GHSA-X424-64QH-5J54
Vulnerability from github – Published: 2026-09-17 20:32 – Updated: 2026-09-17 20:32Summary
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.
{
"affected": [
{
"database_specific": {
"last_known_affected_version_range": "\u003c= 1.11.0"
},
"package": {
"ecosystem": "Packagist",
"name": "react/http"
},
"ranges": [
{
"events": [
{
"introduced": "0.6.0"
},
{
"fixed": "1.11.1"
}
],
"type": "ECOSYSTEM"
}
]
}
],
"aliases": [
"CVE-2026-84997"
],
"database_specific": {
"cwe_ids": [
"CWE-835"
],
"github_reviewed": true,
"github_reviewed_at": "2026-09-17T20:32:39Z",
"nvd_published_at": "2026-09-16T15:18:00Z",
"severity": "HIGH"
},
"details": "### Summary\n\nA 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.\n\nBoth 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.\n\n### Details\n\n`ChunkedDecoder::handleData()` loops `while ($this-\u003ebuffer !== \u0027\u0027)` and relies on the buffer shrinking each iteration. Two states leave the buffer unchanged while the loop condition stays true.\n\n**Terminal-chunk trailer.** After the terminating `0` chunk, any remaining buffer is treated as trailer data to skip:\n\n```php\n} elseif ($this-\u003echunkSize === 0) {\n $this-\u003ebuffer = (string)\\substr($this-\u003ebuffer, $positionCrlf);\n}\n```\n\nWhen the trailer holds no CRLF yet, `strpos()` returns `false`, PHP coerces that to `0` in `substr()`, and the buffer is never advanced. Neither the error guard (which requires a non-zero chunk size) nor the wait guard (which requires fewer than two bytes remaining) can fire, so the loop re-enters with identical state.\n\n**Off-by-one after a completed chunk.** Once a non-terminal chunk has been fully transferred, the \"chunk does not end with a CRLF\" error guard requires `strlen($this-\u003ebuffer) \u003e 2` while the wait guard requires `\u003c 2`. Exactly two non-CRLF bytes slip past both, and because the chunk is already complete nothing is consumed on the next iteration.\n\n### PoC\n\nRun the example server from the `reactphp/reactphp` README and send a malformed request. Note the missing trailing `\\r\\n`:\n\n```\nPOST / HTTP/1.1\\r\\nHost: x\\r\\nTransfer-Encoding: chunked\\r\\n\\r\\n0\\r\\nab\n```\n\nThe PHP process pegs at 100% CPU and stops answering legitimate requests. A body of `1\\r\\nAAB` triggers the second state.\n\nThe 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.\n\n### Impact\n\nDenial of service. The affected process stops responding entirely and has to be killed.\n\nServers 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.",
"id": "GHSA-x424-64qh-5j54",
"modified": "2026-09-17T20:32:39Z",
"published": "2026-09-17T20:32:39Z",
"references": [
{
"type": "WEB",
"url": "https://github.com/reactphp/http/security/advisories/GHSA-x424-64qh-5j54"
},
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2026-84997"
},
{
"type": "WEB",
"url": "https://github.com/reactphp/http/commit/b6d4688790adf3797071fcf88a3fc4225f30486a"
},
{
"type": "WEB",
"url": "https://github.com/FriendsOfPHP/security-advisories/blob/master/react/http/CVE-2026-84997.yaml"
},
{
"type": "PACKAGE",
"url": "https://github.com/reactphp/http"
},
{
"type": "WEB",
"url": "https://github.com/reactphp/http/releases/tag/v1.11.1"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H",
"type": "CVSS_V3"
}
],
"summary": "react/http: A malformed HTTP chunked body can lead to a denial-of-service and peg the CPU"
}
Sightings
| Author | Source | Type | Date | Other |
|---|
Nomenclature
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- Confirmed: The vulnerability has been validated from an analyst's perspective.
- Published Proof of Concept: A public proof of concept is available for this vulnerability.
- Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
- Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
- Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
- Not confirmed: The user expressed doubt about the validity of the vulnerability.
- Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.
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