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Fencer Vulns
medium 5.3

CVE-2026-77639 json

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 unde…

published 2026-08-20 2 mo ago modified 2026-10-18 21 d ago also EUVD-2026-63648

cvss
5.3
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
0.4%
EPSS 0.4% chance of exploitation in 30 days, percentile 33%
public exploits
0
none known

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-2026-022.

Patch / fix

  • torproject Tor 0.4.9.9
  • tor Debian 14, Debian 12 0.4.9.9-1
  • tor Debian 13 0.4.9.11-0+deb13u1

debian advisories DEBIAN-CVE-2026-77639

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2026-77639 updated cvss 5.3

tor: Tor: Denial of Service via compression bomb bypass

  • Red Hat All currently supported Red Hat products
    not affected: vulnerable code not present

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

CISA SSVC

Exploitation
none
Automatable
yes
Technical impact
partial
Assessed
2 mo ago

Exploit prediction

0.39% chance of exploitation in the next 30 days, higher than 33% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by version score vector
CNA 3.1 5.3 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

Affected products

  • torproject Tor

Classification

Weakness
  • CWE-420 Unprotected Alternate Channel

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.