CVE-2026-23829: Github.com/axllent/mailpit Mailpit Has An SMTP Header Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-54wq-72mp-cq7c",
  "modified" : "2026-02-02T14:50:34Z",
  "published" : "2026-01-20T17:54:55Z",
  "aliases" : [ "CVE-2026-23829" ],
  "summary" : "Mailpit has an SMTP Header Injection via Regex Bypass",
  "details" : "# Vulnerability Report: SMTP Header Injection via Regex Bypass\n\n**Vulnerable Code:** `mailpit/internal/smtpd/smtpd.go`\n\n## Executive Summary\nMailpit's SMTP server is vulnerable to **Header Injection** due to an insufficient Regular Expression used to validate `RCPT TO` and `MAIL FROM` addresses. An attacker can inject arbitrary SMTP headers (or corrupt existing ones) by including carriage return characters (`\\r`) in the email address. This header injection occurs because the regex intended to filter control characters fails to exclude `\\r` and `\\n` when used inside a character class.\n\n## RFC Compliance & Design Analysis\n**\"Is this behavior intentional for a testing tool?\"**\nNo. While testing tools are often permissive, this specific behavior violates the core SMTP protocol and fails the developer's own intent.\n\n1.  **RFC 5321 Violation:** The SMTP protocol strictly forbids Control Characters (CR, LF, Null) in the envelope address (`Mailbox`).\n    *   *RFC 5321 Section 4.1.2:* A `Mailbox` consists of an `Atom` or `Quoted-string`. An `Atom` explicitly excludes \"specials, SPACE and CTLs\" (Control Characters).\n2.  **Failed Intent:** The existence of `\\v` in the regex `[^<>\\v]` proves the developer **intended** to block vertical whitespace. The vulnerability is that `\\v` in Go regex (`re2`) inside brackets `[]` matches *only* Vertical Tab, not CR/LF. If the design were to allow everything, the `\\v` exclusion wouldn't exist.\n3.  **Data Corruption:** Allowing `\\r` results in the generation of malformed `.eml` files where the `Received` header is broken. This is not a feature; it's a bug that creates invalid email files.\n4. RFC 5321 also enforces address lengths which are not applied in Mailpit.\n\n## Technical Analysis\n\n### The Flaw\nThe vulnerability exists in the regex definitions used to parse SMTP commands:\n\n```go\n// internal/smtpd/smtpd.go:32-33\nrcptToRE   = regexp.MustCompile(`(?i)TO: ?<([^<>\\v]+)>( |$)(.*)?`)\nmailFromRE = regexp.MustCompile(`(?i)FROM: ?<(|[^<>\\v]+)>( |$)(.*)?`)\n```\n\nThe developer likely intended `[^<>\\v]` to mean \"Match anything that is NOT a `<` OR `>` OR `Vertical Whitespace`\".\n\nHowever, in Go's `regexp` (RE2) syntax, the behavior of `\\v` changes depending on context:\n- **Outside** brackets: `\\v` matches all vertical whitespace: `[\\n\\v\\f\\r\\x85\\u2028\\u2029]`.\n- **Inside** brackets (`[...]`): `\\v` matches **only** the Vertical Tab character (`\\x0B`).\n\n**Result:** The regex `[^<>\\v]` **allows** Carriage Return (`\\r`) and Line Feed (`\\n`) characters to pass through, as they are not `<` or `>` or `\\x0B`.\n\n### Exploit Scenario\n### Exploit Scenario\nWhen Mailpit constructs the `Received` header, it uses the validated recipient address directly:\n\n```go\n// internal/smtpd/smtpd.go:865\nbuffer.WriteString(fmt.Sprintf(\"        for <%s>; %s\\r\\n\", to[0], now))\n```\n\nIf `to[0]` contains `victim\\rINJECTED-HEADER: YES`, the resulting string in memory becomes:\n\n```text\n        for <victim\\rINJECTED-HEADER: YES>; ...\n```\n\nWhile `bufio.ReadString` prevents injecting immediate `\\n` (newlines), `\\r` (Carriage Return) bypasses this check. \n\n**The Result:** The stored EML file contains a \"Bare CR\".\n- **RFC Violation:** RFC 5321 strictly forbids Bare CR. Lines must end in CRLF.\n- **UI Behavior:** Browsers typically render Bare CR as a space, so it may look like `victim INJECTED` in the Mailpit UI.\n- **Real Impact:** The raw email is corrupted. If this email is exported or relayed, downstream systems (Outlook, older MTAs) may interpret the Bare CR as a line break, triggering a full **Header Injection**. Furthermore, Mailpit failing to reject this gives developers a **false sense of security**, as their code might be generating malformed emails that work in Mailpit but fail in production (e.g., with Gmail or Exchange).\n\n### Raw EML Verification\nThe following screenshot of the raw `.eml` file confirms that the `\\r` character successfully broke the `Received` header structure in the stored file, effectively creating a new line for the injected content.\n\n<img width=\"621\" height=\"230\" alt=\"image\" src=\"https://github.com/user-attachments/assets/1611f07e-316d-436a-95d6-9b14c9a8ecc6\" />\n\n<img width=\"1058\" height=\"441\" alt=\"image\" src=\"https://github.com/user-attachments/assets/9543d904-6e0a-4c8b-b283-abbe05b752d0\" />\n\n<img width=\"668\" height=\"196\" alt=\"image\" src=\"https://github.com/user-attachments/assets/907e4467-aab6-4bb4-83ce-743af4f6ba8d\" />\n\n\n\nAs seen in lines  of the screenshot:\n```text\n        for <victim\nINJECTED_VIA_CR:YES>; Tue, 13 Jan ...\n```\nThe `INJECTED_VIA_CR:YES` payload is treated as a start of a new line by the text editor (VS Code), which honors `\\r` as a line break. This proves the injection matches the \"Bare CR\" attack vector.\n\n## Additional Proof of Concepts\n\n### 1. Null Byte Injection (`\\x00`)\nThe regex `[^<>\\v]+` also allows the Null Byte (`\\x00`).\n**Test:** `test_null_byte.py` sent `RCPT TO:<victim\\x00-NULL-BYTE-HERE>`.\n**Result:** Server accepted the message (`250 OK`).\n**Impact:** The API returns an empty `[]` for the To field in the message summary, indicating the parser failure in the UI/API layer. The raw message content confirms the Null Byte is stored in the database.\n\n### 3. Detailed Character Compatibility\nTests (0-127 ASCII) confirm that the regex `[^<>\\v]` blocks **only** the following:\n- `<` (Less Than)\n- `>` (Greater Than)\n- `\\x0B` (Vertical Tab)\n\n**Crucially, it ALLOWS:**\n| Character | Hex | Regex Status | Network Status | Impact |\n| :--- | :--- | :--- | :--- | :--- |\n| **Carriage Return** | `\\r` (`0x0D`) | **ALLOWED** | **Passed** | **Header Injection** |\n| **Line Feed** | `\\n` (`0x0A`) | **ALLOWED** | Blocked* | *Blocked by `bufio.ReadString`, not regex. |\n| **Null Byte** | `\\x00` (`0x00`) | **ALLOWED** | **Passed** | API DoS / Corrupt Data |\n| **Tab** | `\\t` (`0x09`) | **ALLOWED** | **Passed** | Formatting issues |\n| **Delete** | `\\x7F` (`0x7F`) | **ALLOWED** | **Passed** | Potential obfuscation |\n| **Controls** | `0x01`-`0x1F` | **ALLOWED** | **Passed** | (Except `0x0A`, `0x0B`, `0x0D`) |\n\n*This confirms that the regex fails to implement a proper \"Safe Text\" allowlist, defaulting instead to a flawed denylist.*\n\n## Proof of Concept\nThe following Python script demonstrates the injection of a \"bare CR\" into the headers, which is successfully accepted by the server.\n\n```python\nimport socket\n\ndef exploit():\n    s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)\n    s.connect((\"127.0.0.1\", 1025))\n    s.recv(1024)\n    s.send(b\"EHLO test.com\\r\\n\")\n    s.recv(1024)\n    s.send(b\"MAIL FROM:<attacker@evil.com>\\r\\n\")\n    s.recv(1024)\n    \n    # Injecting \\r \n    payload = b\"RCPT TO:<victim\\rX-Injected: Yes>\\r\\n\"\n    s.send(payload)\n    resp = s.recv(1024)\n    print(f\"Server Response: {resp.decode()}\") # Expect 250 OK\n    \n    s.send(b\"DATA\\r\\n\")\n    s.recv(1024)\n    s.send(b\"Subject: Test\\r\\n\\r\\nBody\\r\\n.\\r\\n\")\n    s.recv(1024)\n    s.close()\n    \nexploit()\n```\n\n## Remediation\nUpdate the regex to explicitly exclude `\\r` and `\\n`, or use the correct character class escape for control characters.\n\n**Recommended Fix:**\nUse `\\x00-\\x1F` to exclude all ASCII control characters.\n\n```go\n// Fix: Exclude all control characters explicitly\nrcptToRE   = regexp.MustCompile(`(?i)TO: ?<([^<>\\x00-\\x1f]+)>( |$)(.*)?`)\nmailFromRE = regexp.MustCompile(`(?i)FROM: ?<(|[^<>\\x00-\\x1f]+)>( |$)(.*)?`)\n```\n\nAlternatively, strictly exclude CR and LF:\n```go\nrcptToRE   = regexp.MustCompile(`(?i)TO: ?<([^<>\\r\\n]+)>( |$)(.*)?`)\n```\n## Classification & References\n- **CWE-93:** [Improper Neutralization of CRLF Sequences ('CRLF Injection')](https://cwe.mitre.org/data/definitions/93.html)\n- **CWE-150:** [Improper Neutralization of Escape, Meta, or Control Sequences](https://cwe.mitre.org/data/definitions/150.html)\n- **OWASP:** [Injection Flaws](https://owasp.org/www-community/attacks/Injection_Flaws)\n- **CAPEC-106:** [Command Injection](https://capec.mitre.org/data/definitions/106.html) (Related usage pattern)\n- [[RFC 5321 Section 4.5.3.1 - Size Limits](https://datatracker.ietf.org/doc/html/rfc5321#section-4.5.3.1)](https://datatracker.ietf.org/doc/html/rfc5321#section-4.5.3.1)",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/axllent/mailpit"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "1.28.3"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 1.28.2"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/axllent/mailpit/security/advisories/GHSA-54wq-72mp-cq7c"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-23829"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/axllent/mailpit/commit/36cc06c125954dec6673219dafa084e13cc14534"
  }, {
    "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-150", "CWE-93" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-01-20T17:54:55Z",
    "nvd_published_at" : "2026-01-19T00:15:48Z"
  }
}