GHSA-m982-7q3h-r784

### Summary

A memory-safety vulnerability in Open Babel's PQS parser caused an out-of-bounds (pre-buffer) read when reading a crafted input file.

### Details

The flaw was in the `lowerit` helper used by the PQS parser. A malformed input caused the helper to read one or more bytes before the start of its input buffer.

### Impact

Open Babel is a C++ library and CLI used to read and write chemistry file formats; it is shipped by Linux distributions and embedded in services that may parse untrusted input. Triggering this vulnerability requires the victim to open a malicious PQS file with the `obabel` tool, the `OBConversion` API, or any of the language bindings (Python, Ruby, Java, R, Perl, C#, PHP).

### Affected versions

All releases up to and including 3.1.1.

### Patched version

3.2.0 (released 2026-05-26).

### Patch

Fix commit: https://github.com/openbabel/openbabel/commit/f4a5ebae Fixes consolidated in #2913.

A minimized reproducer for this CVE is checked in under `test/files/fuzz_regress/` and is exercised on every CI build under ASAN+UBSAN by the `fuzzregresstest` harness.

### Credit

Reported via OSS-Fuzz.