Summary
Denial of service in Tendermint
Description
Denial of Service 1
Tendermint 0.33.2 and earlier does not limit the number of P2P connection requests. For each p2p connection, Tendermint allocates XXX bytes. Even though this memory is garbage collected once the connection is terminated (due to duplicate IP or reaching a maximum number of inbound peers), temporary memory spikes can lead to OOM (Out-Of-Memory) exceptions.
Tendermint 0.33.3 (and 0.32.10) limits the total number of P2P incoming connection requests to to p2p.max_num_inbound_peers + len(p2p.unconditional_peer_ids).
Notes:
- Tendermint does not rate limit P2P connection requests per IP (an attacker can saturate all the inbound slots);
- Tendermint does not rate limit HTTP(S) requests. If you expose any RPC endpoints to the public, please make sure to put in place some protection (https://www.nginx.com/blog/rate-limiting-nginx/). We may implement this in the future (https://github.com/tendermint/tendermint/issues/1696).
Denial of Service 2
Tendermint 0.33.2 and earlier does not reclaim activeID of a peer after it's removed in Mempool reactor. This does not happen all the time. It only happens when a connection fails (for any reason) before the Peer is created and added to all reactors. RemovePeer is therefore called before AddPeer, which leads to always growing memory (activeIDs map). The activeIDs map has a maximum size of 65535 and the node will panic if this map reaches the maximum. An attacker can create a lot of connection attempts (exploiting Denial of Service 1), which ultimately will lead to the node panicking.
Tendermint 0.33.3 (and 0.32.10) claims activeID for a peer in InitPeer, which is executed before MConnection is started.
Notes:
InitPeerfunction was added to all reactors to combat a similar issue - https://github.com/tendermint/tendermint/issues/3338;- Denial of Service 2 is independent of Denial of Service 1 and can be executed without it.
Specific Go Packages Affected
github.com/tendermint/tendermint/p2p
Workarounds
No workarounds.
References
- https://hackerone.com/reports/820317 (not disclosed yet)
- https://github.com/tendermint/tendermint/issues/3338
- https://github.com/tendermint/tendermint/issues/1696
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.
Credits
- fudongbai for discovering and reporting Denial of Service 2
- Ethan Buchman (@ebuchman) for writing a test case for Denial of Service 2 and Tess Rinearson (@tessr) for fixing it
- Anton Kaliaev (@melekes) for fixing Denial of Service 1
Impact
- All full nodes (except for validators who are behind closed networks)
- Node's memory usage increases, then it panics either in the mempool or due to OOM.
A write operation targets a memory location beyond the intended buffer boundary. Typical impact: memory corruption, crash, or arbitrary code execution.
CVE-2020-5303 has a CVSS score of 3.1 (Low). The vector is network-reachable, no privileges required, and user interaction required. 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 (0.33.3, 0.31.12, 0.32.10); 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.
Already deployed Kodem?
See it in your environmentNew to Kodem? Get a demo →Remediation advice
- v0.33.3
- v0.32.10
- v0.31.12
Frequently Asked Questions
- What is CVE-2020-5303? CVE-2020-5303 is a low-severity out-of-bounds write vulnerability in github.com/tendermint/tendermint (go), affecting versions >= 0.33.0, < 0.33.3. It is fixed in 0.33.3, 0.31.12, 0.32.10. A write operation targets a memory location beyond the intended buffer boundary.
- How severe is CVE-2020-5303? CVE-2020-5303 has a CVSS score of 3.1 (Low). 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-2020-5303? github.com/tendermint/tendermint (go) versions >= 0.33.0, < 0.33.3 is affected.
- Is there a fix for CVE-2020-5303? Yes. CVE-2020-5303 is fixed in 0.33.3, 0.31.12, 0.32.10. Upgrade to this version or later.
- Is CVE-2020-5303 exploitable, and should I be worried? Whether CVE-2020-5303 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-2020-5303 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-2020-5303?
- Upgrade
github.com/tendermint/tendermintto 0.33.3 or later - Upgrade
github.com/tendermint/tendermintto 0.31.12 or later - Upgrade
github.com/tendermint/tendermintto 0.32.10 or later
- Upgrade