Vulnerability-Lookup

FKIE_CVE-2026-71890

Vulnerability from fkie_nvd - Published: 2026-10-03 09:17 - Updated: 2026-10-03 09:17

Severity

8.7 (High) - CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N/U:Amber

Summary

In Bouncy Castle for Java before 1.86, validation of an MLS (RFC 9420) external commit's proposal list, org.bouncycastle.mls.protocol.Group.validateExternalCachedProposals, counted the proposals by type and bounded the removed leaf index but never established that the removed leaf had anything to do with the joiner. RFC 9420 sec. 12.2 permits at most one Remove proposal in an external commit, with which the joiner removes an old version of themselves, and requires that where one is present the LeafNode in the commit's path field meet the criteria it would have to meet in an Update for the removed leaf, in particular that its credential present identifiers acceptable for the removed participant. The ordinary proposal-list validator's self-remove rule is deliberately not applied on this path, because a resync commit legitimately removes a leaf the joiner owns, but nothing was put in its place. Any party holding the group's public GroupInfo, which is precisely what an external joiner is meant to be given, could therefore commit a Remove naming any member's LeafIndex and have every member apply it, evicting that member and taking over their slot in the ratchet tree. The credential check that should have prevented this existed only in the gRPC interop harness and so protected no other caller of the public Group.externalJoin and Group.handle API. An external commit carrying a Remove is now accepted only when the removed leaf's credential is identical to the one in the joiner's own new leaf, on both the sending and the receiving side.

References

	URL	Tags
	91579145-5d7b-4cc5-b925-a0262ff19630	https://github.com/bcgit/bc-java/commit/7e8bb10eb90baddf3f10b8679f627462b9522d24
	91579145-5d7b-4cc5-b925-a0262ff19630	https://github.com/bcgit/bc-java/wiki/CVE%E2%80%902026%E2%80%9071890

Impacted products

	Vendor	Product	Version

JSON

To clipboard

{
  "affected": [
    {
      "affectedData": [
        {
          "collectionURL": "https://www.bouncycastle.org/download/bouncy-castle-java/",
          "defaultStatus": "unaffected",
          "modules": [
            "mls"
          ],
          "packageName": "bcmls",
          "packageURL": "pkg:maven/org.bouncycastle/bcmls-jdk18on",
          "platforms": [
            "all"
          ],
          "product": "BC-JAVA",
          "programFiles": [
            "Group"
          ],
          "repo": "https://github.com/bcgit/bc-java",
          "vendor": "Legion of the Bouncy Castle Inc.",
          "versions": [
            {
              "lessThan": "1.86",
              "status": "affected",
              "version": "1.73",
              "versionType": "maven"
            }
          ]
        }
      ],
      "source": "91579145-5d7b-4cc5-b925-a0262ff19630"
    }
  ],
  "cveTags": [],
  "descriptions": [
    {
      "lang": "en",
      "value": "In Bouncy Castle for Java before 1.86, validation of an MLS (RFC 9420) external commit\u0027s proposal list, org.bouncycastle.mls.protocol.Group.validateExternalCachedProposals, counted the proposals by type and bounded the removed leaf index but never established that the removed leaf had anything to do with the joiner. RFC 9420 sec. 12.2 permits at most one Remove proposal in an external commit, with which the joiner removes an old version of themselves, and requires that where one is present the LeafNode in the commit\u0027s path field meet the criteria it would have to meet in an Update for the removed leaf, in particular that its credential present identifiers acceptable for the removed participant. The ordinary proposal-list validator\u0027s self-remove rule is deliberately not applied on this path, because a resync commit legitimately removes a leaf the joiner owns, but nothing was put in its place. Any party holding the group\u0027s public GroupInfo, which is precisely what an external joiner is meant to be given, could therefore commit a Remove naming any member\u0027s LeafIndex and have every member apply it, evicting that member and taking over their slot in the ratchet tree. The credential check that should have prevented this existed only in the gRPC interop harness and so protected no other caller of the public Group.externalJoin and Group.handle API. An external commit carrying a Remove is now accepted only when the removed leaf\u0027s credential is identical to the one in the joiner\u0027s own new leaf, on both the sending and the receiving side."
    }
  ],
  "id": "CVE-2026-71890",
  "lastModified": "2026-10-03T09:17:05.607",
  "metrics": {
    "cvssMetricV40": [
      {
        "cvssData": {
          "Automatable": "NOT_DEFINED",
          "Recovery": "NOT_DEFINED",
          "Safety": "NOT_DEFINED",
          "attackComplexity": "LOW",
          "attackRequirements": "NONE",
          "attackVector": "NETWORK",
          "availabilityRequirement": "NOT_DEFINED",
          "baseScore": 8.7,
          "baseSeverity": "HIGH",
          "confidentialityRequirement": "NOT_DEFINED",
          "exploitMaturity": "NOT_DEFINED",
          "integrityRequirement": "NOT_DEFINED",
          "modifiedAttackComplexity": "NOT_DEFINED",
          "modifiedAttackRequirements": "NOT_DEFINED",
          "modifiedAttackVector": "NOT_DEFINED",
          "modifiedPrivilegesRequired": "NOT_DEFINED",
          "modifiedSubAvailabilityImpact": "NOT_DEFINED",
          "modifiedSubConfidentialityImpact": "NOT_DEFINED",
          "modifiedSubIntegrityImpact": "NOT_DEFINED",
          "modifiedUserInteraction": "NOT_DEFINED",
          "modifiedVulnAvailabilityImpact": "NOT_DEFINED",
          "modifiedVulnConfidentialityImpact": "NOT_DEFINED",
          "modifiedVulnIntegrityImpact": "NOT_DEFINED",
          "privilegesRequired": "NONE",
          "providerUrgency": "AMBER",
          "subAvailabilityImpact": "NONE",
          "subConfidentialityImpact": "NONE",
          "subIntegrityImpact": "NONE",
          "userInteraction": "NONE",
          "valueDensity": "NOT_DEFINED",
          "vectorString": "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:Amber",
          "version": "4.0",
          "vulnAvailabilityImpact": "NONE",
          "vulnConfidentialityImpact": "NONE",
          "vulnIntegrityImpact": "HIGH",
          "vulnerabilityResponseEffort": "NOT_DEFINED"
        },
        "source": "91579145-5d7b-4cc5-b925-a0262ff19630",
        "type": "Secondary"
      }
    ]
  },
  "published": "2026-10-03T09:17:05.607",
  "references": [
    {
      "source": "91579145-5d7b-4cc5-b925-a0262ff19630",
      "url": "https://github.com/bcgit/bc-java/commit/7e8bb10eb90baddf3f10b8679f627462b9522d24"
    },
    {
      "source": "91579145-5d7b-4cc5-b925-a0262ff19630",
      "url": "https://github.com/bcgit/bc-java/wiki/CVE%E2%80%902026%E2%80%9071890"
    }
  ],
  "sourceIdentifier": "91579145-5d7b-4cc5-b925-a0262ff19630",
  "vulnStatus": "Received",
  "weaknesses": [
    {
      "description": [
        {
          "lang": "en",
          "value": "CWE-863"
        }
      ],
      "source": "91579145-5d7b-4cc5-b925-a0262ff19630",
      "type": "Secondary"
    }
  ]
}

CVE-2026-71890 (GCVE-0-2026-71890)

Vulnerability from cvelistv5 – Published: 2026-10-03 08:57 – Updated: 2026-10-03 08:57

Title

MLS external commit can remove an arbitrary group member

Summary

Severity

8.7 (High)


                        
                          CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N/U:Amber

CWE

CWE-863 - Incorrect Authorization

Assigner

bcorg mitre root Scorecard

References

2 references

URL	Tags
https://github.com/bcgit/bc-java/wiki/CVE%E2%80%9…	vendor-advisory
https://github.com/bcgit/bc-java/commit/7e8bb10eb…	patch

Impacted products

1 product

Vendor	Product	Version
Legion of the Bouncy Castle Inc.	BC-JAVA	Affected: 1.73 , < 1.86 (maven)

Credits

Yu Bao from the PayPal Cyber Security Team

Show details on NVD website

JSON

To clipboard

{
  "containers": {
    "cna": {
      "affected": [
        {
          "collectionURL": "https://www.bouncycastle.org/download/bouncy-castle-java/",
          "defaultStatus": "unaffected",
          "modules": [
            "mls"
          ],
          "packageName": "bcmls",
          "packageURL": "pkg:maven/org.bouncycastle/bcmls-jdk18on",
          "platforms": [
            "all"
          ],
          "product": "BC-JAVA",
          "programFiles": [
            "Group"
          ],
          "repo": "https://github.com/bcgit/bc-java",
          "vendor": "Legion of the Bouncy Castle Inc.",
          "versions": [
            {
              "lessThan": "1.86",
              "status": "affected",
              "version": "1.73",
              "versionType": "maven"
            }
          ]
        }
      ],
      "credits": [
        {
          "lang": "en",
          "type": "reporter",
          "value": "Yu Bao from the PayPal Cyber Security Team"
        }
      ],
      "descriptions": [
        {
          "lang": "en",
          "supportingMedia": [
            {
              "base64": false,
              "type": "text/html",
              "value": "\u003cp\u003eIn Bouncy Castle for Java before 1.86, validation of an MLS (RFC 9420) external commit\u0027s proposal list, org.bouncycastle.mls.protocol.Group.validateExternalCachedProposals, counted the proposals by type and bounded the removed leaf index but never established that the removed leaf had anything to do with the joiner. RFC 9420 sec. 12.2 permits at most one Remove proposal in an external commit, with which the joiner removes an old version of themselves, and requires that where one is present the LeafNode in the commit\u0027s path field meet the criteria it would have to meet in an Update for the removed leaf, in particular that its credential present identifiers acceptable for the removed participant. The ordinary proposal-list validator\u0027s self-remove rule is deliberately not applied on this path, because a resync commit legitimately removes a leaf the joiner owns, but nothing was put in its place. Any party holding the group\u0027s public GroupInfo, which is precisely what an external joiner is meant to be given, could therefore commit a Remove naming any member\u0027s LeafIndex and have every member apply it, evicting that member and taking over their slot in the ratchet tree. The credential check that should have prevented this existed only in the gRPC interop harness and so protected no other caller of the public Group.externalJoin and Group.handle API. An external commit carrying a Remove is now accepted only when the removed leaf\u0027s credential is identical to the one in the joiner\u0027s own new leaf, on both the sending and the receiving side.\u003c/p\u003e"
            }
          ],
          "value": "In Bouncy Castle for Java before 1.86, validation of an MLS (RFC 9420) external commit\u0027s proposal list, org.bouncycastle.mls.protocol.Group.validateExternalCachedProposals, counted the proposals by type and bounded the removed leaf index but never established that the removed leaf had anything to do with the joiner. RFC 9420 sec. 12.2 permits at most one Remove proposal in an external commit, with which the joiner removes an old version of themselves, and requires that where one is present the LeafNode in the commit\u0027s path field meet the criteria it would have to meet in an Update for the removed leaf, in particular that its credential present identifiers acceptable for the removed participant. The ordinary proposal-list validator\u0027s self-remove rule is deliberately not applied on this path, because a resync commit legitimately removes a leaf the joiner owns, but nothing was put in its place. Any party holding the group\u0027s public GroupInfo, which is precisely what an external joiner is meant to be given, could therefore commit a Remove naming any member\u0027s LeafIndex and have every member apply it, evicting that member and taking over their slot in the ratchet tree. The credential check that should have prevented this existed only in the gRPC interop harness and so protected no other caller of the public Group.externalJoin and Group.handle API. An external commit carrying a Remove is now accepted only when the removed leaf\u0027s credential is identical to the one in the joiner\u0027s own new leaf, on both the sending and the receiving side."
        }
      ],
      "metrics": [
        {
          "cvssV4_0": {
            "Automatable": "NOT_DEFINED",
            "Recovery": "NOT_DEFINED",
            "Safety": "NOT_DEFINED",
            "attackComplexity": "LOW",
            "attackRequirements": "NONE",
            "attackVector": "NETWORK",
            "baseScore": 8.7,
            "baseSeverity": "HIGH",
            "exploitMaturity": "NOT_DEFINED",
            "privilegesRequired": "NONE",
            "providerUrgency": "AMBER",
            "subAvailabilityImpact": "NONE",
            "subConfidentialityImpact": "NONE",
            "subIntegrityImpact": "NONE",
            "userInteraction": "NONE",
            "valueDensity": "NOT_DEFINED",
            "vectorString": "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N/U:Amber",
            "version": "4.0",
            "vulnAvailabilityImpact": "NONE",
            "vulnConfidentialityImpact": "NONE",
            "vulnIntegrityImpact": "HIGH",
            "vulnerabilityResponseEffort": "NOT_DEFINED"
          },
          "format": "CVSS",
          "scenarios": [
            {
              "lang": "en",
              "value": "GENERAL"
            }
          ]
        }
      ],
      "problemTypes": [
        {
          "descriptions": [
            {
              "cweId": "CWE-863",
              "description": "CWE-863 Incorrect Authorization",
              "lang": "en",
              "type": "CWE"
            }
          ]
        }
      ],
      "providerMetadata": {
        "dateUpdated": "2026-10-03T08:57:00.965Z",
        "orgId": "91579145-5d7b-4cc5-b925-a0262ff19630",
        "shortName": "bcorg"
      },
      "references": [
        {
          "tags": [
            "vendor-advisory"
          ],
          "url": "https://github.com/bcgit/bc-java/wiki/CVE%E2%80%902026%E2%80%9071890"
        },
        {
          "tags": [
            "patch"
          ],
          "url": "https://github.com/bcgit/bc-java/commit/7e8bb10eb90baddf3f10b8679f627462b9522d24"
        }
      ],
      "source": {
        "discovery": "EXTERNAL"
      },
      "title": "MLS external commit can remove an arbitrary group member",
      "x_generator": {
        "engine": "bc release-notes template"
      }
    }
  },
  "cveMetadata": {
    "assignerOrgId": "91579145-5d7b-4cc5-b925-a0262ff19630",
    "assignerShortName": "bcorg",
    "cveId": "CVE-2026-71890",
    "datePublished": "2026-10-03T08:57:00.965Z",
    "dateReserved": "2026-08-08T00:06:07.401Z",
    "dateUpdated": "2026-10-03T08:57:00.965Z",
    "state": "PUBLISHED"
  },
  "dataType": "CVE_RECORD",
  "dataVersion": "5.2"
}

Sightings

Author	Source	Type	Date	Other

Nomenclature

Seen: The vulnerability was mentioned, discussed, or observed by the user.
Confirmed: The vulnerability has been validated from an analyst's perspective.
Published Proof of Concept: A public proof of concept is available for this vulnerability.
Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
Not confirmed: The user expressed doubt about the validity of the vulnerability.
Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.

The MITRE ATT&CK techniques below are AI-generated suggestions, inferred from the description of the vulnerability by the CIRCL/vulnerability-attack-technique-classification-roberta-base model, served locally by ML-Gateway. They have not been verified by an analyst and are provided for guidance only.
The approach is described in our paper Mapping CVEs to MITRE ATT&CK Techniques: A Curated Gold-Set Classifier and the Limits of LLM-Assisted Label Expansion.
Browse all ATT&CK techniques and the vulnerabilities related to each.

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FKIE_CVE-2026-71890

CVE-2026-71890 (GCVE-0-2026-71890)

Tags

Sightings

Nomenclature

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