CVE-2026-35201: Rdiscount Rdiscount Has An Out-of-bounds Read Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-6r34-94wq-jhrc",
  "modified" : "2026-05-13T16:20:38Z",
  "published" : "2026-04-06T17:53:59Z",
  "aliases" : [ "CVE-2026-35201" ],
  "summary" : "rdiscount has an Out-of-bounds Read",
  "details" : "### Summary\n\nA signed length truncation bug causes an out-of-bounds read in the default Markdown parse path. Inputs larger than `INT_MAX` are truncated to a signed `int` before entering the native parser, allowing the parser to read past the end of the supplied buffer and crash the process\n\n### Details\n\nIn both public entry points:\n\n- `ext/rdiscount.c:97`\n- `ext/rdiscount.c:136`\n\n`RSTRING_LEN(text)` is passed directly into `mkd_string()`:\n\n```c\nMMIOT *doc = mkd_string(RSTRING_PTR(text), RSTRING_LEN(text), flags);\n```\n\n`mkd_string()` accepts `int len`:\n\n- `ext/mkdio.c:174`\n\n```c\nDocument * mkd_string(const char *buf, int len, mkd_flag_t flags)\n{\n    struct string_stream about;\n\n    about.data = buf;\n    about.size = len;\n\n    return populate((getc_func)__mkd_io_strget, &about, flags & INPUT_MASK);\n}\n```\n\nThe parser stores the remaining input length in a signed `int`:\n\n- `ext/markdown.h:205`\n\n```c\nstruct string_stream {\n    const char *data;\n    int   size;\n};\n```\n\nThe read loop stops only when `size == 0`:\n\n- `ext/mkdio.c:161`\n\n```c\nint __mkd_io_strget(struct string_stream *in)\n{\n    if ( !in->size ) return EOF;\n\n    --(in->size);\n\n    return *(in->data)++;\n}\n```\n\nIf the Ruby string length exceeds `INT_MAX`, the value can truncate to a negative `int`. In that state, the parser continues incrementing `data` and reading past the end of the original Ruby string, causing an out-of-bounds read and native crash.\n\nAffected APIs:\n\n- `RDiscount.new(input).to_html`\n- `RDiscount.new(input).toc_content`\n\n### PoC\n\nCrash via `to_html`:\n\n```sh\nRUBYLIB=lib:ext ruby -e 'require \"rdiscount\"; n=2_200_000_000; s = \"a\" * n; warn \"built=#{s.bytesize}\"; RDiscount.new(s).to_html\"'\n```\nresult:\n\n- `built=2200000000`\n- Ruby terminates with `[BUG] Segmentation fault`\n- top control frame: `CFUNC :to_html`\n\nsame result with `toc_content`\n\n### Impact\n\nThis is an out-of-bounds read with the main issue being reliable denial-of-service. Impacted is limited to deployments parses attacker-controlled Markdown and permits multi-GB inputs.\n\n### Fix\n\njust add a checked length guard before the `mkd_string()` call in both public entry points:\n\n- `ext/rdiscount.c:97`\n- `ext/rdiscount.c:136`\nex: \n```c\nVALUE text = rb_funcall(self, rb_intern(\"text\"), 0);\nlong text_len = RSTRING_LEN(text);\nVALUE buf = rb_str_buf_new(1024);\nCheck_Type(text, T_STRING);\n\nif (text_len > INT_MAX) {\n    rb_raise(rb_eArgError, \"markdown input too large\");\n}\n\nMMIOT *doc = mkd_string(RSTRING_PTR(text), (int)text_len, flags);\n```\n\nThe same guard should be applied in `rb_rdiscount_toc_content()` before its `mkd_string()` call.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "RubyGems",
      "name" : "rdiscount"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "1.3.1.1"
      }, {
        "fixed" : "2.2.7.4"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/davidfstr/rdiscount/security/advisories/GHSA-6r34-94wq-jhrc"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-35201"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/davidfstr/rdiscount/commit/b1a16445e92e0d12c07594dedcdc56f80b317761"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/davidfstr/rdiscount"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/rubysec/ruby-advisory-db/blob/master/gems/rdiscount/CVE-2026-35201.yml"
  }, {
    "type" : "WEB",
    "url" : "http://github.com/davidfstr/rdiscount/releases/tag/2.2.7.4"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-125" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-04-06T17:53:59Z",
    "nvd_published_at" : "2026-04-06T20:16:27Z"
  }
}