Etherpad: x-proxy-path header reflected into admin HTML/JS/CSS (cache-poisoning XSS) and concatenated into redirect (open-redirect)

{
  "id": "190d4b08283cbbc55a5faf493aadaacf59da220f3ac9bfe5f14878e15c28d537",
  "url": "https://osv.dev/vulnerability/CVE-2026-55087",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "title": "Etherpad: x-proxy-path header reflected into admin HTML/JS/CSS (cache-poisoning XSS) and concatenated into redirect (open-redirect)",
    "summary": "Etherpad is a real-time collaborative editor. From 2.1.0 until 3.1.0, Etherpad uses the attacker-controlled x-proxy-path request header in src/node/hooks/express/admin.ts when substituting paths into HTML, JavaScript, and CSS under /admin without sanitization, Vary: x-proxy-path, or Cache-Control: private, no-store. A shared proxy or CDN can cache the resulting response and serve attacker-injected script to an administrator. In src/node/hooks/express/specialpages.ts, version 3.0.0 also accepts a protocol-relative x-proxy-path value when constructing the /p/:pad/timeslider redirect, allowing redirection to an attacker-controlled host. The issues are exploitable when the deployment permits client-supplied x-proxy-path headers to reach Etherpad. This issue is fixed in version 3.1.0.",
    "evidence": "osv:CVE-2026-55087",
    "products": [],
    "severity": "medium",
    "structured": {
      "id": "CVE-2026-55087",
      "aliases": [
        "CVE-2026-55087",
        "GHSA-fjgc-3mj7-8rg8"
      ],
      "metrics": [
        {
          "type": "CVSS_V3",
          "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N"
        }
      ],
      "affected": [
        {
          "ranges": [
            {
              "repo": "https://github.com/ether/etherpad",
              "type": "GIT",
              "events": [
                {
                  "introduced": "c45b7a361e56e6c753cd1c9b8cc4e1017090ce9c"
                },
                {
                  "fixed": "fa1c6b2e43d0510c851643cb7ba67cf042331a37"
                },
                {
                  "fixed": "8c6104c5d5daf41f0d454acc04d42dffa0e0d996"
                }
              ],
              "database_specific": {
                "extracted_events": [
                  {
                    "range": [
                      {
                        "introduced": "2.1.0"
                      },
                      {
                        "fixed": "3.1.0"
                      }
                    ],
                    "source": "AFFECTED_FIELD",
                    "strategy": "StringRangeExpression"
                  },
                  {
                    "range": [
                      {
                        "fixed": "8c6104c5d5daf41f0d454acc04d42dffa0e0d996"
                      }
                    ],
                    "source": "REFERENCES_COMMIT"
                  }
                ]
              }
            }
          ],
          "versions": [
            "v3.0.0",
            "3.0.0",
            "v2.7.3",
            "2.7.3",
            "v2.7.2",
            "2.7.2",
            "v2.7.1",
            "2.7.1",
            "v2.7.0",
            "2.7.0",
            "v2.6.1",
            "2.6.1",
            "v2.6.0",
            "2.6.0",
            "v2.5.3",
            "2.5.3",
            "v2.5.1",
            "2.5.1",
            "v2.5.0",
            "2.5.0",
            "v2.4.2",
            "2.4.2",
            "v2.4.1",
            "2.4.1",
            "v2.4.0",
            "2.4.0",
            "v2.3.2",
            "2.3.2",
            "v2.3.1",
            "2.3.1",
            "v2.3.0",
            "2.3.0",
            "v2.2.7",
            "2.2.7",
            "v2.2.6",
            "2.2.6",
            "v2.2.5",
            "2.2.5",
            "v2.2.4",
            "2.2.4",
            "v2.2.3",
            "2.2.3",
            "v2.2.2",
            "2.2.2",
            "v2.2.1",
            "2.2.1",
            "v2.2.0",
            "2.2.0",
            "v2.1.1",
            "2.1.1",
            "v2.1.0",
            "2.1.0"
          ],
          "database_specific": {
            "source": "https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-55087.json"
          }
        }
      ],
      "modified": "2026-10-28T02:30:41.541466125Z",
      "provider": "osv",
      "references": [
        "https://github.com/ether/etherpad/tree/3.1.0",
        "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/55xxx/CVE-2026-55087.json",
        "https://github.com/ether/etherpad/security/advisories/GHSA-fjgc-3mj7-8rg8",
        "https://nvd.nist.gov/vuln/detail/CVE-2026-55087",
        "https://github.com/ether/etherpad/commit/8c6104c5d5daf41f0d454acc04d42dffa0e0d996",
        "https://github.com/ether/etherpad/pull/7784"
      ],
      "provider_metadata": {
        "cwe_ids": [
          "CWE-444",
          "CWE-601",
          "CWE-79"
        ],
        "cna_assigner": "GitHub_M",
        "osv_generated_from": "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/55xxx/CVE-2026-55087.json"
      }
    },
    "related_cves": [
      {
        "value": "CVE-2026-55087",
        "evidence": "/id"
      }
    ],
    "security_relevant": true,
    "severity_evidence": "/severity/0/score"
  },
  "source_id": "core-osv",
  "document_id": 509041,
  "published_at": "2026-08-19T19:29:16.221+00:00"
}