Summary
Denial of Service in TenderMint
Description
Denial of Service
Tendermint 0.33.0 and above allow block proposers to include signatures for the wrong block. This may happen naturally if you start a network, have it run for some time and restart it without changing the chainID. (It is a misconfiguration to reuse chainIDs.) Correct block proposers will accidentally include signatures for the wrong block if they see these signatures, and then commits won't validate, making all proposed blocks invalid. A malicious validator (even with a minimal amount of stake) can use this vulnerability to completely halt the network.
Tendermint 0.33.6 checks all the signatures are for the block with +2/3 majority before creating a commit.
False Witness
Tendermint 0.33.1 and above are no longer fully verifying commit signatures during block execution - they stop after +2/3. This means proposers can propose blocks that contain valid +2/3 signatures and then the rest of the signatures can be whatever they want. They can claim that all the other validators signed just by including a CommitSig with arbitrary signature data. While this doesn't seem to impact safety of Tendermint per se, it means that Commits may contain a lot of invalid data **.
** This was already true of blocks, since they could include invalid txs filled with garbage, but in that case the application knew that they are invalid and could punish the proposer. But since applications didn't--and don't-- verify commit signatures directly (they trust Tendermint to do that), they won't be able to detect it.
This can impact incentivization logic in the application that depends on the LastCommitInfo sent in BeginBlock, which includes which validators signed. For instance, Gaia incentivizes proposers with a bonus for including more than +2/3 of the signatures. But a proposer can now claim that bonus just by including arbitrary data for the final -1/3 of validators without actually waiting for their signatures. There may be other tricks that can be played because of this.
Tendermint 0.33.6 verifies all the signatures during block execution ***.
*** Please note that the light client does not check nil votes and exits as soon as 2/3+ of the signatures are checked.
Workarounds
No workarounds.
References
For more information
If you have any questions or comments about this advisory:
- Open an issue in tendermint/tendermint
- Email us at [email protected]
More information can be found here.
Impact
- All nodes
- The network stops due to having a commit with a wrong signature.
CVE-2020-15091 has a CVSS score of 6.5 (Medium). The vector is network-reachable, low 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 (0.33.6); 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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- v0.33.6 andn v0.34.0-dev1.0.20200702134149-480b995a3172
Frequently Asked Questions
- What is CVE-2020-15091? CVE-2020-15091 is a medium-severity security vulnerability in github.com/tendermint/tendermint (go), affecting versions < 0.33.6. It is fixed in 0.33.6.
- How severe is CVE-2020-15091? CVE-2020-15091 has a CVSS score of 6.5 (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 github.com/tendermint/tendermint are affected by CVE-2020-15091? github.com/tendermint/tendermint (go) versions < 0.33.6 is affected.
- Is there a fix for CVE-2020-15091? Yes. CVE-2020-15091 is fixed in 0.33.6. Upgrade to this version or later.
- Is CVE-2020-15091 exploitable, and should I be worried? Whether CVE-2020-15091 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-2020-15091 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-2020-15091? Upgrade
github.com/tendermint/tendermintto 0.33.6 or later.