How severe is CVE-2020-8929?

CVE-2020-8929 has a CVSS score of 5.3/10, rated MEDIUM. Review your exposure and patch according to your risk tolerance.

Which packages are affected by CVE-2020-8929?

CVE-2020-8929 affects the following packages: com.google.crypto.tink:tink (Maven). Ecosystems affected: Maven.

How do I fix CVE-2020-8929?

Update com.google.crypto.tink:tink to 1.5.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2020-8929 is resolved across your whole dependency graph.

How do I detect CVE-2020-8929 in my Maven dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for com.google.crypto.tink:tink, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2020-8929 if there is no patch (or I can't update yet)?

If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Is CVE-2020-8929 actively exploited in the wild?

No public exploit code has been indexed for CVE-2020-8929 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2020-8929?

CVE-2020-8929 has an EPSS (Exploit Prediction Scoring System) score of 0.5%, placing it in the 38th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2020-8929?

CVE-2020-8929 is classified as CWE-176 (CWE-176). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2020-8929 published, and has it been updated?

CVE-2020-8929 was published on October 19, 2020 and was last updated on July 9, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

☕

☕ Maven

Not in CISA KEV

MEDIUM severity

CVE-2020-8929 — tink

MEDIUMFix: google/tink@93d839a

CVE-2020-8929 is a medium-severity (CVSS 5.3) CWE-176 vulnerability in com.google.crypto.tink:tink. A fix is available for com.google.crypto.tink:tink — see the affected versions and patch details below.

Ciphertext Malleability Issue in Tink Java

Also known asGHSA-g5vf-v6wf-7w2rPYSEC-2020-142

Published

Oct 19, 2020

Updated

Jul 9, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Oct 7, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

0.5%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs38th percentile — riskier than 38% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

CVE-2020-8929 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.

Where this sits among everything scored

Of 385,738 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.

Real-World Exposure

1 pkg affected

☕com.google.crypto.tink:tink

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Maven packages — download data is not available via public APIs for these ecosystems.

Description

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.

Patches

The issue was fixed in this pull request.

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

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
☕Maven	`com.google.crypto.tink:tink`	all versions	1.5.0`com.google.crypto.tink:tink:1.5.0`

Affected Products

1 product · 1 configurations

Application

tink javagoogle

< 1.5.0

range

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for com.google.crypto.tink:tink, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update com.google.crypto.tink:tink to 1.5.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2020-8929 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Frequently Asked Questions

### 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. ### Patches The issue was fixed in this [pull request