CVE-2021-39228 is a critical-severity (CVSS 9.8) Use After Free vulnerability in tremor-script. 1 public exploit reference exists, so weaponization risk is real. A fix is available for tremor-script — see the affected versions and patch details below.
Memory Safety Issue when using patch or merge on state and assign the result back to state
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
CVE-2021-39228 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.
Where this sits among everything scored
Of 382,795 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.
Real-World Exposure
tremor-scriptReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects crates.io packages — download data is not available via public APIs for these ecosystems.
Description
Impact
This vulnerability is a memory safety Issue when using patch or merge on state and assign the result back to state.
In this case affected versions of Tremor and the tremor-script crate maintains references to memory that might have been freed already. And these memory regions can be accessed by retrieving the state, e.g. send it over TCP or HTTP. This requires the Tremor server (or any other program using tremor-script) to execute a tremor-script script that uses the mentioned language construct.
Details
If affects the following two tremor-script language constructs:
- A Merge where we assign the result back to the target expression
and the expression to be merged needs to reference the
event:
let state = merge state of event end;
- A Patch where we assign the result back to the target expression
and the patch operations used need to reference the
event:
let state = patch state of insert event.key => event.value end;
For constructs like this (it does not matter what it references in the expression to be merged or the patch operations) an optimization was applied to manipulate the target value in-place, instead of cloning it.
Our Value struct, which underpins all event data in tremor-script, is representing strings as borrowed beef::Cow<'lifetime, str>,
that reference the raw data Vec<u8> the event is based upon. We keep this raw byte-array next to the Value structure inside our Event as a self-referential struct,
so we make sure that the structured Value and its references are valid across its whole lifetime.
The optimization was considered safe as long as it was only possible to merge or patch event data or static data.
When state was introduced to tremor-script (in version 0.7.3) a new possibility to keep Value data around for longer than the lifetime of an event emerged.
If event data is merged or patched into state without cloning it first, it can still reference keys or values from
the previous event, which will now be invalid. This allows access to those already freed regions of memory and to get their content out over the wire.
Patches
The issue has been patched in https://crates.io/crates/tremor-script/0.11.6 and https://github.com/tremor-rs/tremor-runtime/releases/tag/v0.11.6 via commit 1a2efcd by removing the optimization and always clone the target expression of a Merge or [Patch](https://www.tremor.rs/docs/tremor-script/index#patch.
Workarounds
If an upgrade is not possible, a possible workaround is to avoid the optimization
by introducing a temporary variable and not immediately reassigning to state:
let tmp = merge state of event end;
let state = tmp
References
The actual fix is applied in this PR: https://github.com/tremor-rs/tremor-runtime/pull/1217
For more information
If you have any questions or comments about this advisory:
- Open an issue on our repository tremor-rs/tremor-runtime
- Please join our discord https://chat.tremor.rs and reach out to the team.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🦀crates.io | tremor-script | ≥ 0.7.3&&< 0.11.6 | 0.11.6cargo update -p tremor-script --precise 0.11.6 |
Affected Products
tremorlinuxfoundationResearch use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for tremor-script, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update tremor-script to 0.11.6 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-39228 is resolved across your whole dependency graph.
Workarounds
Stop feeding it untrusted input: reject or quarantine files and payloads from unverified sources until you can upgrade, restrict accepted formats to the ones you actually need, and run the parsing or decoding step in a least-privileged sandbox or short-lived worker so a crash or corrupted read cannot reach the rest of the process.
Frequently Asked Questions
Is CVE-2021-39228 in your dependencies?
Find it across crates.io, including transitive dependencies.