CVE-2016-2338: An exploitable heap overflow vulnerability exists in the Psych::Emitter start_document function of Ruby

This week

CRITICAL 9.8REMOTEWATCHLISTDOWNGRADEMITIGATE

CVE-2016-2338 is a critical-severity flaw (CVSS 9.8) affecting ruby, debian linux. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

An exploitable heap overflow vulnerability exists in the Psych::Emitter start_document function of Ruby. In Psych::Emitter start_document function heap buffer "head" allocation is made based on tags array length. Specially constructed object passed as element of tags array can increase this array size after mentioned allocation and cause heap overflow.

Why this priority

Critical severity
EPSS top 8%: very likely to be exploited
Public exploit reference
Remotely exploitable, no login, no user action
On your watchlist

A fixed release or distro patch exists.

Risk facts

CVSS score	9.8 (Critical)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	4.7% (top 8%)
CISA exploited list	No
Weakness	CWE-787
Published	2022-09-29
Last modified	2026-06-17

Publisher (CNA): CERT/CC · Vendor: debian · ruby-lang

Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2022-09-29 03:15 UTCPublished CVE record published
2026-06-17 00:43 UTCUpdated Record last modified

ruby-lang ruby DOWNGRADE

Affected: 2.2.2 · 2.3.0

Fixed in: no fixed release known

Safe to downgrade to: a release before 2.2.2

1. Check installed version

apt-cache policy ruby || rpm -q ruby || ruby --version

2. No fixed release: DOWNGRADE to a release before 2.2.2 (Debian / Ubuntu)

apt-cache madison ruby sudo apt install ruby=<VERSION_BEFORE_2.2.2> sudo apt-mark hold ruby

2. No fixed release: DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list ruby sudo dnf downgrade ruby-<VERSION_BEFORE_2.2.2> sudo dnf versionlock add ruby

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

debian debian linux MITIGATE

Affected: 8.0

Fixed in: no fixed release known

Safe to downgrade to: a release before 8.0

Check your OS release

cat /etc/os-release; uname -r

Debian / Ubuntu

sudo apt update && sudo apt full-upgrade

RHEL / Amazon Linux / Fedora

sudo dnf upgrade --refresh

Windows

Install the latest cumulative update from Windows Update

If no patch exists yet

OS downgrades are not safe. Apply the vendor mitigation from the advisory and restrict exposure.

Exact fixes by distribution

A distro patch exists: prefer upgrading over downgrading.

Fedora ELN · libslirp · FIXED · 4.9.7-1.eln159 · FEDORA-2026-209cb94e13 · source

sudo dnf upgrade --advisory=FEDORA-2026-209cb94e13

Fedora 46 · modpoll · FIXED · 1.6.1-1.fc46 · FEDORA-2026-60d403ff15 · source

sudo dnf upgrade --advisory=FEDORA-2026-60d403ff15

RHEL 6 · ruby · DEFERRED

RHEL 7 · ruby · DEFERRED

RHEL 8 · ruby:2.5/ruby · NOT AFFECTED

RHEL 8 · ruby:2.7/ruby · NOT AFFECTED

RHEL 8 · ruby:3.0/ruby · NOT AFFECTED

Amazon Linux 2 · see advisory · FIXED · ALAS2-2025-2990 · source

sudo yum update --advisory=ALAS2-2025-2990

Public exploits and detection

Public code exists for this CVE. Treat unpatched systems as at risk, and only run PoCs in a lab.

GitHub PoC: SpiralBL0CK/CVE-2016-2338-nday (0 stars)

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2016-2338?

ruby: downgrade or apply the listed mitigation; debian linux: mitigate or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2016-2338 being exploited?

CVE-2016-2338 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2016-2338 affect?

CVE-2016-2338 affects ruby, debian linux. Severity is critical (CVSS 9.8). An exploitable heap overflow vulnerability exists in the Psych::Emitter start_document function of Ruby. In Psych::Emitter start_document function heap buffer "head" allocation is made based on tags array length. Specially constructed objec

How urgent is CVE-2016-2338?

Priority 72/100. Critical severity; EPSS top 8%: very likely to be exploited; Public exploit reference; Remotely exploitable, no login, no user action; On your watchlist

Software pages

Debian vulnerabilities · Ruby vulnerabilities

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References

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