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PatchSiren cyber security CVE debrief

CVE-2026-4115 PuTTY CVE debrief

A vulnerability in PuTTY 0.83's Ed25519 signature verification (eddsa_verify in crypto/ecc-ssh.c) allows improper cryptographic signature verification. The issue involves signature malleability where non-canonical signatures may be accepted. The CVSS 4.0 score of 2.9 (LOW) reflects high attack complexity and difficult exploitability. The vulnerability was published March 22, 2026 and last modified April 30, 2026. A patch is available (commit af996b5ec27ab79bae3882071b9d6acf16044549). The vendor responded professionally and released a fix promptly. While proof-of-concept code exists publicly, real-world impact remains unconfirmed.

Vendor
PuTTY
Product
PuTTY
CVSS
LOW 2.9
CISA KEV
Not listed in stored evidence
Original CVE published
2026-03-22
Original CVE updated
2026-04-30
Advisory published
2026-03-22
Advisory updated
2026-04-30

Who should care

Organizations using PuTTY 0.83 for SSH connections with Ed25519 host keys or user authentication; security teams validating cryptographic implementations; developers building Ed25519 signature verification

Technical summary

The eddsa_verify function in PuTTY 0.83's crypto/ecc-ssh.c fails to properly validate Ed25519 signatures, potentially accepting non-canonical (malleable) signatures. This violates RFC 8032 Section 8.4 requirements for signature canonicalization. The attack requires network access but is rated high complexity with difficult exploitability. Successful exploitation could allow signature forgery in limited scenarios, though practical impact remains uncertain.

Defensive priority

LOW

Recommended defensive actions

  • Upgrade to PuTTY version containing commit af996b5ec27ab79bae3882071b9d6acf16044549 or later
  • Verify Ed25519 signature validation in custom SSH implementations against RFC 8032 canonical signature requirements
  • Monitor for updated vendor guidance on real-world exploitability

Evidence notes

The vulnerability affects PuTTY 0.83 specifically, per NVD CPE data. The issue is documented in the vendor's wishlist tracker. RFC 8032 Section 8.4 provides technical context on Ed25519 signature verification requirements.

Sources and references

Verified primary and authoritative sources

  • CVE-2026-4115 CVE Program record

    Publisher, destination, and source semantics verified

    URL: https://www.cve.org/CVERecord?id=CVE-2026-4115

    CVE Program - Official CVE Program record with source-provided CVE metadata.

  • CVE-2026-4115 NVD vulnerability detail

    Publisher, destination, and source semantics verified

    URL: https://nvd.nist.gov/vuln/detail/CVE-2026-4115

    NIST National Vulnerability Database - Official NIST NVD detail page and source-specific vulnerability assessment.

Supplemental references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.