# CVE-2026-103636: Apache DataSketches C++ out-of-bounds read in VarOpt union deserialization

Source: https://junglewise.ai/threats/cve-2026-103636-out-of-bounds-read-in-the-varopt-union-deserialization-of-apache  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-10-10. Updated: 2026-10-16  
License: CC BY 4.0 with a link back

CVE-2026-103636 · Severity: info · Published 2026-10-10

Apache DataSketches C++ is a library for efficient data analysis and sketch algorithms. The VarOpt union deserialization function has an out-of-bounds read vulnerability that can crash applications or expose adjacent memory contents when processing truncated serialized data from untrusted sources.

Technologies: [Apache DataSketches C++](https://junglewise.ai/threats/technologies/datasketches-c). Vendors: [Apache](https://junglewise.ai/threats/vendors/apache).

## Quick answers

**Is CVE-2026-103636 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-103636 being exploited in the wild.

**How severe is CVE-2026-103636?** Informational severity, published 10 October 2026.

**Which products and versions are affected?** Apache DataSketches C++ 2.0.0-incubating through 5.2.0.

**Is there a fix for CVE-2026-103636?** Yes: fixed in 5.3.0. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-103636 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-103636?** Check whether you run Apache DataSketches C++ at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- Fixed in 5.3.0

## Technical details

The var_opt_union::deserialize() function reads a 32-byte preamble without confirming sufficient bytes are available (only 8 bytes were checked), allowing reads up to 24 bytes past the input buffer. Additionally, unsigned integer underflow when computing remaining size for the embedded sketch bypasses subsequent size validation, enabling further out-of-bounds reads. Affected applications must deserialize VarOpt unions from untrusted sources to be vulnerable.

## Affected products

- Apache DataSketches C++ 2.0.0-incubating through 5.2.0

## Timeline

- 2026-10-10: disclosed
- 2026-10-08: patched

## References

- [https://github.com/](https://github.com/)
- [https://github.com/apache/datasketches-cpp/releases/tag/5.3.0](https://github.com/apache/datasketches-cpp/releases/tag/5.3.0)

## Related threats

- [Apache DataSketches C++ out-of-bounds read and write in Count-Min deserialization](https://junglewise.ai/threats/cve-2026-103634-out-of-bounds-read-and-write-in-the-count-min-sketch)
- [Apache DataSketches C++ out-of-bounds read in Theta sketch deserialization](https://junglewise.ai/threats/cve-2026-103635-out-of-bounds-read-in-the-compact-theta-sketch-deserialization)
- [Apache DataSketches C++ out-of-bounds read/write in CPC deserialization](https://junglewise.ai/threats/cve-2026-103513-out-of-bounds-read-and-write-in-the-cpc-sketch-deserialization)
- [Apache DataSketches C++ heap buffer overflow in HLL deserialization](https://junglewise.ai/threats/cve-2026-103501-heap-buffer-overflow-in-the-hll-sketch-deserialization-of-apache)