CVE-2026-61816: Uncontrolled Resource Consumption and Algorithmic Complexity in zbateson/mail-mime-parser

The library zbateson/mail-mime-parser serves as a PHP-based alternative to native IMAP functions and Pear libraries for decoding RFC 822 and MIME compliant email structures. It is commonly implemented in webmail applications, customer support ticketing desks, and mail gateway APIs to process inbound email traffic. Because these systems ingest untrusted payloads directly from the public internet, any processing inefficiencies in the parser represent a significant remote attack vector.

Three independent execution pathways within the parser exhibit super-linear time complexity or unbound memory usage patterns. This vulnerability class, categorized under CWE-400 (Uncontrolled Resource Consumption), allows attackers to systematically bypass physical size constraints (such as a 2 MB or 10 MB maximum request limit) by optimizing payload complexity rather than file volume. An input document of negligible physical size can force host systems to stall processing threads indefinitely or exhaust the available RAM allocation.

The parsing operations in mail-mime-parser are designed to execute lazily. However, the costly super-linear evaluation is forced on the target host as soon as the calling application performs its first full-text read or executes the getAllParts() API. Consequently, passive application-level checks are incapable of stopping the exhaustion cycle once a message is loaded into memory.

The underlying vulnerability stems from three distinct structural mechanics within the parser's logic. These mechanics fail to constrain execution iteration or structure allocation limits, enabling predictable and repeatable algorithmic failure states.

Vector 1: Quadratic Multipart Nesting Depth

During multipart boundary discovery, the parser evaluates each read line against the boundary of the active part and its parent structures. When processing a MIME document with nesting depth $D$, the parser traverses up the proxy hierarchy to verify if a line signifies the end of a parent structure. Under deep recursion, this parent traversal executes dynamically via helper functions. If the message contains $D$ nested multipart objects, evaluating the boundary termination for each line triggers $O(D^2)$ boundary comparisons, translating directly to excessive CPU cycles for relatively low-byte inputs.

Vector 2: Quadratic Sibling MIME Part Appending

When a multipart MIME container processes a sequence of sibling attachments, each node is sequentially registered into the internal tree configuration using PartChildrenContainer::add. The unpatched method used PHP's native \array_splice function to insert elements into the container structure. Because \array_splice causes PHP to rebuild and re-index the internal array from scratch, inserting $N$ sibling parts sequentially results in $O(N^2)$ memory and processor operations. This forces high-frequency array shift operations that lock up the execution thread.

Vector 3: Unbounded Header Buffering

The class HeaderParserService::parse() reads RFC 822 header groups using an unconstrained loop that terminates only when encountering an empty line. Each header line is decoded and mapped to individual PHP objects. Because the parser does not limit either the total count of headers or the aggregate size of the header block, a payload containing tens of thousands of mock headers can easily overwhelm memory boundaries. Due to internal object management structures in PHP, a small array of thousands of small headers can expand exponentially in RAM usage, leading to a kernel Out-Of-Memory (OOM) termination of the parent PHP-FPM pool or worker process.

The mitigation and patch history illustrates how minor architectural details in PHP array and loop implementations can trigger catastrophic resource exhaustion.

Sibling Insertion Refactoring

In the vulnerable version of PartChildrenContainer.php, elements were appended by passing a computed default index directly into \array_splice:

// Vulnerable Implementation
public function add(IMessagePart $part, ?int $position = null) : static
{
    $index = $position ?? \count($this->children);
    \array_splice(
        $this->children,
        $index,
        0,
        [$part]
    );
    return $this;
}

The patch addresses this structural vulnerability by introducing a fast-path bypass. When the position is unspecified or evaluates to the end of the array, the element is appended directly using PHP's native bracket syntax, which executes in $O(1)$ amortized complexity instead of rebuilding the array via \array_splice:

// Patched Implementation in v3.0.6 & v4.0.2
public function add(IMessagePart $part, ?int $position = null) : static
{
    if ($position === null || $position >= \count($this->children)) {
        $this->children[] = $part;
    } else {
        \array_splice(
            $this->children,
            $position,
            0,
            [$part]
        );
    }
    return $this;
}

Header Limitation Mechanisms

The class HeaderParserService.php was altered to enforce maximum thresholds, preventing unbounded memory growth. The patched parser tracks both the total processed headers and the size offset dynamically via \ftell:

// Patched Header Parsing Loop with Limits
if ($count >= $this->maxHeaderCount || \ftell($handle) - $start >= $this->maxHeaderSizeBytes) {
    $container->addError(
        'Header count or total size limit reached while parsing headers',
        LogLevel::ERROR
    );
    break;
}

Exploiting this vulnerability does not require complex memory corruption techniques or bypasses of standard exploit mitigations (such as ASLR or DEP). Instead, the attacker sends a syntactically valid email structured to maximize the algorithmic load on the backend server.

An attacker can craft a payload leveraging Vector A, which nests 300+ multipart groups inside one another. The outer structures are progressively left open, requiring the parser to traverse the entire parent chain recursively for every single character read. A proof-of-concept file payload demonstrating this behavior can be smaller than 200 KB yet require up to 30 seconds of high-priority CPU time on modern infrastructure.

Alternatively, an attacker can target Vector C by supplying thousands of mock headers (e.g., X-Header-1: A\r\n, X-Header-2: B\r\n, etc.). Because PHP applications generally share worker pools (like PHP-FPM or Apache mod_php), exhausting all available processes or triggering OOM failures on the pool quickly leads to an application-wide HTTP 502/504 Bad Gateway or Service Unavailable state.

To remediate these issues, the vendor introduced configuration properties in di_config.php to define and enforce threshold caps. By default, the parser now restricts the maximum MIME part nesting depth to 256, the maximum header count to 1000, and the maximum cumulative header size to 1,048,576 bytes.

While these limits successfully disrupt the exploit vectors, a minor residual risk remains. If an attacker passes a single, continuous header line of several megabytes, the loop validation in HeaderParserService::parse is not triggered until after the entire line has been read and loaded into a single string buffer. If the system memory configuration is low, a single massive string operation could still trigger an unexpected OOM crash before the boundary verification is reached.