CVE-2026-103902 in DigiDoc Android
Summary
by MITRE • 10/12/2026
Improper verification of cryptographic signature vulnerability in DigiDoc libdigidocpp, DigiDoc DigiDoc4, DigiDoc DigiDoc Android, and DigiDoc DigiDoc iOS. The vulnerability allows an attacker to create forged digital signatures that are displayed a valid by affected DigiDoc software.This issue affects libdigidocpp: from 4.0.0 before 4.5.0; DigiDoc4: from 4.6.0 before 4.11.0; DigiDoc Android: from 2.7.0 before 3.1.0; DigiDoc iOS: from 2.7.0 before 3.1.0.
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Analysis
by VulDB Data Team • 10/12/2026
The identified vulnerability represents a critical failure in the cryptographic verification logic within the libdigidocpp library and its associated client applications, including DigiDoc4 Desktop, DigiDoc Android, and DigiDoc iOS. This flaw fundamentally undermines the integrity of digital signatures processed by these systems, which are widely used for secure electronic identification and trusted services across Estonia and other regions relying on this ecosystem. The core technical issue stems from improper verification mechanisms that fail to rigorously validate all components of a cryptographic signature before accepting it as authentic. In a properly implemented system, every element of the signed data structure must be checked against the public key certificate to ensure no tampering has occurred during transmission or storage. However, in the affected versions ranging from libdigidocpp 4.0.0 up to but not including 4.5.0, and corresponding client software updates prior to version 3.1.0 for mobile platforms and 4.11.0 for desktop clients, this validation process contains a logical gap that allows malformed or maliciously crafted signature structures to bypass security checks.
From a technical perspective, the vulnerability likely involves an incomplete parsing of the CMS (Cryptographic Message Syntax) structure used by DigiDoc files. When processing these documents, the software may accept signatures where certain attributes are missing, incorrectly formatted, or where the hash algorithm identifiers do not strictly match expected standards. This allows an attacker to construct a forged digital signature that appears valid within the application interface. The attacker can manipulate the underlying data payload while maintaining a syntactically correct but semantically invalid cryptographic wrapper. Because the verification routine does not enforce strict adherence to all relevant RFC specifications and internal consistency checks, the software erroneously concludes that the document has been signed by a trusted authority with unaltered content. This constitutes a severe breach of trust in digital signature validation mechanisms, as it decouples the visual representation of validity from the actual cryptographic reality of the data integrity.
The operational impact of this vulnerability is profound for any organization or individual relying on DigiDoc software for legally binding transactions, government services, or secure communications. An adversary with access to a vulnerable version can generate documents that display as digitally signed and valid within the user interface, yet contain altered content that differs from what was originally intended by the signer. This enables fraud, unauthorized data modification, and potential legal disputes where evidence of authenticity is compromised. Since DigiDoc files are often used for submitting tax returns, banking transactions, and official applications, the ability to forge signatures without detection poses a significant risk to financial security and regulatory compliance. Users may unknowingly approve or submit documents that have been tampered with after initial signing, believing them to be authentic due to the misleading validity indicators provided by the compromised software.
This vulnerability aligns closely with CWE-347, which describes Improper Verification of Cryptographic Signature, a category of flaws where an application fails to verify signatures correctly or uses weak algorithms that can be broken. Furthermore, in the context of the MITRE ATT&CK framework, this flaw facilitates techniques related to Defense Evasion and potentially Data Manipulation, as it allows attackers to bypass security controls designed to ensure data integrity. The exploitation does not require complex privilege escalation but rather a sophisticated understanding of the CMS structure and the specific validation gaps present in the older library versions. It highlights the importance of rigorous input validation and strict adherence to cryptographic standards when implementing digital signature verification logic.
To mitigate this risk, immediate action is required for all users running affected software versions. Organizations should upgrade libdigidocpp to version 4.5.0 or later, which includes patches that enforce stricter signature verification protocols. Correspondingly, desktop users must update DigiDoc4 Desktop to version 4.11.0 or newer, while mobile platform users on Android and iOS need to install versions 3.1.0 or higher of the respective applications. These updates address the logical flaws in the cryptographic validation routines by ensuring that all signature components are thoroughly checked against expected standards before marking a document as valid. Until these patches are applied, it is advisable for high-risk operations to employ additional verification methods, such as out-of-band confirmation with signers or manual inspection of document hashes using independent tools, to mitigate the risk of accepting forged signatures.