Summary
Integer Overflow in Chunked Transfer-Encoding
hyper's HTTP server and client code had a flaw that could trigger an integer overflow when decoding chunk sizes that are too big. This allows possible data loss, or if combined with an upstream HTTP proxy that allows chunk sizes larger than hyper does, can result in "request smuggling" or "desync attacks".
Vulnerability
Example:
GET / HTTP/1.1
Host: example.com
Transfer-Encoding: chunked
f0000000000000003
abc
0
hyper only reads the rightmost 64-bit integer as the chunk size. So it reads f0000000000000003 as 3. A loss of data can occur since hyper would then read only 3 bytes of the body. Additionally, an HTTP request smuggling vulnerability would occur if using a proxy which instead has prefix truncation in the chunk size, or that understands larger than 64-bit chunk sizes.
Read more about desync attacks: https://portswigger.net/research/http-desync-attacks-request-smuggling-reborn
Workarounds
Besides upgrading hyper, you can take the following options:
- Reject requests manually that contain a
Transfer-Encodingheader. - Ensure any upstream proxy rejects
Transfer-Encodingchunk sizes greater than what fits in 64-bit unsigned integers.
Credits
This issue was initially reported by Mattias Grenfeldt and Asta Olofsson.
Impact
To determine if vulnerable to data loss, these things must be true:
- Using HTTP/1.1. Since HTTP/2 does not use chunked encoding, it is not vulnerable.
- Using hyper as a server or client. The body would be improperly truncated in either case.
- Users send requests or responses with chunk sizes greater than 18 exabytes.
To determine if vulnerable to desync attacks, these things must be true:
- Using an upstream proxy that allows chunks sizes larger than 64-bit. If the proxy rejects chunk sizes that are too large, that request won't be forwarded to hyper.
An arithmetic operation produces a value that exceeds the integer type's maximum, causing it to wrap to an unexpected small value. Typical impact: incorrect size calculations leading to heap overflows or logic errors.
CVE-2021-32714 has a CVSS score of 5.9 (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 (0.14.10); 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.
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We have released the following patch versions:
- v0.14.10 (to be released when this advisory is published)
Frequently Asked Questions
- What is CVE-2021-32714? CVE-2021-32714 is a medium-severity integer overflow or wraparound vulnerability in hyper (rust), affecting versions < 0.14.10. It is fixed in 0.14.10. An arithmetic operation produces a value that exceeds the integer type's maximum, causing it to wrap to an unexpected small value.
- How severe is CVE-2021-32714? CVE-2021-32714 has a CVSS score of 5.9 (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 hyper are affected by CVE-2021-32714? hyper (rust) versions < 0.14.10 is affected.
- Is there a fix for CVE-2021-32714? Yes. CVE-2021-32714 is fixed in 0.14.10. Upgrade to this version or later.
- Is CVE-2021-32714 exploitable, and should I be worried? Whether CVE-2021-32714 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-2021-32714 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-2021-32714? Upgrade
hyperto 0.14.10 or later.