Summary
react/http: A malformed HTTP chunked body can lead to a denial-of-service and peg the CPU
A malformed HTTP message using Transfer-Encoding: chunked can drive React\Http\Io\ChunkedDecoder into an infinite loop, pegging a CPU core and freezing the event loop. Because ReactPHP is single-threaded, one such message stalls the entire process for every client until it is killed.
Both directions are affected. ChunkedDecoder decodes chunked request bodies for React\Http\HttpServer and chunked response bodies for React\Http\Browser, so a server can be attacked by a malicious client and a client can be attacked by a malicious or compromised server.
Details
ChunkedDecoder::handleData() loops while ($this->buffer !== '') and relies on the buffer shrinking each iteration. Two states leave the buffer unchanged while the loop condition stays true.
Terminal-chunk trailer. After the terminating 0 chunk, any remaining buffer is treated as trailer data to skip:
} elseif ($this->chunkSize === 0) {
$this->buffer = (string)\substr($this->buffer, $positionCrlf);
}
When the trailer holds no CRLF yet, strpos() returns false, PHP coerces that to 0 in substr(), and the buffer is never advanced. Neither the error guard (which requires a non-zero chunk size) nor the wait guard (which requires fewer than two bytes remaining) can fire, so the loop re-enters with identical state.
Off-by-one after a completed chunk. Once a non-terminal chunk has been fully transferred, the "chunk does not end with a CRLF" error guard requires strlen($this->buffer) > 2 while the wait guard requires < 2. Exactly two non-CRLF bytes slip past both, and because the chunk is already complete nothing is consumed on the next iteration.
PoC
Run the example server from the reactphp/reactphp README and send a malformed request. Note the missing trailing \r\n:
POST / HTTP/1.1\r\nHost: x\r\nTransfer-Encoding: chunked\r\n\r\n0\r\nab
The PHP process pegs at 100% CPU and stops answering legitimate requests. A body of 1\r\nAAB triggers the second state.
The client side is reachable the same way: a Browser request to a server that answers with Transfer-Encoding: chunked and either malformed body shape hangs the client process.
Impact
Denial of service. The affected process stops responding entirely and has to be killed.
Servers behind a reverse proxy that parses and re-frames HTTP, such as a typical nginx setup, are not affected on the server side, because the proxy normalises the request before it reaches PHP. That mitigation does not extend to the client side: outbound requests made with Browser reach the remote server directly, so an application fetching attacker-influenced URLs is affected regardless of what sits in front of it.
CVE-2026-84997 has a CVSS score of 7.5 (High). 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 (1.11.1); 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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Frequently Asked Questions
- What is CVE-2026-84997? CVE-2026-84997 is a high-severity security vulnerability in react/http (composer), affecting versions >= 0.6.0, <= 1.11.0. It is fixed in 1.11.1.
- How severe is CVE-2026-84997? CVE-2026-84997 has a CVSS score of 7.5 (High). 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 react/http are affected by CVE-2026-84997? react/http (composer) versions >= 0.6.0, <= 1.11.0 is affected.
- Is there a fix for CVE-2026-84997? Yes. CVE-2026-84997 is fixed in 1.11.1. Upgrade to this version or later.
- Is CVE-2026-84997 exploitable, and should I be worried? Whether CVE-2026-84997 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-2026-84997 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-2026-84997? Upgrade
react/httpto 1.11.1 or later.