until the patch is applied: disable the affected feature, apply WAF rules, or restrict access via network ACLs.
Search webserver/proxy logs for unusual request patterns matching this CVE's known IOCs. Run grep -r 'py7zr' . against your dependency files (package-lock.json, requirements.txt, go.sum) to find affected services.
Response actions
Concrete steps and command examples for SOC/SRE teams to execute in order
-
1Identify exposure identify
grep -r 'py7zr' . | grep -v node_modulesリポジトリと本番環境の依存ファイル (package-lock.json / requirements.txt / go.sum / Gemfile.lock 等) で
py7zrを grep し、稼働しているサービス・バージョンを把握する。 -
2Apply patch patch
Upgrade py7zr to 1.1.3ステージング環境で 1.1.3 に上げて回帰テスト → 本番反映。回帰テストはアプリの主要ハッピーパスを含めること。
-
3Post-deployment verification verify
Confirm patched version is live in productionパッチ適用後、ステージングで PoC または同等の悪用パターンを再現して脆弱性が閉じたことを確認。本番ではアクセスログ・WAF でアラート再発が無いか継続監視。
Affected packages
Fix status by distribution
Fixed packages published by each distribution for this CVE
- Debian DEBIAN-CVE-2026-55206 py7zr
- Ubuntu UBUNTU-CVE-2026-55206 py7zr
References
- package github.com
- web github.com
- web github.com
- web github.com
- web security-advisories@github.com