🚨 Ruby json: JSON generator heap buffer overflow when streaming to an IO

Summary

JSON.dump(obj, io) and JSON::State#generate(obj, io) can write past the
internal JSON generator buffer when a streamed object contains an
attacker-controlled string near 16 KB. The issue is a heap out-of-bounds write
in the IO-streaming path and is demonstrated as a reliable process crash /
denial of service.

This was triaged on HackerOne as report #3785370. The issue was confirmed there
and I was asked to open it here.

Details

Root cause is in ext/json/fbuffer/fbuffer.h, fbuffer_do_inc_capa().

On the IO path, the buffer is grown to FBUFFER_IO_BUFFER_SIZE (16383), but the
early return checks total capacity instead of remaining capacity:

if (RB_UNLIKELY(fb->io)) {
    if (fb->capa < FBUFFER_IO_BUFFER_SIZE) {
        fbuffer_realloc(fb, FBUFFER_IO_BUFFER_SIZE);
    } else {
        fbuffer_flush(fb);
    }
<span class="pl-k">if</span> (<span class="pl-en">RB_LIKELY</span>(<span class="pl-s1">requested</span> <span class="pl-c1">&lt;</span> <span class="pl-s1">fb</span><span class="pl-c1">-&gt;</span><span class="pl-c1">capa</span>)) {
    <span class="pl-k">return</span>;
}

}

If fb->len already contains JSON syntax bytes, and a string flush has
16383 - fb->len <= requested < 16383, this check returns even though there is
not enough space left. fbuffer_append_reserved() then writes past the buffer:

MEMCPY(fb->ptr + fb->len, newstr, char, len);

The minimal fix is to compare against the remaining capacity:

-        if (RB_LIKELY(requested < fb->capa)) {
+        if (RB_LIKELY(requested <= fb->capa - fb->len)) {
             return;
         }

PoC

require "json"
require "stringio"
io = StringIO.new

big = "a" * 16385

big[16382] = '"'          # escapable byte near the buffer boundary
JSON.dump([big], io)

Verified results:

Ruby 4.0.5 / bundled json 2.18.0:
malloc(): invalid size (unsorted)
.../json/common.rb:956: [BUG] Aborted
ruby/ruby master c78418b7a0 / json 2.19.8 / ASan:

heap-buffer-overflow WRITE of size 16382

fbuffer_append_reserved  ext/json/fbuffer/fbuffer.h:145

search_flush             ext/json/generator/generator.c:139

convert_UTF8_to_JSON     ext/json/generator/generator.c:231

raw_generate_json_string ext/json/generator/generator.c:922

cState_m_generate        ext/json/generator/generator.c:1891

Control: the same data through JSON.dump([big]) without an IO argument returns
normally. The bug is specific to the IO-streaming path.

Impact

A remote attacker can trigger a heap out-of-bounds write if they control a
string field that an application serializes through JSON.dump(obj, io) or
JSON::State#generate(obj, io). The demonstrated impact is reliable denial of
service. I am not claiming code execution or information disclosure.

🚨 Ruby JSON has a format string injection vulnerability

Impact

A format string injection vulnerability than that lead to denial of service attacks or information disclosure, when the allow_duplicate_key: false parsing option is used to parse user supplied documents.

This option isn't the default, if you didn't opt-in to use it, you are not impacted.

Patches

Patched in 2.19.2.

Workarounds

The issue can be avoided by not using the allow_duplicate_key: false parsing option.

🚨 Ruby JSON has a format string injection vulnerability

Impact

A format string injection vulnerability than that lead to denial of service attacks or information disclosure, when the allow_duplicate_key: false parsing option is used to parse user supplied documents.

This option isn't the default, if you didn't opt-in to use it, you are not impacted.

Patches

Patched in 2.19.2.

Workarounds

The issue can be avoided by not using the allow_duplicate_key: false parsing option.

🚨 Ruby JSON has a format string injection vulnerability

Impact

A format string injection vulnerability than that lead to denial of service attacks or information disclosure, when the allow_duplicate_key: false parsing option is used to parse user supplied documents.

This option isn't the default, if you didn't opt-in to use it, you are not impacted.

Patches

Patched in 2.19.2.

Workarounds

The issue can be avoided by not using the allow_duplicate_key: false parsing option.

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