CVE-2013-4206: Heap-based buffer underflow in the modmul function in sshbn.c in PuTTY before 0.63 allows remote SSH servers t

Q: How do I fix CVE-2013-4206?

putty: downgrade or apply the listed mitigation; putty: upgrade to newer than 0.62. Exact commands for each OS are on this page.

Q: Is CVE-2013-4206 being exploited?

CVE-2013-4206 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

Q: What does CVE-2013-4206 affect?

CVE-2013-4206 affects putty, putty. Severity is medium (CVSS 6.8). Heap-based buffer underflow in the modmul function in sshbn.c in PuTTY before 0.63 allows remote SSH servers to cause a denial of service (crash) and possibly trigger memory corruption or code execution via a crafted DSA signature, which is

Q: How urgent is CVE-2013-4206?

Priority 45/100. No elevated risk signals yet.

This month

MEDIUM 6.8DOWNGRADEUPGRADE

CVE-2013-4206 is a medium-severity flaw (CVSS 6.8) affecting putty, putty. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

Heap-based buffer underflow in the modmul function in sshbn.c in PuTTY before 0.63 allows remote SSH servers to cause a denial of service (crash) and possibly trigger memory corruption or code execution via a crafted DSA signature, which is not properly handled when performing certain bit-shifting operations during modular multiplication.

Why this priority

No elevated risk signals yet

A fixed release or distro patch exists.

Risk facts

CVSS score	6.8 (Medium)
EPSS exploit chance	2.5% (top 15%)
CISA exploited list	No
Weakness	CWE-119
Published	2013-08-19
Last modified	2026-06-16

Publisher (CNA): Red Hat, Inc. · Vendor: putty · simon tatham

Last updated 2026-06-16 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2013-08-19 23:55 UTCPublished CVE record published
2026-06-16 23:56 UTCUpdated Record last modified

putty putty DOWNGRADE

Affected: 0.45 · 0.46 · 0.47 · 0.48 · 0.49 · 0.50 · 0.51 · 0.52

Fixed in: no fixed release known

Safe to downgrade to: a release before 0.45

1. Check installed version

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

2. No fixed release: DOWNGRADE to a release before 0.45 (Debian / Ubuntu)

apt-cache madison putty sudo apt install putty=<VERSION_BEFORE_0.45> sudo apt-mark hold putty

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

dnf --showduplicates list putty sudo dnf downgrade putty-<VERSION_BEFORE_0.45> sudo dnf versionlock add putty

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.

simon tatham putty UPGRADE

Affected: <= 0.62 · 0.53

Fixed in: newer than 0.62

Safe to downgrade to: no unaffected older release

1. Check installed version

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

2. UPGRADE to newer than 0.62 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade putty

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade putty

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

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-2013-4206?

putty: downgrade or apply the listed mitigation; putty: upgrade to newer than 0.62. Exact commands for each OS are on this page.

Is CVE-2013-4206 being exploited?

CVE-2013-4206 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-2013-4206 affect?

CVE-2013-4206 affects putty, putty. Severity is medium (CVSS 6.8). Heap-based buffer underflow in the modmul function in sshbn.c in PuTTY before 0.63 allows remote SSH servers to cause a denial of service (crash) and possibly trigger memory corruption or code execution via a crafted DSA signature, which is

How urgent is CVE-2013-4206?

Priority 45/100. No elevated risk signals yet.

Software pages

PuTTY vulnerabilities

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References

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