{"id":"CVE-2026-21859","aliases":["GHSA-8v65-47jx-7mfr","GO-2026-4284"],"url":"https://o3.security/vulnerability/CVE-2026-21859","summary":"Mailpit Proxy Endpoint is Vulnerable to Server-Side Request Forgery (SSRF)","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)","published":"2026-01-07T23:24:07.869Z","modified":"2026-08-12T03:51:20.110219603Z","cvss":{"score":5.8,"severity":"MEDIUM","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:N/A:N"},"epss":null,"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"Go","name":"github.com/axllent/mailpit","fixedVersion":"1.28.1"}],"fix":{"url":"https://github.com/axllent/mailpit/commit/3b9b470c093b3d20b7d751722c1c24f3eed2e19d","label":"axllent/mailpit@3b9b470"},"references":[{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/21xxx/CVE-2026-21859.json"},{"type":"ADVISORY","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":"FIX","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"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-08-12T03:51:20.110219603Z"}}