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Subject: BUG #19602: Vuln46: citext split_part silently returns NULL for a
 zero field position instead of raising core sp
To: pgsql-bugs@lists.postgresql.org
From: PG Bug reporting form <noreply@postgresql.org>
Cc: 1217816127@qq.com
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Date: Mon, 03 Aug 2026 06:58:12 +0000
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The following bug has been logged on the website:

Bug reference:      19602
Logged by:          Yuelin Wang
Email address:      1217816127@qq.com
PostgreSQL version: 19beta2
Operating system:   Linux (Ubuntu 24.04, x86_64)
Description:       =20

## Vuln46: citext split_part silently returns NULL for a zero field position
instead of raising core split_part's error

### Summary

citext.split_part(citext, citext, int) is implemented in SQL as an array
subscript expression (regexp_split_to_array(...))[$3] rather than by calling
pg_catalog.split_part. Postgres array subscripting silently returns NULL for
an out of range index such as 0, so citext's split_part diverges from core
split_part, which explicitly raises "field position must not be zero" for a
zero field argument.

CWE: CWE-1284. Severity: Low.

### PoC

```sql
CREATE EXTENSION citext;
SELECT split_part('abc~@~def~@~ghi'::citext, '~@~', 0) IS NULL AS is_null_0;
SELECT split_part('abc~@~def~@~ghi'::citext, '~@~', 0);
SELECT pg_catalog.split_part('abc~@~def~@~ghi', '~@~', 0);
```

### Result

Real captured output from the independent verification run:

```
CREATE EXTENSION
 is_null_0
-----------
 t
(1 row)

 split_part
------------
=20
(1 row)

ERROR:  field position must not be zero
```

### Impact

An application that relies on split_part raising an error for a zero field
position to catch a programming or input validation bug will instead
silently receive NULL when operating on citext values, potentially masking
the underlying logic error rather than failing loudly.