CVE-2020-8929 in Tink
Summary
by MITRE • 10/19/2020
A mis-handling of invalid unicode characters in the Java implementation of Tink versions prior to 1.5 allows an attacker to change the ID part of a ciphertext, which result in the creation of a second ciphertext that can decrypt to the same plaintext. This can be a problem with encrypting deterministic AEAD with a single key, and rely on a unique ciphertext-per-plaintext.
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Analysis
by VulDB Data Team • 06/05/2025
The vulnerability described in CVE-2020-8929 represents a critical weakness in the Java implementation of Google Tink cryptographic library versions prior to 1.5. This flaw specifically targets the handling of invalid unicode characters within the library's cryptographic operations, creating a scenario where attackers can manipulate ciphertext identifiers without detection. The issue stems from improper validation and processing of malformed unicode sequences during the encryption and decryption processes, which fundamentally undermines the security assumptions of the cryptographic system.
The technical flaw manifests when the Tink library encounters invalid unicode characters during cryptographic operations. Under normal circumstances, such characters should trigger proper error handling or rejection of the input. However, the vulnerability allows attackers to exploit a mis-handling mechanism where the library fails to properly validate unicode sequences, enabling manipulation of the ciphertext identifier portion. This misbehavior creates a scenario where a single plaintext can produce multiple distinct ciphertexts that are all capable of decrypting to the identical plaintext, directly violating the fundamental security principle that each plaintext should map to a unique ciphertext when using deterministic authenticated encryption.
The operational impact of this vulnerability is particularly severe for systems implementing deterministic authenticated encryption with a single key. When applications rely on the uniqueness of ciphertexts to ensure security properties or to prevent certain types of attacks, this vulnerability effectively nullifies those protections. The ability to generate a second ciphertext that decrypts to the same plaintext creates opportunities for replay attacks, key compromise scenarios, and undermines cryptographic proofs of security that depend on the uniqueness of ciphertexts. This is especially problematic in systems where ciphertext uniqueness is used as a security control or where deterministic encryption is employed for data indexing or retrieval purposes.
The vulnerability aligns with CWE-20, which describes improper input validation, and represents a specific implementation flaw in how the library handles unicode character sequences. From an ATT&CK perspective, this vulnerability could be leveraged as part of a privilege escalation or data integrity attack, potentially allowing adversaries to manipulate cryptographic outputs without detection. The weakness creates a situation where an attacker can effectively bypass cryptographic guarantees that are fundamental to secure communications and data protection. Organizations using affected versions of Tink should immediately upgrade to version 1.5 or later, which includes proper unicode validation and handling mechanisms. Additionally, systems should implement monitoring for unusual cryptographic behavior patterns and consider implementing additional integrity checks beyond what the library provides to detect potential exploitation attempts.