Vulnerability-Lookup

CVE-2026-63466 (GCVE-0-2026-63466)

Vulnerability from cvelistv5 – Published: 2026-08-21 18:21 – Updated: 2026-08-21 21:43

Title

Unleash: Global Mustache.escape override disables HTML escaping process-wide, enabling Slack/Teams link-injection via unrestricted username

Summary

Unleash is an open-source feature management platform. Prior to 8.0.3, FeatureEventFormatterMd.format in src/lib/addons/feature-event-formatter-md.ts assigns Mustache.escape to an identity function before rendering action and path templates. Because Mustache.escape is process-wide, the assignment disables escaping for subsequent Mustache.render calls in email-service.ts, webhook.ts, datadog.ts, and new-relic.ts. An editor-level user can place Slack or Microsoft Teams link syntax in an unrestricted username, trigger a feature event, and inject an attacker-labeled link into a trusted outbound notification channel, while other Mustache sinks remain unescaped until restart. This issue is fixed in version 8.0.3.

Severity

4.1 (Medium)


                        
                          CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:N/I:L/A:N

SSVC

Exploitation: none Automatable: no Technical Impact: partial

CISA Coordinator · CISA-ADP (v2.0.3)

Decision recorded 2026-08-21 20:58 UTC

CWE

CWE-116 - Improper Encoding or Escaping of Output

Assigner

GitHub_M mitre root

References

3 references

URL	Tags
https://github.com/Unleash/unleash/security/advis…	x_refsource_CONFIRM
https://github.com/Unleash/unleash/commit/002012c…	x_refsource_MISC
https://github.com/Unleash/unleash/releases/tag/v8.0.3	x_refsource_MISC

Impacted products

1 product

Vendor	Product	Version	CPE status
Unleash	unleash	Affected: < 8.0.3	guessed

Show details on NVD website

JSON

To clipboard

{
  "containers": {
    "adp": [
      {
        "metrics": [
          {
            "other": {
              "content": {
                "id": "CVE-2026-63466",
                "options": [
                  {
                    "Exploitation": "none"
                  },
                  {
                    "Automatable": "no"
                  },
                  {
                    "Technical Impact": "partial"
                  }
                ],
                "role": "CISA Coordinator",
                "timestamp": "2026-08-21T20:58:22.218056Z",
                "version": "2.0.3"
              },
              "type": "ssvc"
            }
          }
        ],
        "providerMetadata": {
          "dateUpdated": "2026-08-21T21:43:35.526Z",
          "orgId": "134c704f-9b21-4f2e-91b3-4a467353bcc0",
          "shortName": "CISA-ADP"
        },
        "title": "CISA ADP Vulnrichment"
      }
    ],
    "cna": {
      "affected": [
        {
          "product": "unleash",
          "vendor": "Unleash",
          "versions": [
            {
              "status": "affected",
              "version": "\u003c 8.0.3"
            }
          ]
        }
      ],
      "descriptions": [
        {
          "lang": "en",
          "value": "Unleash is an open-source feature management platform. Prior to 8.0.3, FeatureEventFormatterMd.format in src/lib/addons/feature-event-formatter-md.ts assigns Mustache.escape to an identity function before rendering action and path templates. Because Mustache.escape is process-wide, the assignment disables escaping for subsequent Mustache.render calls in email-service.ts, webhook.ts, datadog.ts, and new-relic.ts. An editor-level user can place Slack or Microsoft Teams link syntax in an unrestricted username, trigger a feature event, and inject an attacker-labeled link into a trusted outbound notification channel, while other Mustache sinks remain unescaped until restart. This issue is fixed in version 8.0.3."
        }
      ],
      "metrics": [
        {
          "cvssV3_1": {
            "attackComplexity": "LOW",
            "attackVector": "NETWORK",
            "availabilityImpact": "NONE",
            "baseScore": 4.1,
            "baseSeverity": "MEDIUM",
            "confidentialityImpact": "NONE",
            "integrityImpact": "LOW",
            "privilegesRequired": "LOW",
            "scope": "CHANGED",
            "userInteraction": "REQUIRED",
            "vectorString": "CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:N/I:L/A:N",
            "version": "3.1"
          }
        }
      ],
      "problemTypes": [
        {
          "descriptions": [
            {
              "cweId": "CWE-116",
              "description": "CWE-116: Improper Encoding or Escaping of Output",
              "lang": "en",
              "type": "CWE"
            }
          ]
        }
      ],
      "providerMetadata": {
        "dateUpdated": "2026-08-21T18:21:46.526Z",
        "orgId": "a0819718-46f1-4df5-94e2-005712e83aaa",
        "shortName": "GitHub_M"
      },
      "references": [
        {
          "name": "https://github.com/Unleash/unleash/security/advisories/GHSA-w4mq-xh27-6xpx",
          "tags": [
            "x_refsource_CONFIRM"
          ],
          "url": "https://github.com/Unleash/unleash/security/advisories/GHSA-w4mq-xh27-6xpx"
        },
        {
          "name": "https://github.com/Unleash/unleash/commit/002012cfdbedd2e9b7db9dc83b9f549f761db22e",
          "tags": [
            "x_refsource_MISC"
          ],
          "url": "https://github.com/Unleash/unleash/commit/002012cfdbedd2e9b7db9dc83b9f549f761db22e"
        },
        {
          "name": "https://github.com/Unleash/unleash/releases/tag/v8.0.3",
          "tags": [
            "x_refsource_MISC"
          ],
          "url": "https://github.com/Unleash/unleash/releases/tag/v8.0.3"
        }
      ],
      "source": {
        "advisory": "GHSA-w4mq-xh27-6xpx",
        "discovery": "UNKNOWN"
      },
      "title": "Unleash: Global Mustache.escape override disables HTML escaping process-wide, enabling Slack/Teams link-injection via unrestricted username"
    }
  },
  "cveMetadata": {
    "assignerOrgId": "a0819718-46f1-4df5-94e2-005712e83aaa",
    "assignerShortName": "GitHub_M",
    "cveId": "CVE-2026-63466",
    "datePublished": "2026-08-21T18:21:46.526Z",
    "dateReserved": "2026-07-16T21:37:45.768Z",
    "dateUpdated": "2026-08-21T21:43:35.526Z",
    "state": "PUBLISHED"
  },
  "dataType": "CVE_RECORD",
  "dataVersion": "5.2",
  "vulnerability-lookup:meta": {
    "epss": {
      "cve": "CVE-2026-63466",
      "date": "2026-09-29",
      "epss": "0.00319",
      "percentile": "0.22395"
    },
    "nvd": {
      "cve": {
        "affected": [
          {
            "affectedData": [
              {
                "product": "unleash",
                "vendor": "Unleash",
                "versions": [
                  {
                    "status": "affected",
                    "version": "\u003c 8.0.3"
                  }
                ]
              }
            ],
            "source": "security-advisories@github.com"
          }
        ],
        "cveTags": [],
        "descriptions": [
          {
            "lang": "en",
            "value": "Unleash is an open-source feature management platform. Prior to 8.0.3, FeatureEventFormatterMd.format in src/lib/addons/feature-event-formatter-md.ts assigns Mustache.escape to an identity function before rendering action and path templates. Because Mustache.escape is process-wide, the assignment disables escaping for subsequent Mustache.render calls in email-service.ts, webhook.ts, datadog.ts, and new-relic.ts. An editor-level user can place Slack or Microsoft Teams link syntax in an unrestricted username, trigger a feature event, and inject an attacker-labeled link into a trusted outbound notification channel, while other Mustache sinks remain unescaped until restart. This issue is fixed in version 8.0.3."
          }
        ],
        "id": "CVE-2026-63466",
        "lastModified": "2026-09-09T21:20:38.860",
        "metrics": {
          "cvssMetricV31": [
            {
              "cvssData": {
                "attackComplexity": "LOW",
                "attackVector": "NETWORK",
                "availabilityImpact": "NONE",
                "baseScore": 4.1,
                "baseSeverity": "MEDIUM",
                "confidentialityImpact": "NONE",
                "integrityImpact": "LOW",
                "privilegesRequired": "LOW",
                "scope": "CHANGED",
                "userInteraction": "REQUIRED",
                "vectorString": "CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:N/I:L/A:N",
                "version": "3.1"
              },
              "exploitabilityScore": 2.3,
              "impactScore": 1.4,
              "source": "security-advisories@github.com",
              "type": "Secondary"
            }
          ],
          "ssvcV203": [
            {
              "source": "134c704f-9b21-4f2e-91b3-4a467353bcc0",
              "ssvcData": {
                "id": "CVE-2026-63466",
                "options": [
                  {
                    "exploitation": "none"
                  },
                  {
                    "automatable": "no"
                  },
                  {
                    "technicalImpact": "partial"
                  }
                ],
                "role": "CISA Coordinator",
                "timestamp": "2026-08-21T20:58:22.218056Z",
                "version": "2.0.3"
              }
            }
          ]
        },
        "published": "2026-08-21T19:17:32.117",
        "references": [
          {
            "source": "security-advisories@github.com",
            "url": "https://github.com/Unleash/unleash/commit/002012cfdbedd2e9b7db9dc83b9f549f761db22e"
          },
          {
            "source": "security-advisories@github.com",
            "url": "https://github.com/Unleash/unleash/releases/tag/v8.0.3"
          },
          {
            "source": "security-advisories@github.com",
            "url": "https://github.com/Unleash/unleash/security/advisories/GHSA-w4mq-xh27-6xpx"
          }
        ],
        "sourceIdentifier": "security-advisories@github.com",
        "vulnStatus": "Deferred",
        "weaknesses": [
          {
            "description": [
              {
                "lang": "en",
                "value": "CWE-116"
              }
            ],
            "source": "security-advisories@github.com",
            "type": "Secondary"
          }
        ]
      }
    },
    "vulnrichment": {
      "containers": {
        "adp": [
          {
            "metrics": [
              {
                "other": {
                  "content": {
                    "id": "CVE-2026-63466",
                    "options": [
                      {
                        "Exploitation": "none"
                      },
                      {
                        "Automatable": "no"
                      },
                      {
                        "Technical Impact": "partial"
                      }
                    ],
                    "role": "CISA Coordinator",
                    "timestamp": "2026-08-21T20:58:22.218056Z",
                    "version": "2.0.3"
                  },
                  "type": "ssvc"
                }
              }
            ],
            "providerMetadata": {
              "dateUpdated": "2026-08-21T20:58:23.502Z",
              "orgId": "134c704f-9b21-4f2e-91b3-4a467353bcc0",
              "shortName": "CISA-ADP"
            },
            "title": "CISA ADP Vulnrichment"
          }
        ],
        "cna": {
          "affected": [
            {
              "product": "unleash",
              "vendor": "Unleash",
              "versions": [
                {
                  "status": "affected",
                  "version": "\u003c 8.0.3"
                }
              ]
            }
          ],
          "descriptions": [
            {
              "lang": "en",
              "value": "Unleash is an open-source feature management platform. Prior to 8.0.3, FeatureEventFormatterMd.format in src/lib/addons/feature-event-formatter-md.ts assigns Mustache.escape to an identity function before rendering action and path templates. Because Mustache.escape is process-wide, the assignment disables escaping for subsequent Mustache.render calls in email-service.ts, webhook.ts, datadog.ts, and new-relic.ts. An editor-level user can place Slack or Microsoft Teams link syntax in an unrestricted username, trigger a feature event, and inject an attacker-labeled link into a trusted outbound notification channel, while other Mustache sinks remain unescaped until restart. This issue is fixed in version 8.0.3."
            }
          ],
          "metrics": [
            {
              "cvssV3_1": {
                "attackComplexity": "LOW",
                "attackVector": "NETWORK",
                "availabilityImpact": "NONE",
                "baseScore": 4.1,
                "baseSeverity": "MEDIUM",
                "confidentialityImpact": "NONE",
                "integrityImpact": "LOW",
                "privilegesRequired": "LOW",
                "scope": "CHANGED",
                "userInteraction": "REQUIRED",
                "vectorString": "CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:N/I:L/A:N",
                "version": "3.1"
              }
            }
          ],
          "problemTypes": [
            {
              "descriptions": [
                {
                  "cweId": "CWE-116",
                  "description": "CWE-116: Improper Encoding or Escaping of Output",
                  "lang": "en",
                  "type": "CWE"
                }
              ]
            }
          ],
          "providerMetadata": {
            "dateUpdated": "2026-08-21T18:21:46.526Z",
            "orgId": "a0819718-46f1-4df5-94e2-005712e83aaa",
            "shortName": "GitHub_M"
          },
          "references": [
            {
              "name": "https://github.com/Unleash/unleash/security/advisories/GHSA-w4mq-xh27-6xpx",
              "tags": [
                "x_refsource_CONFIRM"
              ],
              "url": "https://github.com/Unleash/unleash/security/advisories/GHSA-w4mq-xh27-6xpx"
            },
            {
              "name": "https://github.com/Unleash/unleash/commit/002012cfdbedd2e9b7db9dc83b9f549f761db22e",
              "tags": [
                "x_refsource_MISC"
              ],
              "url": "https://github.com/Unleash/unleash/commit/002012cfdbedd2e9b7db9dc83b9f549f761db22e"
            },
            {
              "name": "https://github.com/Unleash/unleash/releases/tag/v8.0.3",
              "tags": [
                "x_refsource_MISC"
              ],
              "url": "https://github.com/Unleash/unleash/releases/tag/v8.0.3"
            }
          ],
          "source": {
            "advisory": "GHSA-w4mq-xh27-6xpx",
            "discovery": "UNKNOWN"
          },
          "title": "Unleash: Global Mustache.escape override disables HTML escaping process-wide, enabling Slack/Teams link-injection via unrestricted username"
        }
      },
      "cveMetadata": {
        "assignerOrgId": "a0819718-46f1-4df5-94e2-005712e83aaa",
        "assignerShortName": "GitHub_M",
        "cveId": "CVE-2026-63466",
        "datePublished": "2026-08-21T18:21:46.526Z",
        "dateReserved": "2026-07-16T21:37:45.768Z",
        "dateUpdated": "2026-08-21T21:43:35.526Z",
        "state": "PUBLISHED"
      },
      "dataType": "CVE_RECORD",
      "dataVersion": "5.2"
    }
  }
}

FKIE_CVE-2026-63466

Vulnerability from fkie_nvd - Published: 2026-08-21 19:17 - Updated: 2026-09-09 21:20

Severity

4.1 (Medium) - CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:N/I:L/A:N

Summary

References

	URL	Tags
	security-advisories@github.com	https://github.com/Unleash/unleash/commit/002012cfdbedd2e9b7db9dc83b9f549f761db22e
	security-advisories@github.com	https://github.com/Unleash/unleash/releases/tag/v8.0.3
	security-advisories@github.com	https://github.com/Unleash/unleash/security/advisories/GHSA-w4mq-xh27-6xpx

Impacted products

	Vendor	Product	Version

JSON

To clipboard

{
  "affected": [
    {
      "affectedData": [
        {
          "product": "unleash",
          "vendor": "Unleash",
          "versions": [
            {
              "status": "affected",
              "version": "\u003c 8.0.3"
            }
          ]
        }
      ],
      "source": "security-advisories@github.com"
    }
  ],
  "cveTags": [],
  "descriptions": [
    {
      "lang": "en",
      "value": "Unleash is an open-source feature management platform. Prior to 8.0.3, FeatureEventFormatterMd.format in src/lib/addons/feature-event-formatter-md.ts assigns Mustache.escape to an identity function before rendering action and path templates. Because Mustache.escape is process-wide, the assignment disables escaping for subsequent Mustache.render calls in email-service.ts, webhook.ts, datadog.ts, and new-relic.ts. An editor-level user can place Slack or Microsoft Teams link syntax in an unrestricted username, trigger a feature event, and inject an attacker-labeled link into a trusted outbound notification channel, while other Mustache sinks remain unescaped until restart. This issue is fixed in version 8.0.3."
    }
  ],
  "id": "CVE-2026-63466",
  "lastModified": "2026-09-09T21:20:38.860",
  "metrics": {
    "cvssMetricV31": [
      {
        "cvssData": {
          "attackComplexity": "LOW",
          "attackVector": "NETWORK",
          "availabilityImpact": "NONE",
          "baseScore": 4.1,
          "baseSeverity": "MEDIUM",
          "confidentialityImpact": "NONE",
          "integrityImpact": "LOW",
          "privilegesRequired": "LOW",
          "scope": "CHANGED",
          "userInteraction": "REQUIRED",
          "vectorString": "CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:N/I:L/A:N",
          "version": "3.1"
        },
        "exploitabilityScore": 2.3,
        "impactScore": 1.4,
        "source": "security-advisories@github.com",
        "type": "Secondary"
      }
    ],
    "ssvcV203": [
      {
        "source": "134c704f-9b21-4f2e-91b3-4a467353bcc0",
        "ssvcData": {
          "id": "CVE-2026-63466",
          "options": [
            {
              "exploitation": "none"
            },
            {
              "automatable": "no"
            },
            {
              "technicalImpact": "partial"
            }
          ],
          "role": "CISA Coordinator",
          "timestamp": "2026-08-21T20:58:22.218056Z",
          "version": "2.0.3"
        }
      }
    ]
  },
  "published": "2026-08-21T19:17:32.117",
  "references": [
    {
      "source": "security-advisories@github.com",
      "url": "https://github.com/Unleash/unleash/commit/002012cfdbedd2e9b7db9dc83b9f549f761db22e"
    },
    {
      "source": "security-advisories@github.com",
      "url": "https://github.com/Unleash/unleash/releases/tag/v8.0.3"
    },
    {
      "source": "security-advisories@github.com",
      "url": "https://github.com/Unleash/unleash/security/advisories/GHSA-w4mq-xh27-6xpx"
    }
  ],
  "sourceIdentifier": "security-advisories@github.com",
  "vulnStatus": "Deferred",
  "weaknesses": [
    {
      "description": [
        {
          "lang": "en",
          "value": "CWE-116"
        }
      ],
      "source": "security-advisories@github.com",
      "type": "Secondary"
    }
  ]
}

GHSA-W4MQ-XH27-6XPX

Vulnerability from github – Published: 2026-08-21 19:14 – Updated: 2026-08-21 19:14

Summary

Unleash: Global Mustache.escape override disables HTML escaping process-wide, enabling Slack/Teams link-injection via unrestricted username

Details

Vulnerability Details

File: src/lib/addons/feature-event-formatter-md.ts Line: 355 (in v8.0.1; format() method)

Root Cause

FeatureEventFormatterMd.format() does:

Mustache.escape = (text) => text;
const text = Mustache.render(action, context);

mustache (pinned ^4.2.0, confirmed installed 4.2.0) keeps escape as a module-level singleton (mustache.js: mustache.escape = escapeHtml;), read by every Mustache.render() call in the process unless a per-call config.escape override is passed (var escape = this.getConfigEscape(config) || mustache.escape;). Node's module cache guarantees every import Mustache from 'mustache' in the process — feature-event-formatter-md.ts, email-service.ts, webhook.ts, datadog.ts, new-relic.ts — shares the same object instance.

This assignment therefore permanently disables HTML escaping for every other Mustache.render() call in the same Node process (including email-service.ts templates) from the moment any single notification addon (Webhook, Slack legacy, Microsoft Teams, Datadog, New Relic) first formats any event, for the remaining lifetime of the process.

feature-event-formatter-md-events.ts (EVENT_MAP) confirms the blast radius: nearly every event's action template interpolates attacker-controlled values with single-mustache (intended-to-be-escaped) syntax, most importantly {{user}}, which is event.createdBy — the acting account's username (or email if set; src/lib/util/extract-user.ts: extractUsernameFromUser). Neither username nor name have any charset/length validation anywhere in the codebase (create-user-schema.ts, create-invited-user-schema.ts, user-service.ts:289 only does Joi.assert(name, Joi.string(), 'Name') — a type check, nothing more).

Slack's own API docs require &, <, > to be replaced with &, <, > before sending user-generated text, specifically so Slack's mrkdwn parser does not interpret it as <url|label> link syntax. Mustache's default escapeHtml happens to produce exactly those entities, so this was (likely unintentionally) the application's only defense against link-injection in chat notifications — and it is unconditionally switched off by the same code path that depends on it.

Attack Scenario

Admin has a Webhook, Slack (legacy), Microsoft Teams, Datadog, or New Relic integration configured (a very common production setup for flag-change notifications).
Attacker has (or self-registers, if public signup is enabled — POST /invite/:token/signup is permission: NONE) any Editor-level account and sets username to e.g. evil<https://attacker.example/urgent-rollback|Click here to view incident>.
Attacker performs any ordinary write action (create/update/toggle a feature flag — routine, no special privilege beyond Editor on one project).
The configured addon's handleEvent() calls this.msgFormatter.format(event), which mutates the global escape function and immediately renders the {{user}}-containing template with escaping disabled.
The resulting message — containing the attacker's raw <url|label> Slack link syntax — is POSTed to the team's Slack/Teams channel or webhook endpoint and rendered as a real, clickable, attacker-labeled hyperlink inside a trusted automated notification feed.

Vulnerable Code

Mustache.escape = (text) => text;

const text = Mustache.render(action, context);
const url = path
    ? `${this.unleashUrl}${Mustache.render(path, context)}`
    : undefined;

Impact

Stored markdown/link-injection (phishing-link injection) into any configured outbound notification channel (Slack legacy, MS Teams, Webhook default markdown, Datadog, New Relic), using an attacker-controlled username — no admin privilege required, only Editor on a single project, and potentially reachable through public self-signup.
Secondary: loss of HTML escaping for any other reachable Mustache single-mustache placeholder process-wide until restart (increases severity of any other currently-unreached or future Mustache sink, e.g. email templates).
Tertiary: a custom Webhook bodyTemplate that interpolates raw event/user fields directly into a JSON string literal (rather than the pre-escaped eventJson field the code already provides for this purpose) can have its JSON structure broken by an attacker-controlled " character once the global escape function is neutered.

Recommended Fix

Never mutate the shared Mustache.escape global. Pass a local escape function via Mustache's per-call render option instead (supported and typed in @types/mustache@4.2.6's RenderOptions.escape):

const renderConfig = { escape: (text: string) => text };
const text = Mustache.render(action, context, undefined, renderConfig);
const url = path
    ? `${this.unleashUrl}${Mustache.render(path, context, undefined, renderConfig)}`
    : undefined;

Verification

Dynamically confirmed on v8.0.1 in a local Docker lab (official unleashorg/unleash-server:8.0.1 image + Postgres 15): - Created a Webhook addon with the addon UI's own placeholder bodyTemplate ({{event.createdBy}} etc.), pointed at a local listener. - Created an Editor-role user with username = evil2<https://attacker.example/urgent-rollback|Click here to view incident> (accepted with HTTP 201, no sanitization). - Logged in as that user and created a feature flag (ordinary Editor action). - Captured webhook payload: "createdBy": "evil2<https://attacker.example/urgent-rollback|Click here to view incident>" — <, >, | completely unescaped, live Slack link-injection syntax. - Control test with the same pinned mustache@4.2.0 package confirmed the default (pre-bug) output for the same string would have been evil2<https://attacker.example/urgent-rollback|Click here to view incident> — i.e. the single global assignment is solely responsible for the unescaped output observed live.

Severity

4.1 (Medium)


                  
                    CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:N/I:L/A:N

Show details on source website

JSON

To clipboard

{
  "affected": [
    {
      "package": {
        "ecosystem": "npm",
        "name": "unleash-server"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "8.0.3"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-63466"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-116"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-08-21T19:14:52Z",
    "nvd_published_at": null,
    "severity": "MODERATE"
  },
  "details": "## Vulnerability Details\n\n**File**: `src/lib/addons/feature-event-formatter-md.ts`\n**Line**: 355 (in v8.0.1; `format()` method)\n\n### Root Cause\n\n`FeatureEventFormatterMd.format()` does:\n\n```ts\nMustache.escape = (text) =\u003e text;\nconst text = Mustache.render(action, context);\n```\n\n`mustache` (pinned `^4.2.0`, confirmed installed `4.2.0`) keeps `escape` as a **module-level singleton** (`mustache.js`: `mustache.escape = escapeHtml;`), read by every `Mustache.render()` call in the process unless a per-call `config.escape` override is passed (`var escape = this.getConfigEscape(config) || mustache.escape;`). Node\u0027s module cache guarantees every `import Mustache from \u0027mustache\u0027` in the process \u2014 `feature-event-formatter-md.ts`, `email-service.ts`, `webhook.ts`, `datadog.ts`, `new-relic.ts` \u2014 shares the same object instance.\n\nThis assignment therefore **permanently disables HTML escaping for every other `Mustache.render()` call in the same Node process** (including `email-service.ts` templates) from the moment any single notification addon (Webhook, Slack legacy, Microsoft Teams, Datadog, New Relic) first formats any event, for the remaining lifetime of the process.\n\n`feature-event-formatter-md-events.ts` (`EVENT_MAP`) confirms the blast radius: nearly every event\u0027s `action` template interpolates attacker-controlled values with single-mustache (intended-to-be-escaped) syntax, most importantly `{{user}}`, which is `event.createdBy` \u2014 the acting account\u0027s `username` (or `email` if set; `src/lib/util/extract-user.ts`: `extractUsernameFromUser`). Neither `username` nor `name` have any charset/length validation anywhere in the codebase (`create-user-schema.ts`, `create-invited-user-schema.ts`, `user-service.ts:289` only does `Joi.assert(name, Joi.string(), \u0027Name\u0027)` \u2014 a type check, nothing more).\n\nSlack\u0027s own API docs require `\u0026`, `\u003c`, `\u003e` to be replaced with `\u0026amp;`, `\u0026lt;`, `\u0026gt;` before sending user-generated text, specifically so Slack\u0027s mrkdwn parser does not interpret it as `\u003curl|label\u003e` link syntax. Mustache\u0027s default `escapeHtml` happens to produce exactly those entities, so this was (likely unintentionally) the application\u0027s only defense against link-injection in chat notifications \u2014 and it is unconditionally switched off by the same code path that depends on it.\n\n### Attack Scenario\n1. Admin has a Webhook, Slack (legacy), Microsoft Teams, Datadog, or New Relic integration configured (a very common production setup for flag-change notifications).\n2. Attacker has (or self-registers, if public signup is enabled \u2014 `POST /invite/:token/signup` is `permission: NONE`) any **Editor**-level account and sets `username` to e.g. `evil\u003chttps://attacker.example/urgent-rollback|Click here to view incident\u003e`.\n3. Attacker performs any ordinary write action (create/update/toggle a feature flag \u2014 routine, no special privilege beyond Editor on one project).\n4. The configured addon\u0027s `handleEvent()` calls `this.msgFormatter.format(event)`, which mutates the global escape function and immediately renders the `{{user}}`-containing template with escaping disabled.\n5. The resulting message \u2014 containing the attacker\u0027s raw `\u003curl|label\u003e` Slack link syntax \u2014 is POSTed to the team\u0027s Slack/Teams channel or webhook endpoint and rendered as a real, clickable, attacker-labeled hyperlink inside a trusted automated notification feed.\n\n### Vulnerable Code\n```ts\nMustache.escape = (text) =\u003e text;\n\nconst text = Mustache.render(action, context);\nconst url = path\n    ? `${this.unleashUrl}${Mustache.render(path, context)}`\n    : undefined;\n```\n\n### Impact\n- Stored markdown/link-injection (phishing-link injection) into any configured outbound notification channel (Slack legacy, MS Teams, Webhook default markdown, Datadog, New Relic), using an attacker-controlled username \u2014 no admin privilege required, only Editor on a single project, and potentially reachable through public self-signup.\n- Secondary: loss of HTML escaping for any other reachable Mustache single-mustache placeholder process-wide until restart (increases severity of any other currently-unreached or future Mustache sink, e.g. email templates).\n- Tertiary: a custom Webhook `bodyTemplate` that interpolates raw event/user fields directly into a JSON string literal (rather than the pre-escaped `eventJson` field the code already provides for this purpose) can have its JSON structure broken by an attacker-controlled `\"` character once the global escape function is neutered.\n\n### Recommended Fix\nNever mutate the shared `Mustache.escape` global. Pass a local escape function via Mustache\u0027s per-call render option instead (supported and typed in `@types/mustache@4.2.6`\u0027s `RenderOptions.escape`):\n\n```ts\nconst renderConfig = { escape: (text: string) =\u003e text };\nconst text = Mustache.render(action, context, undefined, renderConfig);\nconst url = path\n    ? `${this.unleashUrl}${Mustache.render(path, context, undefined, renderConfig)}`\n    : undefined;\n```\n\n### Verification\nDynamically confirmed on v8.0.1 in a local Docker lab (official `unleashorg/unleash-server:8.0.1` image + Postgres 15):\n- Created a Webhook addon with the addon UI\u0027s own placeholder `bodyTemplate` (`{{event.createdBy}}` etc.), pointed at a local listener.\n- Created an Editor-role user with `username = evil2\u003chttps://attacker.example/urgent-rollback|Click here to view incident\u003e` (accepted with HTTP 201, no sanitization).\n- Logged in as that user and created a feature flag (ordinary Editor action).\n- Captured webhook payload: `\"createdBy\": \"evil2\u003chttps://attacker.example/urgent-rollback|Click here to view incident\u003e\"` \u2014 `\u003c`, `\u003e`, `|` completely unescaped, live Slack link-injection syntax.\n- Control test with the same pinned `mustache@4.2.0` package confirmed the default (pre-bug) output for the same string would have been `evil2\u0026lt;https:\u0026#x2F;\u0026#x2F;attacker.example\u0026#x2F;urgent-rollback|Click here to view incident\u0026gt;` \u2014 i.e. the single global assignment is solely responsible for the unescaped output observed live.",
  "id": "GHSA-w4mq-xh27-6xpx",
  "modified": "2026-08-21T19:14:52Z",
  "published": "2026-08-21T19:14:52Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/Unleash/unleash/security/advisories/GHSA-w4mq-xh27-6xpx"
    },
    {
      "type": "WEB",
      "url": "https://github.com/Unleash/unleash/commit/002012cfdbedd2e9b7db9dc83b9f549f761db22e"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/Unleash/unleash"
    },
    {
      "type": "WEB",
      "url": "https://github.com/Unleash/unleash/releases/tag/v8.0.3"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:N/I:L/A:N",
      "type": "CVSS_V3"
    }
  ],
  "summary": "Unleash: Global Mustache.escape override disables HTML escaping process-wide, enabling Slack/Teams link-injection via unrestricted username"
}

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.

Related by attack behaviour

Vulnerabilities whose description is nearest to this one in the vector space of the CIRCL/vulnerability-attack-technique-biencoder model. This is a similarity search over the bi-encoder space (plain cosine), not a classification, and it has no measured accuracy.

Action not permitted

CVE-2026-63466 (GCVE-0-2026-63466)

FKIE_CVE-2026-63466

GHSA-W4MQ-XH27-6XPX

Vulnerability Details

Root Cause

Attack Scenario

Vulnerable Code

Impact

Recommended Fix

Verification

Tags

Sightings

Nomenclature

Related by attack behaviour