CVE-2024-2757: PHP mb_encode_mimeheader DoS in PHP 8.3

Background / Overview​

What exactly is CVE‑2024‑2757?​

The technical summary​

• Vulnerable function: mb_encode_mimeheader() (mbstring extension).
• Affected upstream range: PHP 8.3.* builds prior to the patched release. The official GitHub security advisory and PHP release notes identify the fix in the 8.3.x security update. (github.com) (php.net)
• Failure mode: infinite loop / unreachable exit condition when the input contains particular patterns — broadly described as long sequences of non‑space characters followed by a space — which causes the function to fail to break/advance the internal folding pointer.
• Impact: Denial of Service (availability impact only). CVSS v3.1 base score reported around 7.5 (High) by multiple trackers, with the vector indicating remote exploitation requiring no privileges. (github.com)

Reproduction and PoC​

Why this is dangerous in the real world​

Where mb_encode_mimeheader gets called​

• Mail routines in web applications: contact forms, password reset emails, notification systems.
• Framework mailer abstractions: popular frameworks and libraries call the PHP mail encoding helpers; the advisory explicitly notes CakePHP 5’s usage as an example of how a framework can inherit the risk. If the framework or application passes untrusted user input directly into header encoding calls, it becomes a potential remote DoS target. (github.com)
• Email gateways and microservices: any service that accepts user input and encodes headers for outgoing mail.

Typical exploitation path​

• Attacker sends POST/PUT data or crafts fields (subject, display name, etc.) with long runs of non‑space characters finishing with a space.
• Target application receives the input and calls mb_encode_mimeheader(), either directly or through a library/framework.
• PHP process enters an endless loop, consumes CPU and/or thread capacity; repeated requests can exhaust pool workers (FastCGI, php-fpm), queue up responses, and produce a sustained DoS condition.

Verification and cross‑checking the facts​

• Am I running an affected interpreter?
• Does any code in my service invoke mb_encode_mimeheader() (directly or via a library)?

• PHP project advisory and changelog on php.net and the GitHub security advisory (GHSA‑fjp9‑9hwx‑59fq) — the code fix and advisory text are authoritative. (github.com) (php.net)
• National / vendor trackers — NVD, Ubuntu, Debian, Red Hat, and vendor security trackers provide CVSS scoring and distribution packaging status. Use these to map the upstream CVE to distro package versions you might be running in production.

Detection and hunting: how to find exposure in your estate​

1) Identify interpreter versions​

• Command line quick check: php -v (or php --version). Look for PHP 8.3.x and check the exact micro version.
• For containers/build pipelines, inspect images (docker images or your CI manifests) for base images that include PHP 8.3.

2) Find code paths that call the function​

• Grep across code repos and vendor directories:
• Search for mb_encode_mimeheader( and for framework mailer bindings that ultimately call it (e.g., check CakePHP, PHPMailer wrappers, or any custom mail helper).
• Audit templating and mail‑building modules that accept user input into header fields (subject, display name).

3) Observe runtime symptoms​

• High CPU load or processes stuck at 100% with long runtimes in php-fpm or FastCGI pools.
• Elevated request durations and connection queues on front‑end HTTP servers.
• Mail‑worker processes consuming CPU without progress.

4) Check packages and vendor advisories​

• Consult distro advisories: Ubuntu, Debian, Alpine, Red Hat trackers will list the fixed package version numbers for their repositories; container vendors and appliance vendors will issue their own notices.

Practical remediation steps (fast checklist)​

• Patch first, mitigate second.
• Upgrade to the PHP security release that includes the mb_encode_mimeheader fix. The PHP project published a security release (8.3.6 at the time of this advisory) and urges all 8.3 users to upgrade. If you run distribution packages, apply your vendor’s security updates (they may map the upstream fix into their packaging). (php.net) (github.com)
• If you cannot patch immediately, apply temporary mitigations:
• Introduce input validation and length limits on user-supplied fields that can become headers (subject, from/display name).
• Reject or transform header strings with long runs of non‑white‑space characters. For example, limit sequences of consecutive non‑space characters to a safe threshold (e.g., 200 characters) or insert safe breaking characters before passing to mb_encode_mimeheader().
• Ensure web server and PHP execution timeouts are strict: short max_execution_time, request timeouts at the front end (Nginx, Apache), and a small per‑request CPU quota if supported by process manager.
• Throttle mail endpoints and add rate limiting at the HTTP front door (WAF / reverse proxy) to reduce repeated automated exercise.
• Isolate mail encoding to a separate worker:
• Move mail encoding and delivery into an isolated worker process / queue worker (e.g., background queue like RabbitMQ, Redis Queue) so any single mail job looping does not take down the front‑end web handlers.
• Rebuild container images and CI pipelines:
• Update base images and rebuild service images to include the patched interpreter. Push changes through your CI/CD pipeline with regression testing for mail flows.
• Vendor mappings:
• Check your OS vendor security page (Ubuntu, Debian, Alpine, RHEL) to confirm package numbers and to apply vendor packages rather than blindly installing upstream binaries.

Safe wrapper example (practical code)​

<?php
function safe_mb_encode_mimeheader(string $text, string $charset = 'UTF-8', string $encoding = 'B', string $linefeed = "\r\n"): string {
    // Maximum allowed run of non-space characters before we insert a break
    $maxRun = 200;

    // If there is any very long run of non-whitespace characters, insert a space
    $text = preg_replace_callback('/([^\s]{' . ($maxRun + 1) . ',})/u', function($m) use ($maxRun) {
        $s = $m[0];
        // Insert spaces every $maxRun characters to avoid pathological input
        return trim(chunk_split($s, $maxRun, ' '));
    }, $text);

    // Optionally cap overall header length
    $maxTotal = 2000;
    if (mb_strlen($text, $charset) > $maxTotal) {
        $text = mb_substr($text, 0, $maxTotal - 3, $charset) . '...';
    }

    // Now call the encoding helper
    return mb_encode_mimeheader($text, $charset, $encoding, $linefeed);
}
?>

• The wrapper inserts spaces into long no‑space runs so the original mbstring folding logic can find a valid break.
• Choose conservative thresholds to balance compatibility and safety.
• Test with your mail clients and recipients to ensure header semantics remain acceptable.

Operational guidance for common environments​

For PHP‑FPM / FastCGI pools​

• Limit max_children and set process_recycling policies to avoid runaway worker accumulation.
• Monitor for zombie or long‑running workers and automate graceful restarts if CPU or wall clock thresholds are breached.

For containerized services​

• Rebuild images from patched upstream PHP builds or vendor images (Alpine, Debian, Ubuntu packages frequently publish fixed images; check vendor advisories).
• Avoid running unpatched interpreters in production images — rebuild and redeploy images with clear CVE labels.

For Windows + IIS hosters​

• Replace or update PHP Windows binaries with the official php.net security release or vendor‑supplied Windows builds.
• If you rely on packaged appliances or vendor solutions, check their security advisories and timelines for patch delivery.

For managed hosting / PaaS​

• Contact your provider to confirm they have applied the patch to the PHP runtime; require confirmation of patched microversions or ask for a planned patch window.

Critical analysis — strengths of the response and residual risks​

Strengths​

• The bug is specific and localized — it affects a single function and its line‑wrapping logic. That makes the remediation straightforward: upgrade to a security release from PHP or install vendor packages that include the fix. The PHP project published a targeted fix and issued a security release, giving operators a clear remediation path. (github.com) (php.net)
• The attack surface is limited to contexts where the function is invoked with user‑controlled input, so systems that do not use mb_encode_mimeheader() or that sanitize header inputs are inherently lower risk.

Residual risks and caveats​

• Hidden call sites: Many frameworks and libraries abstract mail building, so teams may have blind spots where the function is called indirectly. Static grep searches are helpful but not sufficient — audit dependency chains and third‑party packages used for mail delivery (framework mailers, plugins, CMS extensions).
• Packaging mismatches: The upstream PHP project, distro maintainers, and third‑party vendors sometimes label releases differently or backport fixes to other microversions. This produces inconsistent public statements about “fixed in 8.3.5 vs 8.3.6.” For operational certainty, prefer the PHP project's release notes and GitHub advisory for the code fix and then map to your OS vendor package using the vendor’s security tracker. We flag this as a verification point: cross‑check php.net/GHSA entries with your distro vendor metadata before declaring systems remediated. (github.com)
• Persistent availability impact: If an attacker can plant crafted input into persisted data stores (e.g., a database of user profiles) and an ongoing background job encodes headers from that store, the DoS condition could recur even after an immediate attack subsides — until the software is patched or the bad data is sanitized. That is a persistent availability risk that requires data hygiene as well as binary patching.

Recommended checklist (prioritized)​

• Inventory: Locate every PHP 8.3 interpreter in production, staging, test, and CI. Record exact microversions. (High priority)
• Patch: Apply the PHP security release (8.3.x security release per php.net) or vendor package that includes the mb_encode_mimeheader fix. Rebuild containers and redeploy. (Critical)
• Short‑term hardening: Add input validation and the safe wrapper for header encoding; enforce request rate limits and short execution timeouts. (High)
• Audit dependencies: Search for mb_encode_mimeheader() usage in your application, plugins, and composer packages; escalate any library that calls it to required patching or mitigation. (High)
• Monitoring: Add alerts for long‑running PHP workers, high CPU per php‑fpm process, and unusually high mail‑worker durations. (Medium)
• Data hygiene: Scan persisted fields (stored subjects, display names) for suspicious long runs of non‑space characters and sanitize them if found. (Medium)

Final verdict and action plan​

• Treat this as a high‑priority stability/security release and schedule the patch immediately.
• If you cannot patch in the next maintenance window, apply the temporary mitigations (input validation, wrapper, timeouts, rate limiting, isolation).
• Confirm the patch at both the interpreter level (php -v shows the patched microversion) and at the distribution package level (vendor security update applied).