Summary
Buffer overflow in SmallVec::insert_many
A bug in the SmallVec::insert_many method caused it to allocate a buffer that was smaller than needed. It then wrote past the end of the buffer, causing a buffer overflow and memory corruption on the heap. This bug was only triggered if the iterator passed to insert_many yielded more items than the lower bound returned from its size_hint method.
The flaw was corrected in smallvec 0.6.14 and 1.6.1, by ensuring that additional space is always reserved for each item inserted. The fix also simplified the implementation of insert_many to use less unsafe code, so it is easier to verify its correctness.
Impact
A write operation targets a memory location beyond the intended buffer boundary. Typical impact: memory corruption, crash, or arbitrary code execution.
CVE-2021-25900 has a CVSS score of 9.8 (Critical). 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 (0.6.14, 1.6.1); 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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smallvec to 0.6.14 or later; smallvec to 1.6.1 or later
Kodem Kai can prioritize this vulnerability in your dependency tree and generate a fix recommendation.
Frequently Asked Questions
- What is CVE-2021-25900? CVE-2021-25900 is a critical-severity out-of-bounds write vulnerability in smallvec (rust), affecting versions >= 0.6.3, < 0.6.14. It is fixed in 0.6.14, 1.6.1. A write operation targets a memory location beyond the intended buffer boundary.
- How severe is CVE-2021-25900? CVE-2021-25900 has a CVSS score of 9.8 (Critical). 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 smallvec are affected by CVE-2021-25900? smallvec (rust) versions >= 0.6.3, < 0.6.14 is affected.
- Is there a fix for CVE-2021-25900? Yes. CVE-2021-25900 is fixed in 0.6.14, 1.6.1. Upgrade to this version or later.
- Is CVE-2021-25900 exploitable, and should I be worried? Whether CVE-2021-25900 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-2021-25900 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-2021-25900?
- Upgrade
smallvecto 0.6.14 or later - Upgrade
smallvecto 1.6.1 or later
- Upgrade