Summary
Memory Safety Issue when using patch or merge on state and assign the result back to state
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 theevent:
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 theevent:
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.
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.
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.
Memory is accessed after it has been freed, leading to undefined behavior in native code. Typical impact: memory corruption, crash, or potential code execution.
CVE-2021-39228 has a CVSS score of 6.5 (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 (0.11.6); 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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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.
Frequently Asked Questions
- What is CVE-2021-39228? CVE-2021-39228 is a medium-severity use after free vulnerability in tremor-script (rust), affecting versions >= 0.7.3, < 0.11.6. It is fixed in 0.11.6. Memory is accessed after it has been freed, leading to undefined behavior in native code.
- How severe is CVE-2021-39228? CVE-2021-39228 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 tremor-script are affected by CVE-2021-39228? tremor-script (rust) versions >= 0.7.3, < 0.11.6 is affected.
- Is there a fix for CVE-2021-39228? Yes. CVE-2021-39228 is fixed in 0.11.6. Upgrade to this version or later.
- Is CVE-2021-39228 exploitable, and should I be worried? Whether CVE-2021-39228 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-2021-39228 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-2021-39228? Upgrade
tremor-scriptto 0.11.6 or later.