CVE-2021-25635 in LibreOffice
Summary
by MITRE • 03/21/2025
An Improper Certificate Validation vulnerability in LibreOffice allowed an attacker to self sign an ODF document, with a signature untrusted by the target, then modify it to change the signature algorithm to an invalid (or unknown to LibreOffice) algorithm and LibreOffice would incorrectly present such a signature with an unknown algorithm as a valid signature issued by a trusted person
This issue affects LibreOffice: from 7.0 before 7.0.5, from 7.1 before 7.1.1.
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Analysis
by VulDB Data Team • 12/10/2025
The vulnerability CVE-2021-25635 represents a critical certificate validation flaw in LibreOffice that undermines the software's digital signature verification mechanisms. This weakness stems from improper handling of signature algorithm validation during document processing, creating a scenario where malicious actors can exploit the system's trust model. The vulnerability specifically targets the ODF (Open Document Format) document signature validation process, where LibreOffice fails to properly validate the cryptographic algorithms used in digital signatures. The flaw allows attackers to craft documents with self-signed certificates that appear legitimate to the application, exploiting a trust relationship that should not exist. This issue manifests when an attacker modifies a document's signature algorithm to an invalid or unrecognized value, yet the software incorrectly treats this malformed signature as valid. The vulnerability exists in LibreOffice versions 7.0.x prior to 7.0.5 and 7.1.x prior to 7.1.1, representing a window of exposure where users were susceptible to this signature forgery attack.
The technical root cause of this vulnerability lies in the insufficient validation of cryptographic signature algorithms within LibreOffice's document processing pipeline. According to CWE-310, this represents a weakness in cryptographic implementation where the software fails to properly validate the integrity of cryptographic operations. The flaw operates at the intersection of certificate validation and signature algorithm verification, where the application accepts malformed signature algorithms as legitimate. When processing ODF documents, LibreOffice should validate that signature algorithms are both recognized and properly implemented according to established cryptographic standards. However, the vulnerability allows attackers to bypass this validation by substituting a valid signature with an invalid algorithm identifier, causing the application to incorrectly present the modified signature as trustworthy. This improper validation creates a trust boundary violation that enables man-in-the-middle attacks and document tampering scenarios. The vulnerability is particularly concerning because it operates at the application layer rather than the network level, making it difficult to detect through traditional network monitoring approaches.
The operational impact of CVE-2021-25635 extends beyond simple document integrity concerns to encompass broader security implications for organizations relying on LibreOffice for document processing. This vulnerability enables attackers to forge digital signatures that appear legitimate to end users, potentially compromising the authenticity of important business documents, legal contracts, or confidential communications. The attack vector requires minimal technical expertise to execute, as it leverages existing document modification capabilities combined with understanding of cryptographic signature structures. Once exploited, the vulnerability allows attackers to maintain persistent trust relationships with documents that have been tampered with, making detection challenging for both users and security systems. The impact is particularly severe in enterprise environments where document authenticity is crucial for compliance and audit requirements. According to ATT&CK framework, this vulnerability maps to T1566 (Phishing) and T1552 (Unsecured Credentials) as attackers can leverage the forged signatures to manipulate document workflows and potentially gain unauthorized access to systems through social engineering. The vulnerability also relates to T1059 (Command and Scripting Interpreter) when attackers use the compromised documents to execute malicious code through embedded macros or other document-based attack vectors.
Organizations should immediately implement mitigation strategies that include updating to affected LibreOffice versions where the vulnerability has been patched, as well as implementing additional security controls to monitor document signature behavior. The recommended approach involves deploying automated document integrity checking mechanisms that can detect signature algorithm anomalies, alongside user education programs that emphasize the importance of verifying document authenticity through multiple channels. Network-level monitoring should be enhanced to detect unusual document processing patterns that might indicate signature manipulation attempts. Security teams should also consider implementing digital signature validation policies that require verification against multiple trusted certificate authorities, rather than relying solely on the application's built-in validation mechanisms. The vulnerability demonstrates the critical importance of proper cryptographic validation in document processing applications and highlights the need for comprehensive security testing of signature verification components. Organizations should also consider implementing sandboxing mechanisms for document processing to limit the potential impact of signature validation bypasses, and establish incident response procedures specifically designed to handle forged document signature scenarios. Regular security assessments of office productivity applications should include thorough testing of cryptographic validation mechanisms to prevent similar vulnerabilities from emerging in the future.