CVE-2020-8929: Tink Ciphertext Malleability Issue Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-g5vf-v6wf-7w2r",
  "modified" : "2025-06-05T16:44:52Z",
  "published" : "2020-10-16T00:51:24Z",
  "aliases" : [ "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)",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N"
  }, {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Maven",
      "name" : "com.google.crypto.tink:tink"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "1.5.0"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "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/google/tink/commit/93d839a5865b9d950dffdc9d0bc99b71280a8899"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/pypa/advisory-database/tree/main/vulns/tink/PYSEC-2020-142.yaml"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-176", "CWE-327" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2020-10-16T00:49:43Z",
    "nvd_published_at" : "2020-10-19T13:15:13Z"
  }
}