---
id: CVE-2026-77639
title: "Tor before 0.4.9.9 was prone to a\_compression bomb bypass where an attacker could concatenate many gzip or zlib sub-streams, each just under the per-stream detection threshold, to avoid the compression bomb check entirely"
summary: "Tor before 0.4.9.9 was prone to a\_compression bomb bypass where an attacker could concatenate many gzip or zlib sub-streams, each just under the per-stream detection threshold, to avoid the compression bomb check entirely. This is TROVE-…"
severity: medium
cvss: 5.3
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L'
cwe:
  - CWE-420
vendor: torproject
product: tor
affected:
  - tor < 0.4.9.9
patched:
  - tor 0.4.9.9
published: '2026-08-20'
updated: '2026-09-08'
sourceUpdated: '2026-09-08T18:33:36.520'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-77639'
references:
  - url: 'https://gitlab.torproject.org/tpo/core/tor/-/raw/tor-0.4.9.9/ChangeLog'
    label: cve@mitre.org
tags:
  - nvd
epss: 0.0039
epssPercentile: 0.30444
ingestedAt: '2026-09-08T19:08:49.591Z'
---

## Overview

Tor before 0.4.9.9 was prone to a compression bomb bypass where an attacker could concatenate many gzip or zlib sub-streams, each just under the per-stream detection threshold, to avoid the compression bomb check entirely. This is TROVE-2026-022.

## Affected

- `tor < 0.4.9.9`

## Remediation

Upgrade past the affected range:

- `tor 0.4.9.9`