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Fencer Vulns
medium 4.0

CVE-2026-80213 json

An issue was discovered in the resolv gem before 0.7.2 for Ruby.

published 2026-08-27 2 mo ago modified 2026-09-09 2 mo ago also EUVD-2026-67139

cvss
4.0
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
0.3%
EPSS 0.3% chance of exploitation in 30 days, percentile 19%
public exploits
0
none known

Summary

An issue was discovered in the resolv gem before 0.7.2 for Ruby. Resolv::DNS::MessageEncoder wrote a DNS label's length into a single octet without checking its range. A label longer than 255 octets had its length stored modulo 256 but the label data was written unchanged, and thus the bytes on the wire described a different name than the one the application asked to encode. RFC 1035 section 2.3.4 limits a label to 63 octets, and the two high bits of the length octet are reserved for compression pointers. put_string packed the length with put_pack("C", d.length) and put_label used it for labels, and thus any value from 0 to 255 could end up as a label length octet, including the reserved 0x40-0xBF range and the 0xC0-0xFF pointer range. Resolv::DNS::Name.create did not check per-label or total name length either, and thus an attacker-controlled hostname reached the encoder unchanged. An application that resolves an attacker-controlled hostname sends a query whose wire bytes name a domain the attacker chose. A hostname suffix that the application validates against an allowlist becomes padding that never appears on the wire, and thus allowlist and egress checks can be bypassed. The recursive resolver caches the response under the attacker's name, and DNS logs record that name rather than the one the application asked for. A label length whose low octet lands in the 0xC0-0xFF range produces a length octet that conforming parsers read as the start of a compression pointer, with the following attacker-controlled byte as the offset.

Patch / fix

  • Ruby resolv 0.3.2, 0.7.2
  • ruby3.3 Debian 13 3.3.13-0+deb13u1

debian advisories DEBIAN-CVE-2026-80213

  • ruby4.0 hummingbird 1 4.0.6-37.3.hum1
  • ruby4.0-libs hummingbird 1 4.0.6-37.3.hum1
  • rubygem4.0-irb hummingbird 1 1.16.0-37.3.hum1
  • rubygem4.0-rbs hummingbird 1 3.10.0-37.3.hum1
  • rubygem4.0-rss hummingbird 1 0.3.2-37.3.hum1
  • rubygem4.0-json hummingbird 1 2.18.0-37.3.hum1
  • rubygem4.0-racc hummingbird 1 1.8.1-37.3.hum1
  • rubygem4.0-rake hummingbird 1 13.3.1-37.3.hum1

red hat advisories RHSA-2026:62563RHSA-2026:62562RHSA-2026:57986RHSA-2026:57994RHSA-2026:57565

  • ruby Alpine 3.21 3.3.13-r0
  • ruby Alpine 3.24, Alpine 3.23, Alpine 3.22 3.4.12-r0

alpine advisories ALPINE-CVE-2026-80213

Alpine secdb © Alpine Linux, from https://secdb.alpinelinux.org via OSV; changed: normalised to fixed package versions per release. CC BY-SA 4.0

  • ruby Azure Linux 3.0 3.3.5-9

azure linux advisories AZL-98229

Azure Linux vulnerability data, Copyright (c) Microsoft Corporation, from https://github.com/microsoft/AzureLinuxVulnerabilityData via OSV, under the MIT licence; changed: normalised to fixed package versions per release. MIT

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-2026-80213 updated cvss 4.0

resolv: Resolv gem: DNS allowlist and egress bypass, and cache poisoning via crafted hostnames

  • Red Hat Lightspeed Core lightspeed-core/rag-tool-cpu-rhel9, lightspeed-core/rag-tool-cuda-12.9-rhel9
    affected: workaround, no fix available “Fix deferred”
  • Red Hat 3scale API Management Platform 2 3scale-amp2/backend-rhel8, 3scale-amp2/toolbox-rhel9 Red Hat Enterprise Linux 6 ruby, ruby-ri, ruby-irb, ruby-libs, ruby-rdoc, ruby-tcltk, +1 more
    affected: workaround, no fix planned “Out of support scope”
  • Red Hat Enterprise Linux 7 ruby, ruby-irb, rubygems, ruby-libs, ruby-tcltk, rubygem-json, +7 more Red Hat Enterprise Linux 8 ruby, ruby-irb, rubygems, ruby-libs, rubygem-pg, rubygem-irb, +26 more Red Hat Enterprise Linux 9 ruby, rubygems, ruby-libs, rubygem-irb, rubygem-rbs, rubygem-rss, +17 more Red Hat Enterprise Linux 10 ubi10/ruby-33, ubi10/ruby-40, ruby 3.3.12-14.el10_2, ruby4.0 4.0.6-36.el10_2
    affected: workaround, no fix available “Fix deferred”
  • Red Hat Hardened Images ruby4.0 4.0.6-37.2.hum1, ruby4.0 4.0.6-37.3.hum1, ruby3.3 3.3.10-23.5.hum1, ruby3.3 3.3.10-23.6.hum1, ruby3.4 3.4.10-31.6.hum1, ruby4.0-libs 4.0.6-37.2.hum1, +48 more
    fixed: vendor fix RHSA-2026:57986
  • Red Hat Hardened Images rubygem3.3-rbs 3.4.0-23.5.hum1, rubygem3.3-rss 0.3.1-23.5.hum1, rubygem3.4-rbs 3.8.0-31.6.hum1, rubygem3.4-rss 0.3.1-31.6.hum1, rubygem4.0-rss 0.3.2-37.2.hum1, rubygem3.3-irb 1.13.1-23.5.hum1, +45 more
    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
no
Technical impact
partial
Assessed
2 mo ago

Exploit prediction

0.27% chance of exploitation in the next 30 days, higher than 19% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by version score vector
CNA 3.1 4.0 CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:N/I:L/A:N

Affected products

  • Ruby resolv

Classification

Weakness

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.