HighCVE-2026-42789

    Non-CA certificate accepted as intermediate issuer in public_key path validation (CVE-2026-42789)

    Published Last modified Sources: Red Hat, SUSE, Debian, BSI, CVE.org, EUVD, OSV, Msrc Update Guide

    What is affected, what to do

    Improper Following of a Certificate's Chain of Trust vulnerability in Erlang OTP public_key (pubkey_cert module) allows a non-CA certificate to be accepted as an intermediate issuer, enabling certificate chain forgery. In lib/public_key/src/pubkey_cert.erl, pubkey_cert:validate_extensions/7 contains two flaws that together allow a certificate with basicConstraints cA:false and no keyUsage extension to be used as an intermediate issuer in a chain passed to public_key:pkix_path_validation/3: the cA:false clause recurses into the remaining extensions without rejecting the certificate when it is in issuer position, and the keyUsage check only fires when the extension is present, so a certificate lacking keyUsage entirely bypasses the keyCertSign enforcement.

    Update erlang\/otp to version 27.3.4.12, Erlang/OTP to version 1.15.1.7.

    Affected
    • Erlang OTP gte 17.0..*
    • Erlang OTP gte 0.22..*

    Not sure whether and how you are affected? Ask a question about this vulnerability

    Metrics

    8.0

    Priority

    XIEM Effective CVSS: highest available CVSS rating from NVD and vendor data.

    Source: Vendor (CNA), CVSS v3

    4.8

    CVSS v3

    Severity Medium on the scale from 0 to 10.

    Source: Red Hat, SUSE, Debian, BSI, CVE.org, EUVD, OSV, Msrc Update Guide

    0.4 %

    EPSS score

    Low — model estimates < 1% exploitation likelihood.

    Source: FIRST EPSS, percentile 32.3

    Show all metrics
    Severity
    High
    Priority (0–325)

    The priority score is a consolidated metric that condenses several individual signals into a single, action-guiding ranking: technical severity (CVSS), exploitation likelihood (EPSS), known-exploited status (KEV) and the context of the affected asset. The aim is to answer the only truly important question in vulnerability management: what first?

    Read in glossary
    61.31 Methodology
    Exploit maturity

    An exploit is code or a technique that specifically uses a concrete vulnerability to force unintended behaviour — such as executing code or escalating privileges. It is the tool that turns a theoretical flaw into a practical attack.

    Read in glossary
    no public PoC known
    XIEM Effective CVSS

    XIEM® is the tiered managed-security operating model from NEOSEC and a registered trademark of NEOSEC GmbH. It extends classic SIEM — the central collection, normalisation and correlation of security-relevant events — with endpoint forensics, network visibility and deception technology, and consolidates them into three building tiers: Sentry, Orchestrate and Command.

    Read in glossary
    8.0 Methodology
    CVSS v3

    CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions.

    Read in glossary
    4.8
    CVSS v4

    CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions.

    Read in glossary
    7.0
    EPSS score

    The EPSS score is the absolute model output for an individual vulnerability: the estimated probability of real-world exploitation within the next 30 days, expressed as a value between 0 and 1 (i.e. 0 % to 100 %). A score of 0.42 means „the model estimates a 42 % chance of real-world exploitation within 30 days“.

    Read in glossary
    0.4 %
    Low — model estimates < 1% exploitation likelihood.
    EPSS percentile

    The EPSS percentile is the relative position of a vulnerability within all EPSS-scored CVEs, expressed as a value between 0 and 1. A percentile of 0.90 means „this CVE has a higher EPSS score than 90 % of all other scored CVEs“. Unlike the EPSS score, it is not a probability — but a positioning within a ranking.

    Read in glossary
    32.3
    Elevated — this CVE ranks above at least 10% of all CVEs scored today.
    KEV status

    The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.

    Read in glossary
    —
    Published
    CISA assessment (SSVC)
    Exploitation: none known · not automatable · Technical impact: partial
    Source: CISA Vulnrichment, as of 27 May 2026
    CWE

    CWE is a catalogue of typical weakness types in software and hardware — such as flawed input validation, unsafe memory handling or poor authentication. Where a CVE names a concrete vulnerability in a concrete product, a CWE describes the underlying class of defect.

    Read in glossary
    CWE-295, CWE-296, CWE-295

    Detection rules

    No NEOSEC detection rule is available for this vulnerability yet.

    Classification under NIS2 and CRA

    Rule-based classification from exploitation status and severity. It does not replace a case-by-case assessment.

    NIS2

    NIS2 is EU Directive (EU) 2022/2555 to raise the level of cybersecurity across the Union. It substantially broadens obligations compared with the first NIS directive: more sectors, mandatory risk management, staged reporting duties and personal accountability of senior management.

    Read in glossary
    High technical severity: assess and remediate the vulnerability in vulnerability handling under Art. 21(2)(e) NIS2 and document the decision.
    CRA

    The CRA is EU Regulation (EU) 2024/2847 setting horizontal cybersecurity requirements for „products with digital elements“ — from industrial controllers and software to connected consumer devices. For the first time manufacturers must ensure security across the entire product lifecycle, handle vulnerabilities and provide security updates.

    Read in glossary
    From 11.12.2027, Annex I Part II CRA requires manufacturers of affected products with digital elements to handle vulnerabilities without delay and provide security updates.
    ISO/IEC 27001

    ISO/IEC 27001 is the internationally leading standard for an information security management system (ISMS). It describes a risk-based, process-oriented framework to systematically plan, implement, review and improve information security.

    Read in glossary
    Control A.8.8, management of technical vulnerabilities: assess relevance to your systems, plan remediation and keep evidence.

    Description

    Improper Following of a Certificate's Chain of Trust vulnerability in Erlang OTP public_key (pubkey_cert module) allows a non-CA certificate to be accepted as an intermediate issuer, enabling certificate chain forgery. In lib/public_key/src/pubkey_cert.erl, pubkey_cert:validate_extensions/7 contains two flaws that together allow a certificate with basicConstraints cA:false and no keyUsage extension to be used as an intermediate issuer in a chain passed to public_key:pkix_path_validation/3: the cA:false clause recurses into the remaining extensions without rejecting the certificate when it is in issuer position, and the keyUsage check only fires when the extension is present, so a certificate lacking keyUsage entirely bypasses the keyCertSign enforcement. Any party holding an end-entity certificate with basicConstraints cA:false and no keyUsage extension, issued by any CA in the victim's trust store, can use that certificate's private key to sign forged leaf certificates for arbitrary identities. public_key:pkix_path_validation/3 accepts the resulting chain, and by extension every TLS or mTLS endpoint built on the OTP ssl application that relies on the default verifier is affected, including server identity verification on the client side and client certificate verification on mTLS servers. This issue affects OTP from OTP 17.0 before OTP 27.3.4.19, OTP 28.5.0.8, and OTP 29.1.2, corresponding to public_key from 0.22 before 1.17.1.7, 1.20.3.6, and 1.21.8. Whether OTP before OTP 17.0, corresponding to public_key before 0.22, is affected is unknown.

    Source: redhat_csaf, suse_csaf, Debian Security Advisories, BSI WID Portal, BSI CSAF, cvelistv5, ENISA EUVD, NVD (NIST), OSV (Open Source Vulnerabilities), Microsoft Security Update Guide, CVE.org (MITRE)

    Affected products

    The following versions are affected. Older releases of the same product line are also vulnerable unless stated otherwise.

    Affected products
    Affected versionsFixed in
    ErlangOTP
    • ≥ 0.22
    • ≥ 17.0
    • —
    • —
    erlangerlang\/otp
    • 27.0 – < 27.3.4.12
    • 28.0 – < 28.5.0.1
    • 17.0 – < 26.2.5.21
    • 29.0 – < 29.0.1
    • 27.3.4.12
    • 28.5.0.1
    • 26.2.5.21
    • 29.0.1
    Open SourceErlang/OTP
    • < 1.15.1.7
    • < 26.2.5.21
    • < 28.5.0.1
    • < 1.20.3.1
    • < 27.3.4.12
    • < 29.0.1
    • < 1.17.1.3
    • < 1.21.1
    • 1.15.1.7
    • 26.2.5.21
    • 28.5.0.1
    • 1.20.3.1
    • 27.3.4.12
    • 29.0.1
    • 1.17.1.3
    • 1.21.1

    Vendors and products

    Affected operating systems

    Affected operating systems
    Linuxdebianerlangtrixie

    Weakness classes (CWE)

    • CWE-295Improper Certificate Validation

      Base

      The product does not validate, or incorrectly validates, a certificate.

      Entry at MITRE CWE-295

    • CWE-296Improper Following of a Certificate's Chain of Trust

      Base

      The product does not follow, or incorrectly follows, the chain of trust for a certificate back to a trusted root certificate.

      Entry at MITRE CWE-296

    • CWE-295Improper Certificate Validation

      Base

      The product does not validate, or incorrectly validates, a certificate.

      Entry at MITRE CWE-295

    References & sources

    Changes to this CVE

    Who changed this CVE and when: NVD (analysis, affected products, CVSS), the CVE publisher (CNA) and additional providers such as CISA (ADP).

    1. Record modifiedEEF

      Description added or changed

      Show 9 original changes
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Descriptionchanged
      Improper Following of a Certificate's Chain of Trust vulnerability in Erlang OTP public_key (pubkey_cert module) allows a non-CA certificate to be accepted as an intermediate issuer, enabling certificate chain forgery. In lib/public_key/src/pubkey_cert.erl, pubkey_cert:validate_extensions/7 contains two flaws that together allow a certificate with basicConstraints cA:false and no keyUsage extension to be used as an intermediate issuer in a chain passed to public_key:pkix_path_validation/3: the cA:false clause recurses into the remaining extensions without rejecting the certificate when it is in issuer position, and the keyUsage check only fires when the extension is present, so a certificate lacking keyUsage entirely bypasses the keyCertSign enforcement. Any party holding an end-entity certificate with basicConstraints cA:false and no keyUsage extension, issued by any CA in the victim's trust store, can use that certificate's private key to sign forged leaf certificates for arbitrary identities. public_key:pkix_path_validation/3 accepts the resulting chain, and by extension every TLS or mTLS endpoint built on the OTP ssl application that relies on the default verifier is affected, including server identity verification on the client side and client certificate verification on mTLS servers. This issue affects OTP from OTP 17.0 before OTP 26.2.5.21, OTP 27.3.4.12, OTP 28.5.0.1, and OTP 29.0.1, corresponding to public_key from 0.22 before 1.15.1.7, 1.17.1.3, 1.20.3.1, and 1.21.1. Whether OTP before OTP 17.0, corresponding to public_key before 0.22, is affected is unknown.
      Improper Following of a Certificate's Chain of Trust vulnerability in Erlang OTP public_key (pubkey_cert module) allows a non-CA certificate to be accepted as an intermediate issuer, enabling certificate chain forgery. In lib/public_key/src/pubkey_cert.erl, pubkey_cert:validate_extensions/7 contains two flaws that together allow a certificate with basicConstraints cA:false and no keyUsage extension to be used as an intermediate issuer in a chain passed to public_key:pkix_path_validation/3: the cA:false clause recurses into the remaining extensions without rejecting the certificate when it is in issuer position, and the keyUsage check only fires when the extension is present, so a certificate lacking keyUsage entirely bypasses the keyCertSign enforcement. Any party holding an end-entity certificate with basicConstraints cA:false and no keyUsage extension, issued by any CA in the victim's trust store, can use that certificate's private key to sign forged leaf certificates for arbitrary identities. public_key:pkix_path_validation/3 accepts the resulting chain, and by extension every TLS or mTLS endpoint built on the OTP ssl application that relies on the default verifier is affected, including server identity verification on the client side and client certificate verification on mTLS servers. This issue affects OTP from OTP 17.0 before OTP 27.3.4.19, OTP 28.5.0.8, and OTP 29.1.2, corresponding to public_key from 0.22 before 1.17.1.7, 1.20.3.6, and 1.21.8. Whether OTP before OTP 17.0, corresponding to public_key before 0.22, is affected is unknown.
      Affected versions (publisher)added
      —
      Affected value modified. <a href="https://github.com/CVEProject/cvelistV5/blob/main/cves/2026/42xxx/CVE-2026-42789.json">CVE-2026-42789</a>
      Referenceadded
      —
      https://github.com/erlang/otp/commit/0a05b7ca33fb0b3f0a6cb9ae34252bb3a74032b8
      Referenceadded
      —
      https://github.com/erlang/otp/commit/75ca4c0dc2bbdb286ac3c3fe89cb4b4d2b4dc333
      Referenceadded
      —
      https://github.com/erlang/otp/commit/82af606b5437c4de1276dbcc8855a47e88231226
      Referenceremoved
      https://github.com/erlang/otp/commit/471cd2f664300a95353c467873800bbe706005db
      —
      Referenceremoved
      https://github.com/erlang/otp/commit/59c8d824386b2eb1614ff9340624843ef6aca0fd
      —
      Reference typeremoved
      https://github.com/erlang/otp/commit/471cd2f664300a95353c467873800bbe706005db Types: Patch
      —
      Reference typeremoved
      https://github.com/erlang/otp/commit/59c8d824386b2eb1614ff9340624843ef6aca0fd Types: Patch
      —

      Source identifier: 6b3ad84c-e1a6-4bf7-a703-f496b71e49db

    2. Record modifiedEEF

      Description added or changed

      Show 2 original changes
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Descriptionchanged
      Improper Following of a Certificate's Chain of Trust vulnerability in Erlang OTP public_key (pubkey_cert module) allows a non-CA certificate to be accepted as an intermediate issuer, enabling certificate chain forgery. In lib/public_key/src/pubkey_cert.erl, pubkey_cert:validate_extensions/7 contains two flaws that together allow a certificate with basicConstraints cA:false and no keyUsage extension to be used as an intermediate issuer in a chain passed to public_key:pkix_path_validation/3: the cA:false clause recurses into the remaining extensions without rejecting the certificate when it is in issuer position, and the keyUsage check only fires when the extension is present, so a certificate lacking keyUsage entirely bypasses the keyCertSign enforcement. Any party holding an end-entity certificate with basicConstraints cA:false and no keyUsage extension, issued by any CA in the victim's trust store, can use that certificate's private key to sign forged leaf certificates for arbitrary identities. public_key:pkix_path_validation/3 accepts the resulting chain, and by extension every TLS or mTLS endpoint built on the OTP ssl application that relies on the default verifier is affected, including server identity verification on the client side and client certificate verification on mTLS servers. This issue affects OTP from OTP 17.0 before OTP 29.0.1, OTP 28.5.0.1, OTP 27.3.4.12 and OTP 26.2.5.21, corresponding to public_key from 0.22 before 1.21.1, 1.20.3.1, 1.17.1.3 and 1.15.1.7.
      Improper Following of a Certificate's Chain of Trust vulnerability in Erlang OTP public_key (pubkey_cert module) allows a non-CA certificate to be accepted as an intermediate issuer, enabling certificate chain forgery. In lib/public_key/src/pubkey_cert.erl, pubkey_cert:validate_extensions/7 contains two flaws that together allow a certificate with basicConstraints cA:false and no keyUsage extension to be used as an intermediate issuer in a chain passed to public_key:pkix_path_validation/3: the cA:false clause recurses into the remaining extensions without rejecting the certificate when it is in issuer position, and the keyUsage check only fires when the extension is present, so a certificate lacking keyUsage entirely bypasses the keyCertSign enforcement. Any party holding an end-entity certificate with basicConstraints cA:false and no keyUsage extension, issued by any CA in the victim's trust store, can use that certificate's private key to sign forged leaf certificates for arbitrary identities. public_key:pkix_path_validation/3 accepts the resulting chain, and by extension every TLS or mTLS endpoint built on the OTP ssl application that relies on the default verifier is affected, including server identity verification on the client side and client certificate verification on mTLS servers. This issue affects OTP from OTP 17.0 before OTP 26.2.5.21, OTP 27.3.4.12, OTP 28.5.0.1, and OTP 29.0.1, corresponding to public_key from 0.22 before 1.15.1.7, 1.17.1.3, 1.20.3.1, and 1.21.1. Whether OTP before OTP 17.0, corresponding to public_key before 0.22, is affected is unknown.
      Affected versions (publisher)added
      —
      Affected value modified. <a href="https://github.com/CVEProject/cvelistV5/blob/main/cves/2026/42xxx/CVE-2026-42789.json">CVE-2026-42789</a>

      Source identifier: 6b3ad84c-e1a6-4bf7-a703-f496b71e49db

    3. Record modifiedEEF

      Show 18 original changes
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Referenceadded
      —
      https://cna.erlef.org/cves/CVE-2026-42789.html
      Referenceadded
      —
      https://github.com/erlang/otp/commit/471cd2f664300a95353c467873800bbe706005db
      Referenceadded
      —
      https://github.com/erlang/otp/commit/59c8d824386b2eb1614ff9340624843ef6aca0fd
      Referenceadded
      —
      https://github.com/erlang/otp/security/advisories/GHSA-c99q-jmpx-v8qq
      Referenceadded
      —
      https://osv.dev/vulnerability/EEF-CVE-2026-42789
      Referenceadded
      —
      https://www.erlang.org/doc/system/versions.html#order-of-versions
      Referenceremoved
      https://cna.erlef.org/cves/CVE-2026-42789.html
      —
      Referenceremoved
      https://github.com/erlang/otp/commit/471cd2f664300a95353c467873800bbe706005db
      —
      Referenceremoved
      https://github.com/erlang/otp/commit/59c8d824386b2eb1614ff9340624843ef6aca0fd
      —
      Referenceremoved
      https://github.com/erlang/otp/security/advisories/GHSA-c99q-jmpx-v8qq
      —
      Referenceremoved
      https://osv.dev/vulnerability/EEF-CVE-2026-42789
      —
      Referenceremoved
      https://www.erlang.org/doc/system/versions.html#order-of-versions
      —
      Reference typeremoved
      https://cna.erlef.org/cves/CVE-2026-42789.html Types: Third Party Advisory
      —
      Reference typeremoved
      https://github.com/erlang/otp/commit/471cd2f664300a95353c467873800bbe706005db Types: Patch
      —
      Reference typeremoved
      https://github.com/erlang/otp/commit/59c8d824386b2eb1614ff9340624843ef6aca0fd Types: Patch
      —
      Reference typeremoved
      https://github.com/erlang/otp/security/advisories/GHSA-c99q-jmpx-v8qq Types: Vendor Advisory
      —
      Reference typeremoved
      https://osv.dev/vulnerability/EEF-CVE-2026-42789 Types: Third Party Advisory
      —
      Reference typeremoved
      https://www.erlang.org/doc/system/versions.html#order-of-versions Types: Product
      —

      Source identifier: 6b3ad84c-e1a6-4bf7-a703-f496b71e49db

    4. Record modifiedRed Hat ADP

      Show 11 original changes
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Affected versions (publisher)added
      —
      Affected value modified. <a href="https://github.com/CVEProject/cvelistV5/blob/main/cves/2026/42xxx/CVE-2026-42789.json">CVE-2026-42789</a>
      Referenceadded
      —
      https://access.redhat.com/errata/RHSA-2026:39809
      Referenceadded
      —
      https://access.redhat.com/errata/RHSA-2026:54757
      Referenceadded
      —
      https://access.redhat.com/security/cve/CVE-2026-42789
      Referenceadded
      —
      https://bugzilla.redhat.com/show_bug.cgi?id=2482093
      Referenceadded
      —
      https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-42789.json
      Referenceremoved
      https://access.redhat.com/errata/RHSA-2026:39809
      —
      Referenceremoved
      https://access.redhat.com/errata/RHSA-2026:54757
      —
      Referenceremoved
      https://access.redhat.com/security/cve/CVE-2026-42789
      —
      Referenceremoved
      https://bugzilla.redhat.com/show_bug.cgi?id=2482093
      —
      Referenceremoved
      https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-42789.json
      —

      Source identifier: 0b0ca135-0b70-47e7-9f44-1890c2a1c46c

    5. Record modifiedEEF

      Description added or changed

      Show the original change
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Descriptionchanged
      Improper Following of a Certificate's Chain of Trust vulnerability in Erlang OTP public_key (pubkey_cert module) allows a non-CA certificate to be accepted as an intermediate issuer, enabling certificate chain forgery. In lib/public_key/src/pubkey_cert.erl, pubkey_cert:validate_extensions/7 contains two flaws that together allow a certificate with basicConstraints cA:false and no keyUsage extension to be used as an intermediate issuer in a chain passed to public_key:pkix_path_validation/3: the cA:false clause recurses into the remaining extensions without rejecting the certificate when it is in issuer position, and the keyUsage check only fires when the extension is present, so a certificate lacking keyUsage entirely bypasses the keyCertSign enforcement. Any party holding an end-entity certificate with basicConstraints cA:false and no keyUsage extension, issued by any CA in the victim's trust store, can use that certificate's private key to sign forged leaf certificates for arbitrary identities. public_key:pkix_path_validation/3 accepts the resulting chain, and by extension every TLS or mTLS endpoint built on the OTP ssl application that relies on the default verifier is affected, including server identity verification on the client side and client certificate verification on mTLS servers. This issue affects OTP from OTP 17.0 before OTP 26.2.5.21, 27.3.4.12, 28.5.0.1, and 29.0.1 corresponding to public_key from 0.22 before 1.15.1.7, 1.17.1.3, 1.20.3.1, and 1.21.1.
      Improper Following of a Certificate's Chain of Trust vulnerability in Erlang OTP public_key (pubkey_cert module) allows a non-CA certificate to be accepted as an intermediate issuer, enabling certificate chain forgery. In lib/public_key/src/pubkey_cert.erl, pubkey_cert:validate_extensions/7 contains two flaws that together allow a certificate with basicConstraints cA:false and no keyUsage extension to be used as an intermediate issuer in a chain passed to public_key:pkix_path_validation/3: the cA:false clause recurses into the remaining extensions without rejecting the certificate when it is in issuer position, and the keyUsage check only fires when the extension is present, so a certificate lacking keyUsage entirely bypasses the keyCertSign enforcement. Any party holding an end-entity certificate with basicConstraints cA:false and no keyUsage extension, issued by any CA in the victim's trust store, can use that certificate's private key to sign forged leaf certificates for arbitrary identities. public_key:pkix_path_validation/3 accepts the resulting chain, and by extension every TLS or mTLS endpoint built on the OTP ssl application that relies on the default verifier is affected, including server identity verification on the client side and client certificate verification on mTLS servers. This issue affects OTP from OTP 17.0 before OTP 29.0.1, OTP 28.5.0.1, OTP 27.3.4.12 and OTP 26.2.5.21, corresponding to public_key from 0.22 before 1.21.1, 1.20.3.1, 1.17.1.3 and 1.15.1.7.

      Source identifier: 6b3ad84c-e1a6-4bf7-a703-f496b71e49db

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