PatchTheCVECVE-2026-77639: Tor before 0.4.9.9 was prone to a compression bomb bypass where an attacker could concatenate many gzip or zli
CVE-2026-77639 is a medium-severity flaw (CVSS 5.3) affecting tor. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.
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.
Why this priority
- Remotely exploitable, no login, no user action
A fixed release or distro patch exists.
Risk facts
| CVSS score | 5.3 (Medium) |
| Attack vector | Network (remote) |
| Attack complexity | Low |
| Privileges needed | None |
| User interaction | None |
| EPSS exploit chance | 0.4% (top 67%) |
| CISA exploited list | No |
| Weakness | CWE-420 |
| Published | 2026-08-20 |
| Last modified | 2026-10-18 |
Last updated 2026-10-18 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score
Am I affected?
Timeline
- Published CVE record published
- Updated Record last modified
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.
Quick answers
How do I fix CVE-2026-77639?
tor: upgrade to 0.4.9.9. Exact commands for each OS are on this page.
Is CVE-2026-77639 being exploited?
CVE-2026-77639 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.
What does CVE-2026-77639 affect?
CVE-2026-77639 affects tor. Severity is medium (CVSS 5.3). 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-202
How urgent is CVE-2026-77639?
Priority 32/100. Remotely exploitable, no login, no user action
Software pages
Related CVEs
- CVE-2026-44603 LOW 3.7 UPGRADE Tor before 0.4.9.7 has an out-of-bounds read by one byte via a malformed BEGIN cell, aka TROVE-2026-007.
- CVE-2026-44597 LOW 3.7 UPGRADE Tor before 0.4.9.7 has an out-of-bounds read when an END, a TRUNCATE, or a TRUNCATED cell lacks a reason in it
- CVE-2026-44600 LOW 3.7 UPGRADE Tor before 0.4.9.7 mishandles accounting of the conflux out-of-order queue during the clearing of a queue, aka
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- CVE-2026-44602 LOW 3.7 UPGRADE Tor before 0.4.9.7 has a NULL pointer dereference when a CERT cell is received out of order, aka TROVE-2026-00