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.
CVE-2013-4206
MEDIUMCVSS v2 · 6.86.8—
EchelonGraph scoreLOW confidence
This CVE was only ever scored under CVSS v2.0. NVD CVSS v2.0 base 6.8 — v2 rates that MEDIUM (v2 bands: LOW 0.0–3.9 / MEDIUM 4.0–6.9 / HIGH 7.0–10.0; v2 has no CRITICAL band). Vector AV:N/AC:M/Au:N/C:P/I:P/A:P. CVSS v2 is an older, coarser metric than v3.1/v4.0 (no Scope, no User Interaction, no Attack Requirements), so it is not directly comparable to a v3 or v4 score. NVD has never re-scored this record under v3. EPSS predicts 2.5% exploitation probability in the next 30 days, which is reflected in EG Risk but does not lift the v2 base. Confidence: LOW.
Sources: epss, nvd_v2
6.8EG
EchelonGraph verdictMonitorLow exploitation likelihood right now — keep watching.
- Lower severity and no public exploit yet
CISA-KEV: Not listedEPSS PROB: 2.5%CVSS v2: 6.8Exploit: None knownExposed services: —
A fix is available — apply it.
Internet exposure
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- CVSS v2.0 (legacy)
- 6.8MEDIUMNVD never assigned a CVSS v3 rating to this CVE
- EG Score
- 6.8MEDIUMlow confidence
- EG Risk
- 36EG Risk 36/100CISA SSVC
EG Risk is EchelonGraph's 0–100 priority score: it fuses intrinsic severity with real-world exploitation and automatability so you can rank equal-severity CVEs and fix the most dangerous first. Higher = act sooner. Distinct from the 0–10 EG Score (severity).
How it’s computedSeverity68% × 45%Exploitation2% × 40%Automatability30% × 15%CISA SSVC: Track at low or medium mission impact; Track or Track* or Attend at high (mission-essential systems).Action: A fix is available. Apply it within your standard update timelines at low or medium mission impact and sooner than that at high. - EPSS PROB
- 2.5%
- EPSS %ILE
- 83rd
- KEV
- Not listed
CISA SSVCTrack at low or medium mission impact; Track or Track* or Attend at high (mission-essential systems).
A fix is available. Apply it within your standard update timelines at low or medium mission impact and sooner than that at high.
Exploitation none (no KEV listing, exploit record or EPSS ≥ 50%) · Automatable unknown (not published for this CVE) · Technical impact unknown (not published for this CVE). Mission impact is CISA's Mission & Well-being decision point, and only you can judge it: high means the affected system is essential to your organisation's mission, or its compromise could cause irreversible harm to people. CISA's decision table
Published
August 19, 2013
Last Modified
April 29, 2026
Advisory Details (1)
0
NONE
2013-08-19 23:55:08.723+00
Affected: [{"url": "http://lists.opensuse.org/opensuse-updates/2013-08/msg00035.html", "source": "[email protected]"}, {"url": "http://secunia.com/advisories/54379", "tags": ["Vendor Advisory"], "source": "[email protected]"}, {"url": "http://secunia.com/advisories/54533", "source": "[email protected]"}, {"url": "http://svn.tartarus.org/sgt/putty/sshbn.c?sortby=date&r1=9977&r2=9976&pathrev=9977", "tags": ["Patch"], "source": "[email protected]"}, {"url": "http://www.chiark.greenend.org.uk/~sgtatham/putty/wishlist/vuln-modmul.html", "tags": ["Vendor Advisory"], "source": "[email protected]"}, {"url": "http://www.debian.org/security/2013/dsa-2736", "source": "[email protected]"}, {"url": "http://www.openwall.com/lists/oss-security/2013/08/06/11", "source": "[email protected]"}, {"url": "http://lists.opensuse.org/opensuse-updates/2013-08/msg00035.html", "source": "af854a3a-2127-422b-91ae-364da2661108"}, {"url": "http://secunia.com/advisories/54379", "tags": ["Vendor Advisory"], "source": "af854a3a-2127-422b-91ae-364da2661108"}, {"url": "http://secunia.com/advisories/54533", "source": "af854a3a-2127-422b-91ae-364da2661108"}, {"url": "http://svn.tartarus.org/sgt/putty/sshbn.c?sortby=date&r1=9977&r2=9976&pathrev=9977", "tags": ["Patch"], "source": "af854a3a-2127-422b-91ae-364da2661108"}, {"url": "http://www.chiark.greenend.org.uk/~sgtatham/putty/wishlist/vuln-modmul.html", "tags": ["Vendor Advisory"], "source": "af854a3a-2127-422b-91ae-364da2661108"}, {"url": "http://www.debian.org/security/2013/dsa-2736", "source": "af854a3a-2127-422b-91ae-364da2661108"}, {"url": "http://www.openwall.com/lists/oss-security/2013/08/06/11", "source": "af854a3a-2127-422b-91ae-364da2661108"}]
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.Vendor Advisories for CVE-2013-4206
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Related CVEs
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Frequently asked(4)
What is CVE-2013-4206?
CVE-2013-4206 is a medium vulnerability published on August 19, 2013. 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…
When was CVE-2013-4206 disclosed?
CVE-2013-4206 was first published on August 19, 2013, with the most recent update on April 29, 2026. EchelonGraph re-ingests CVE updates from NVD on a 2-hour cycle, so this page reflects the latest published state.
Is CVE-2013-4206 actively exploited?
CVE-2013-4206 is not currently on CISA's Known Exploited Vulnerabilities catalog. FIRST EPSS estimates a 2.5% probability of exploitation in the next 30 days (83rd percentile of EPSS-scored CVEs).
How do I remediate CVE-2013-4206?
A fix for CVE-2013-4206 is available: update to the fixed version the vendor names in its advisory.
Dependency Blast Radius
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