CVE-2026-21859: Github.com/axllent/mailpit Mailpit Proxy Endpoint Has Server-Side Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-8v65-47jx-7mfr",
  "modified" : "2026-02-02T14:51:32Z",
  "published" : "2026-01-06T17:44:29Z",
  "aliases" : [ "CVE-2026-21859" ],
  "summary" : "Mailpit Proxy Endpoint has Server-Side Request Forgery (SSRF) vulnerability",
  "details" : "## Summary\n\nA Server-Side Request Forgery (SSRF) vulnerability exists in Mailpit's `/proxy` endpoint that allows attackers to make requests to internal network resources.\n\n\n## Description\n\nThe `/proxy` endpoint allows requests to internal network resources. While it validates `http://` and `https://` schemes, it does not block internal IP addresses, allowing attackers to access internal services and APIs.\n\n## Proof of Concept\n\n### Basic SSRF Request\n```\nGET /proxy?url=http://127.0.0.1:8025/api/v1/info\n```\n\nThis returns internal API data including database path and runtime statistics.\n \n## Impact Assessment\n\n### 1. Internal Network Scanning\nAttacker can probe and discover internal services on the network.\n\n### 2. Information Disclosure\nAccess to internal API data, database paths, and runtime statistics.\n\n### 3. Email Content Access\nAbility to read all captured emails via internal API endpoints.\n\n### 4. Cloud Metadata Access\nIf deployed in cloud environments (AWS/GCP/Azure), potential access to instance metadata services (e.g., `http://169.254.169.254/`).\n\n## Attack Scenarios\n\n### Scenario 1: Development Environment Exposure\nIf Mailpit is accidentally exposed to the internet, attackers can leverage SSRF to access internal development resources and services.\n\n### Scenario 2: Container Escape Information\nIn containerized deployments, SSRF can reveal container metadata and internal service configurations.\n\n### Scenario 3: Lateral Movement\nIn corporate networks, SSRF can be used to discover and interact with internal services, facilitating lateral movement.\n\n## Mitigating Factors\nThis vulnerability is limited to HTTP GET requests with minimal headers. Additionally, Mailpit's web UI & API should be protected by basic authentication when exposed to the internet, which prevents access to the proxy endpoint. \n\n## References\n\n- [MDN Web Security - SSRF Attacks](https://developer.mozilla.org/en-US/docs/Web/Security/Attacks/SSRF)\n- [CWE-918: Server-Side Request Forgery](https://cwe.mitre.org/data/definitions/918.html)\n- [OWASP SSRF Prevention Cheat Sheet](https://cheatsheetseries.owasp.org/cheatsheets/Server_Side_Request_Forgery_Prevention_Cheat_Sheet.html)",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:N/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/axllent/mailpit"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "1.28.1"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 1.28.0"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/axllent/mailpit/security/advisories/GHSA-8v65-47jx-7mfr"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-21859"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/axllent/mailpit/commit/3b9b470c093b3d20b7d751722c1c24f3eed2e19d"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/axllent/mailpit"
  }, {
    "type" : "WEB",
    "url" : "https://pkg.go.dev/vuln/GO-2026-4284"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-918" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-01-06T17:44:29Z",
    "nvd_published_at" : "2026-01-08T00:16:00Z"
  }
}