Mail: Email address spoofing via malformed RFC 2047 encoded-words

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

5.3 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N

EPSS

• CVE-2026-63435: 0.45% estimated exploitation activity in the next 30 days; percentile 39.00 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary Mail::Utilities.qvaluedecode and Mail::Utilities.bvaluedecode decoded only the first RFC 2047 encoded-word in a string and used an overly greedy pattern to match the charset token. A crafted, malformed encoded-word embedded in an address display name or local part could cause the decoded output to differ from what a human reviewer or downstream parser would expect, allowing an attacker to spoof the apparent sender/recipient address. Details Both decoders used a single String#match against a pattern such as /\=\?(.+)?\?[Qq]\?(.)\?\=/m. Two problems: 1. Single match, dropped remainder. Only the first =?charset?Q?...?= (or ?B?) word was decoded. Any additional encoded-words or surrounding text were not handled consistently, so the decoded result could silently omit or alter parts of the input. 2. Greedy charset capture. (.+)? is greedy and matches across ? delimiters, so a malformed word could span more of the string than a strict RFC 2047 parse would, changing the boundary…

Published by the source

2 Sep 2026, 22:02 UTC

Last modified by the source

10 Sep 2026, 03:50 UTC

Source last checked

26 Oct 2026, 15:52 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-mvxr-6m87-mv2q

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-mvxr-6m87-mv2q

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# Patch Next advisory brief: CVE-2026-63435

Advisory: Mail: Email address spoofing via malformed RFC 2047 encoded-words
IDs: CVE-2026-63435, GHSA-mvxr-6m87-mv2q
Record: https://patchnext.com/advisories/cve-2026-63435 (revision 2, source checked 26 Oct 2026, 15:52 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: mail 2.9.1. A vendor fix does not mean a deployment is fixed.

## Affected components
- RubyGems mail: affected < 2.9.1; fixed in 2.9.1

## Suggested inspection (read-only)
- RubyGems mail: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on mail.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-mvxr-6m87-mv2q (OSV record GHSA-mvxr-6m87-mv2q lists affected ranges: RubyGems mail >=0 <2.9.1 (fixed))
- Verified: https://github.com/advisories/GHSA-mvxr-6m87-mv2q (GitHub-reviewed advisory GHSA-mvxr-6m87-mv2q lists affected ranges: RubyGems mail >=0 <2.9.1)

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