CVE-2021-25636

What it actually does

Improper certificate validation allows an attacker-supplied ODF document to contain both X509Data and KeyValue elements under KeyInfo in documentsignatures.xml or macrosignatures.xml; LibreOffice performs verification against the KeyValue yet reports success using the unrelated X509Data value, bypassing signature integrity checks.

Why it matters to the business

Organizations that rely on digitally signed ODF documents for legal, compliance, or workflow authenticity may accept altered files as genuine, creating risk of undetected document tampering and downstream process integrity failures; the issue is limited to older 7.2 releases and has shown no widespread exploitation.

How an attack unfolds

An attacker crafts or modifies an ODF file, inserts both X509Data and KeyValue children into the KeyInfo element of the signature streams, signs the document with a controlled key, and distributes the file; when a victim opens it in a vulnerable LibreOffice version the application reports the signature as valid while actually having verified against attacker-controlled material.

How to fix it

Upgrade LibreOffice 7.2 installations to version 7.2.5 or later; for Fedora packages apply the corresponding fixed updates from the distribution; disable macro execution and require out-of-band signature verification until patches are deployed.

How to confirm the fix worked

Confirm LibreOffice version is 7.2.5 or newer; create a test ODF document containing both X509Data and KeyValue elements and verify that the application now rejects the signature rather than reporting it valid.

Typical attack path

1. Obtain or create an ODF document
2. Extract and edit documentsignatures.xml or macrosignatures.xml to include both X509Data and KeyValue under KeyInfo
3. Apply a digital signature using attacker-controlled material
4. Repackage and distribute the document via email, file share, or web download
5. Victim opens the document in an affected LibreOffice release, triggering the flawed validation and acceptance of the altered file

How to detect it

Review file-open and macro-execution logs from LibreOffice on endpoints for documents whose signature streams contain both X509Data and KeyValue elements; correlate with network or email logs showing receipt of ODF attachments from untrusted sources around the time of document processing.

What your security team should do

• Prioritize upgrade of all LibreOffice 7.2 instances to 7.2.5 or newer within the next maintenance window
• Inventory and re-verify any externally received signed ODF documents opened since the vulnerability disclosure
• Add LibreOffice version checks to endpoint configuration compliance scans
• Educate document-handling teams on the risk of accepting digitally signed files from untrusted sources until patches are applied