# CVE-2026-80213: Other in resolv

> An attacker can bypass hostname allowlists and poison DNS caches by exploiting a length encoding flaw in the Ruby resolv gem.

- **Severity:** Medium 4.0 (`CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:N/I:L/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 27 Aug 2026
- **Updated:** 27 Aug 2026
- **Also known as:** ALPINE-CVE-2026-80213, AZL-98229, BELL-CVE-2026-80213, CLEANSTART-2026-NB50742, GHSA-jr95-x3fr-rcg4, OESA-2026-3868

## What is CVE-2026-80213?

CVE-2026-80213 is a medium-severity vulnerability in Ruby resolv, published on 27 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

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.

Weakness: CWE-197 (Numeric Truncation Error)

## Which versions are affected by CVE-2026-80213?


## How do I fix CVE-2026-80213?

Upgrade to a fixed version.

## Is CVE-2026-80213 being exploited?

There is no known exploitation of CVE-2026-80213 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-80213 and fix it if it is affected.

What it is: CVE-2026-80213 is a medium-severity vulnerability in Ruby resolv, published on 27 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-80213 in the wild and no public exploit code.

Affected product: Ruby resolv. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-80213/ (Markdown: https://insights.sbomify.com/CVE-2026-80213/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-80213
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-80213
- OSV: https://osv.dev/vulnerability/CVE-2026-80213

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-80213/
Summaries are generated and may contain mistakes; the official description above is the record.