---
id: CVE-2026-103636
title: >-
  Out-of-bounds read in the VarOpt union deserialization of Apache DataSketches
  C++ (repo: datasketches-cpp).


  var_opt_union::deserialize() read the 32-byte preamble of a non-empty union
  after checking that only 8 bytes were available, so …
summary: >-
  Out-of-bounds read in the VarOpt union deserialization of Apache DataSketches
  C++ (repo: datasketches-cpp).


  var_opt_union::deserialize() read the 32-byte preamble of a non-empty union
  after checking that only 8 bytes were available, so …
severity: high
cvss: 7.5
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H'
cwe:
  - CWE-125
  - CWE-191
vendor: Apache Software Foundation
product: apache/datasketches-cpp
affected:
  - apache/datasketches-cpp >= 2.0.0-incubating <= 5.2.0
published: '2026-10-10'
updated: '2026-10-16'
sourceUpdated: '2026-10-16T16:16:53.970'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-103636'
references:
  - url: 'https://github.com/apache/datasketches-cpp/releases/tag/5.3.0'
    label: security@apache.org
  - url: 'https://lists.apache.org/thread/znn94s463w7wy125khcywxsp82y2qkq5'
    label: security@apache.org
  - url: 'http://www.openwall.com/lists/oss-security/2026/10/10/6'
    label: af854a3a-2127-422b-91ae-364da2661108
tags:
  - nvd
  - cve.org
epss: 0.0038
epssPercentile: 0.31775
ssvc:
  exploitation: none
  automatable: 'yes'
  technicalImpact: partial
  timestamp: '2026-10-16T15:31:05.074830Z'
ingestedAt: '2026-10-10T11:31:21.932Z'
---

## Overview

Out-of-bounds read in the VarOpt union deserialization of Apache DataSketches C++ (repo: datasketches-cpp).

var_opt_union::deserialize() read the 32-byte preamble of a non-empty union after checking that only 8 bytes were available, so a truncated serialized union could cause a read of up to 24 bytes past the end of the input. For such inputs, the size remaining for the embedded sketch was also computed by an unsigned subtraction that could wrap around, so the embedded sketch's own size checks no longer limited reads to the input. The bytes read can become part of the deserialized union's state. This can cause a crash (denial of service) and could expose adjacent memory contents.

This issue affects Apache DataSketches C++: from 2.0.0-incubating before 5.3.0. Only applications that deserialize VarOpt unions from untrusted sources are affected.

Users are recommended to upgrade to version 5.3.0, which fixes this issue.

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.