# CVE-2026-124959: nmslib hnswlib and Spotify Voyager buffer overflow in loadIndex

Source: https://junglewise.ai/threats/cve-2026-124959-the-loadindex-function-in-nmslib-hnswlib-and-spotify-voyager  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-11-09. Updated: 2026-11-09  
License: CC BY 4.0 with a link back

CVE-2026-124959 · Severity: info · Published 2026-11-09

nmslib hnswlib and Spotify Voyager are vector search libraries that load index files to perform fast approximate nearest-neighbor searches. The loadIndex function does not validate that the element count in a crafted index file matches the allocated buffer capacity, allowing an attacker to cause a buffer overflow and crash the application.

Vendors: [Spotify](https://junglewise.ai/threats/vendors/spotify).

## Quick answers

**Is CVE-2026-124959 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-124959 being exploited in the wild.

**How severe is CVE-2026-124959?** Informational severity, published 9 November 2026.

**Which products and versions are affected?** nmslib hnswlib; Spotify Voyager.

**Is there a fix for CVE-2026-124959?** No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.

**How is CVE-2026-124959 attacked?** It is attacked over the network or with local access, in a software library.

**Am I affected by CVE-2026-124959?** Check whether you run Nmslib Hnswlib or Spotify Voyager. 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.

## Technical details

The loadIndex function in both hnswlib and Voyager fails to validate that the element count in a serialized HNSW index does not exceed the stored capacity, resulting in a heap buffer overflow. An attacker can supply a malicious index file with an inflated element count to trigger out-of-bounds writes. The vulnerability requires an attacker to provide a crafted index file and the application to load it, but no authentication or special privileges are needed.

## Affected products

- nmslib hnswlib
- Spotify Voyager

## Timeline

- 2026-11-09: disclosed

## References

- [https://github.com/nmslib/hnswlib](https://github.com/nmslib/hnswlib)
- [https://github.com/spotify/voyager](https://github.com/spotify/voyager)