Discount is an implementation of John Gruber's Markdown markup language in C. From 1.3.1.1 to before 2.2.7.4, a 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. This vulnerability is fixed in 2.2.7.4.

Affected: RubyGems: rdiscount

### Summary

A 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

### Details

In both public entry points:

- `ext/rdiscount.c:97` - `ext/rdiscount.c:136`

`RSTRING_LEN(text)` is passed directly into `mkd_string()`:

```c MMIOT *doc = mkd_string(RSTRING_PTR(text), RSTRING_LEN(text), flags); ```

`mkd_string()` accepts `int len`:

- `ext/mkdio.c:174`

```c Document * mkd_string(const char *buf, int len, mkd_flag_t flags) { struct string_stream about;

about.data = buf; about.size = len;

return populate((getc_func)__mkd_io_strget, &about, flags & INPUT_MASK); } ```

The parser stores the remaining input length in a signed `int`:

- `ext/markdown.h:205`

```c struct string_stream { const char *data; int size; }; ```

The read loop stops only when `size == 0`:

- `ext/mkdio.c:161`

```c int __mkd_io_strget(struct string_stream *in) { if ( !in->size ) return EOF;

--(in->size);

return *(in->data)++; } ```

If 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.

Affected APIs:

- `RDiscount.new(input).to_html` - `RDiscount.new(inp…

Affected: Ubuntu:16.04:LTS: discount, Ubuntu:18.04:LTS: discount, Ubuntu:20.04:LTS: discount, Ubuntu:22.04:LTS: discount, Ubuntu:24.04:LTS: discount, Ubuntu:25.10: discount, Ubuntu:26.04:LTS: discount

Discount is an implementation of John Gruber's Markdown markup language in C. From 1.3.1.1 to before 2.2.7.4, a 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. This vulnerability is fixed in 2.2.7.4.