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How CVE-2026-3408 Works
Open Babel is a widely used open-source C++ library and command-line tool for converting and manipulating chemical file formats. It is included in major Linux distributions and integrated into numerous scientific workflows, web services, and language bindings (Python, Ruby, Java, R, Perl, C, PHP) that process chemistry data.
The vulnerability resides in the CDXML (ChemDraw XML) file format parser, specifically within the `OBAtom::GetExplicitValence` function defined in src/atom.cpp. CDXML is a structured XML-based format used to represent molecular structures, and the parser must construct in-memory atom objects from the XML elements.
When the CDXML parser encounters a `fragment` element in a malformed input file, it attempts to process the atoms referenced within that fragment. The parser calls `_pmol->GetAtom(atoms
)` to retrieve an `OBAtom` pointer for each atom ID listed in the fragment. In a correctly formed file, every referenced atom ID exists in the molecule, and `GetAtom` returns a valid pointer. However, a crafted CDXML file can reference an atom ID that does not exist in the molecule. In vulnerable versions (up to 3.1.1), the parser does not validate the return value of `GetAtom` before passing it to <code>OBAtomAssignTypicalImplicitHydrogens</code>. If `GetAtom` returns `nullptr` (because the ID is out of range), the subsequent call to `OBAtomAssignTypicalImplicitHydrogens` dereferences this NULL pointer. The crash occurs inside the call chain that ultimately invokes `OBAtom::GetExplicitValence` on the NULL atom object, leading to a NULL pointer dereference. This is a classic CWE-476 NULL Pointer Dereference vulnerability. The attack requires the victim to open a malicious CDXML file using the `obabel` CLI tool, the `OBConversion` API, or any of the language bindings. The crash results in a denial of service (application termination), but in some contexts, a NULL dereference could potentially be leveraged for more severe outcomes if the NULL page is mapped. The exploit is publicly available and can be launched remotely. The fix, committed as `openbabel/openbabel@e23a224b` and released in version 3.2.0 on 2026-05-26, adds a simple NULL check: `if (atom != nullptr)` before calling <code>OBAtomAssignTypicalImplicitHydrogens(atom)</code>. A minimized reproducer is checked into the test suite under `test/files/fuzz_regress/` and is run under ASAN+UBSAN on every CI build. <h2 style="color: blue;">DailyCVE Form:</h2> Platform: Open Babel Version: 3.1.1 and below Vulnerability: NULL pointer dereference Severity: MEDIUM (CVSS 4.3) date: 2026-03-02 <h2 style="color: blue;">Prediction: 2026-05-26</h2> <h2 style="color: blue;">What Undercode Say:</h2> <h2 style="color: blue;">Analytics & Bash Commands</h2> [bash] Check installed Open Babel version obabel --version Verify if system is vulnerable (versions <= 3.1.1) dpkg -l | grep openbabel Debian/Ubuntu rpm -qa | grep openbabel RHEL/Fedora Reproducer command (using the minimized test file) obabel test/files/fuzz_regress/cve-2026-3408.cdxml -O /dev/null Run under AddressSanitizer to observe the crash ASAN_OPTIONS=abort_on_error=1:halt_on_error=1 \ obabel malicious.cdxml -O /dev/null GDB backtrace of the crash gdb --args obabel malicious.cdxml -O /dev/null (gdb) run (gdb) bt full
Code Snippet – Vulnerable Code (before patch)
// cdxmlformat.cpp (vulnerable) for (vector<unsigned int>::iterator vit = _handleImplicitHydrogens.begin(); vit != _handleImplicitHydrogens.end(); ++vit) OBAtomAssignTypicalImplicitHydrogens(_pmol->GetAtom(atoms[bash])); // No NULL check – GetAtom returns nullptr for invalid atom IDs
Code Snippet – Patched Code (after e23a224b)
// cdxmlformat.cpp (patched)
for (vector<unsigned int>::iterator vit = _handleImplicitHydrogens.begin();
vit != _handleImplicitHydrogens.end(); ++vit) {
OBAtom atom = _pmol->GetAtom(atoms[bash]);
if (atom != nullptr)
OBAtomAssignTypicalImplicitHydrogens(atom);
}
Exploit:
To trigger CVE-2026-3408, an attacker crafts a CDXML file containing a `fragment` element that references an atom ID which does not exist in the molecule. For example:
<CDXML> <fragment id="1"> <atom id="1" element="C"/> <bond atom1="1" atom2="2" order="1"/> <!-- atom 2 does not exist --> </fragment> </CDXML>
When the parser processes the fragment, it attempts to retrieve atom 2, which is absent, and passes the resulting NULL pointer to OBAtomAssignTypicalImplicitHydrogens, causing a crash. The exploit can be delivered via email, web upload, or any vector where the victim opens the malicious file with Open Babel.
Protection:
- Upgrade to Open Babel 3.2.0 or later, which contains the patch (commit
e23a224b). - Apply the patch manually if upgrading is not immediately possible:
git clone https://github.com/openbabel/openbabel.git cd openbabel git checkout e23a224b8fd9d7c2a7cde9ef4ec6afb4c05aa08a make && sudo make install
- Validate input before parsing: use a schema validator or a pre-parser to check that all atom IDs referenced in fragments exist in the molecule.
- Run with crash mitigation (e.g., `ulimit -c 0` to disable core dumps, or use a sandbox) to limit the impact of a successful trigger.
- Monitor for any unexpected crashes in services that process CDXML files and consider using an intrusion detection system to flag anomalous file patterns.
Impact:
- Denial of Service: The primary impact is application crash, interrupting any workflow or service that relies on Open Babel to process untrusted CDXML input.
- Service Disruption: In web services or batch processing pipelines, a single malicious file can terminate the entire process, potentially leading to repeated downtime if the input is re‑submitted.
- Potential for Further Exploitation: While NULL pointer dereferences typically cause a crash, in environments where the NULL page is mapped (e.g., some embedded systems or with certain kernel configurations), an attacker might be able to control the dereferenced memory and escalate to arbitrary code execution.
- Widespread Exposure: Open Babel is embedded in many scientific applications, Linux distributions, and cloud-based chemistry services, making this vulnerability a significant supply-chain risk for the computational chemistry community.
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Sources:
Reported By: github.com
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