Summary
Ciphertext Malleability Issue in Tink Java
Workarounds
The only workaround is to backport the fixing pull request.
Details
Tink 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 by the Java API's default behavior. 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).
Acknowledgements
We'd like to thank Peter Esbensen for finding this issue and raising it internally.
For more information
If you have any questions or comments about this advisory:
- Open an issue in Tink
Impact
Tink'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.
No loss of confidentiality or loss of plaintext integrity occurs due to this problem, only ciphertext integrity is compromised.
The application uses a cryptographic algorithm known to have weaknesses, such as MD5, SHA-1, or DES. Typical impact: compromised confidentiality or integrity of protected data.
CVE-2020-8929 has a CVSS score of 5.3 (Medium). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (1.5.0); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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The issue was fixed in this pull request.
Frequently Asked Questions
- What is CVE-2020-8929? CVE-2020-8929 is a medium-severity use of a broken or risky cryptographic algorithm vulnerability in com.google.crypto.tink:tink (maven), affecting versions < 1.5.0. It is fixed in 1.5.0. The application uses a cryptographic algorithm known to have weaknesses, such as MD5, SHA-1, or DES.
- How severe is CVE-2020-8929? CVE-2020-8929 has a CVSS score of 5.3 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of com.google.crypto.tink:tink are affected by CVE-2020-8929? com.google.crypto.tink:tink (maven) versions < 1.5.0 is affected.
- Is there a fix for CVE-2020-8929? Yes. CVE-2020-8929 is fixed in 1.5.0. Upgrade to this version or later.
- Is CVE-2020-8929 exploitable, and should I be worried? Whether CVE-2020-8929 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2020-8929 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2020-8929? Upgrade
com.google.crypto.tink:tinkto 1.5.0 or later.