# CVE-2026-63435: Mail: Email address spoofing via malformed RFC 2047 encoded-words

> Attackers can spoof email headers via malformed RFC 2047 encoding, bypassing security checks and enabling phishing in applications using the Ruby Mail library.

- **Severity:** Medium 5.3 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/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:** 1 Sep 2026
- **Updated:** 2 Sep 2026
- **Also known as:** CLEANSTART-2026-JD97527, CLEANSTART-2026-UV37642, DEBIAN-CVE-2026-63435, GHSA-mvxr-6m87-mv2q, MINI-3qgm-rhmq-5jh9, MINI-8497-x9jj-x597, MINI-9h65-383c-vr68, MINI-crm6-r22w-vq2x, MINI-fjgh-3hqc-gf4f, MINI-g6fm-x62h-338v, MINI-gvg5-h778-pc73, MINI-hhhm-hph8-qrch, MINI-j2x4-55jq-c48h, MINI-pcrc-hh7g-9p4r, MINI-vvxw-6wqp-6c28, OESA-2026-4104, OESA-2026-4105, OESA-2026-4106, UBUNTU-CVE-2026-63435

## What is CVE-2026-63435?

CVE-2026-63435 is a medium-severity vulnerability in mikel mail, published on 1 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

Mail is an internet library for Ruby designed to handle email generation, parsing, and sending. Prior to 2.9.1, Mail::Utilities.q_value_decode and Mail::Utilities.b_value_decode used a single String#match and an overly greedy charset capture to decode only the first RFC 2047 encoded-word and mishandle surrounding or subsequent text. A crafted malformed encoded-word in an address display name or local part could cross ? delimiters and make decoded From, To, or Reply-To header values differ from the raw values inspected by a human reviewer or downstream parser, enabling apparent sender or recipient spoofing, phishing, or authorization-check bypass. This issue is fixed in version 2.9.1.

Weakness: CWE-436 (Interpretation Conflict)

## Which versions are affected by CVE-2026-63435?

2 open source packages ship an affected version, including mail.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `mail` | RubyGems | All versions | 2.9.1 |
| `mail` | RubyGems | All versions | 2.9.1 |

## How do I fix CVE-2026-63435?

Upgrade mail to 2.9.1 or later.

## Is CVE-2026-63435 being exploited?

There is no known exploitation of CVE-2026-63435 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-63435 and fix it if it is affected.

What it is: CVE-2026-63435 is a medium-severity vulnerability in mikel mail, published on 1 Sep 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-2026-63435 in the wild and no public exploit code.

Affected packages and fixed versions:
- mail (RubyGems): affected all versions; fixed in 2.9.1
- mail (RubyGems): affected all versions; fixed in 2.9.1

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-2026-63435/ (Markdown: https://insights.sbomify.com/CVE-2026-63435/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-63435
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-63435
- OSV: https://osv.dev/vulnerability/CVE-2026-63435

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-63435/
Summaries are generated and may contain mistakes; the official description above is the record.