{"id":"CVE-2020-8929","aliases":["GHSA-g5vf-v6wf-7w2r","PYSEC-2020-142"],"url":"https://o3.security/vulnerability/CVE-2020-8929","summary":"Ciphertext Malleability Issue in Tink Java","details":"### Impact\nTink's Java version before 1.5 under some circumstances allowed attackers to change the key ID part of the ciphertext, resulting in the attacker creating a second ciphertext that will decrypt to the same plaintext. This can be a problem in particular in the case of encrypting with a deterministic AEAD with a single key, and relying on the fact that there is only a single valid ciphertext per plaintext.\n\nNo loss of confidentiality or loss of plaintext integrity occurs due to this problem, only ciphertext integrity is compromised.\n\n### Patches\nThe issue was fixed in this [pull request](https://github.com/google/tink/commit/93d839a5865b9d950dffdc9d0bc99b71280a8899).\n\n### Workarounds\nThe only workaround is to backport the fixing [pull request](https://github.com/google/tink/commit/93d839a5865b9d950dffdc9d0bc99b71280a8899).\n\n### Details\nTink uses the first five bytes of a ciphertext for a version byte and a four byte key ID. Since each key has a well defined prefix, this extends non-malleability properties (but technically not indistinguishability). However, in the Java version this prefix lookup used a hash map indexed by unicode strings instead of the byte array, which means that invalid Unicode characters would be [replaced by U+FFFD](https://en.wikipedia.org/wiki/UTF-8#Invalid_sequences_and_error_handling) by the [Java API's default behavior](https://docs.oracle.com/javase/7/docs/api/java/lang/String.html#String(byte[],%20java.nio.charset.Charset)). This means several different values for the five bytes would result in the same hash table key, which allows an attacker to exchange one invalid byte sequence for another, creating a mutated ciphertext that still decrypts (to the same plaintext).\n\n### Acknowledgements\nWe'd like to thank Peter Esbensen for finding this issue and raising it internally.\n\n### For more information\nIf you have any questions or comments about this advisory:\n* Open an issue in [Tink](https://github.com/google/tink/issues)","published":"2020-10-19T13:15:13.437Z","modified":"2026-07-09T00:38:13.910797Z","cvss":{"score":5.3,"severity":"MEDIUM","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N"},"epss":null,"cisaKev":null,"exploitsKnown":null,"affectedPackages":[{"ecosystem":"Maven","name":"com.google.crypto.tink:tink","fixedVersion":"1.5.0"}],"fix":{"url":"https://github.com/google/tink/commit/93d839a5865b9d950dffdc9d0bc99b71280a8899","label":"google/tink@93d839a"},"references":[{"type":"FIX","url":"https://github.com/google/tink/commit/93d839a5865b9d950dffdc9d0bc99b71280a8899"},{"type":"FIX","url":"https://github.com/google/tink/security/advisories/GHSA-g5vf-v6wf-7w2r"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2020-8929"},{"type":"WEB","url":"https://github.com/pypa/advisory-database/tree/main/vulns/tink/PYSEC-2020-142.yaml"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-07-09T00:38:13.910797Z"}}