Urgent Patch: PHP 8.3/8.4 CVE-2024-11235 Use After Free Risks

A subtle sequence of PHP internals — an exception triggered inside a magic property setter combined with a null‑coalescing assignment — can produce a use‑after‑free in the engine’s shutdown path, leaving unpatched PHP 8.3 and 8.4 builds exposed to high‑impact crashes and, in some scenarios, the risk of remote code execution; operators must treat CVE‑2024‑11235 as an urgent patching priority and follow concrete mitigation and detection steps immediately.

Background / Overview​

CVE‑2024‑11235 was disclosed in March–April 2025 and affects PHP versions in the 8.3 and 8.4 series prior to their March 2025 security releases. The bug is rooted in reference‑count handling inside php_request_shutdown: an exception handler can free variables via cleanup_live_vars, and php_request_shutdown later performs reference counting (zend_gc_refcount / zend_gc_delref) on memory that may already have been freed. This sequence produces a classic use‑after‑free (CWE‑416) condition. The upstream advisory and security fixes were published as part of the March 2025 PHP releases (patched builds: PHP 8.3.19 and PHP 8.4.5).
Multiple vulnerability trackers and vendors list the affected ranges as:

• PHP 8.3.* before 8.3.19
• PHP 8.4.* before 8.4.5.

The practical impact reported by several databases ranges from high availability loss (crashes, sustained DoS) to potential remote code execution (RCE) in environments where an attacker can manipulate memory layout. That potential for RCE is conditional — it depends on heap layout and whether an attacker can reliably shape allocations — but public and vendor advisories treat the scenario as plausible enough to demand prompt remediation.

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Why this matters: engine internals meet real‑world attack surface​

PHP is everywhere in web stacks: application servers, shared hosting panels, packaged container images, and vendor appliances. A vulnerability in the interpreter itself — especially one reachable from networked application code that uses common language features like magic methods and the ??= operator — has outsized operational consequences.

• Widespread exposure: Many web applications use property overloading (magic set/get) patterns or rely on dynamic property assignment. A vulnerable interpreter in a public‑facing application server provides an attacker an avenue to trigger the condition remotely.
• Low privilege, high impact: The vulnerability does not require authentication in many exploitation models; it can be triggered by crafted inputs sent to unprivileged endpoints that execute PHP application logic. Several trackers list the attack vector as network and privileges required as none.
• Operational knock‑on effects: Unscheduled process crashes, repeated restarts, and thread exhaustion lead to degraded service and operational churn (auto‑restarts, cache re‑warm, increased latency) even if RCE is not achieved. These are the primary, immediate availability risks for most deployments.

This is not a purely academic claim: historical patterns show that use‑after‑free and reference counting bugs in core libraries frequently produce denial‑of‑service and, in constrained or groomed environments, can be escalated to memory corruption class exploits. Practical security guidance therefore treats correctness fixes in runtimes as high priority.

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Technical analysis — what exactly goes wrong​

The sequence in plain English​

• Application code triggers a property assignment that invokes a class’s magic __set() method.
• Inside __set(), an exception is raised (either intentionally or via a failure path).
• Exception handling can cause variable cleanup routines to free or otherwise dismantle live variables (cleanup_live_vars) as the engine unwinds.
• Later, php_request_shutdown — the routine that walks and adjusts references during request termination — executes reference counting operations (zend_gc_refcount and zend_gc_delref) on values that may have already been freed by the exception cleanup.
• Because freed memory often still contains allocator metadata (for example zend_mm free‑chunk metadata), subsequent dereferences produce a use‑after‑free: the code reads or updates memory that was returned to the allocator.

Why reference counting matters here​

PHP’s garbage‑collection and memory management rely on reference counters to determine object lifetime. When code decrements a refcount on a pointer that points to freed memory, the engine can both read stale values and write changes into spaces that have been reallocated for other purposes. On modern allocators, freed chunks often retain metadata in predictable places; the advisory highlights the risk that the Zend allocator’s metadata patterns may be co‑opted, making this UAF path more than a simple crash vector.

Triggers: __set, ??= and exceptions​

Public write‑ups and the upstream advisory emphasize a specific triggering pattern: magic property assignment paired with the null coalescing assignment operator (??=) and thrown exceptions. While the operator and magic method are common, the exact exploitability depends on an attacker’s ability to influence memory layout — for example, by causing particular allocation sequences or heap grooming in a long‑lived process. Trackers call the exploitation complexity “high” in some scoring schemes because of that requirement, while others highlight the practical plausibility.

What attackers could achieve — practical vs. theoretical​

• High‑confidence outcome: Denial‑of‑Service — crashes or process instability that remove service availability until recovery or patching. This is straightforward and repeatable in unpatched environments.
• Plausible but conditional outcome: Remote Code Execution — several vendors and trackers state RCE is possible if an attacker can reliably manipulate heap layout and allocator metadata. There is no broadly published, reliable RCE proof‑of‑concept in the public domain at the time of writing, but the vulnerability’s semantics leave open that worst case. Treat RCE as a realistic worst‑case, not as the default assumption.

Note: Where public tracker entries or vendor advisories assert RCE risk, they do so based on the theoretical ability to manipulate allocator metadata given UAF in refcounting. Those claims are reasonable, but they are contingent — responsible defenders should treat them as conditional and prioritize patching and mitigation accordingly.

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Cross‑checks and authoritative references​

The upstream PHP security advisory / GitHub security advisory documents the issue and the patched releases (8.3.19, 8.4.5).
Linux distributions and major vendor advisories (Ubuntu, Red Hat/Rocky, Debian downstream notices) tracked the issue and rolled fixes into their package updates, underscoring the urgency for operators to apply vendor patches.
Independent vulnerability databases (OpenCVE, Wiz, Snyk) summarize the vulnerability, affected versions, and mitigation guidance and report high CVSS/impact readings in at least some scoring frameworks — these provide additional corroboration for the technical facts and remediation actions.
Historical analyses of similar use‑after‑free and allocation‑failure bugs highlight how subtle control over allocator state or memory pressure can make a theoretical bug practical in multi‑tenant or constrained environments; these operational lessons are worth reading for teams deciding how urgently to act.

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Exploitability and real‑world risk — a balanced view​

No vulnerability sits in a vacuum. To triage CVE‑2024‑11235 sensibly, operators must weigh exploitability, exposure, and mitigations.

• Attack surface: Network‑facing PHP applications with endpoints that execute user‑influenced logic (for example, form handlers, upload processors, API endpoints) are the primary concern. If your environment exposes PHP‑powered services directly to the internet or untrusted networks, treat exposure as high risk.
• Environment specifics: Shared hosting, container hosts with multiple tenants, or constrained VMs with aggressive memory caps increase the chance that an attacker can nudge allocator behavior or trigger the necessary heap conditions. Conversely, single‑tenant, well‑provisioned hosts with fast patch cycles lower exploitation probability.
• Triggers and reproducibility: While crash‑only PoCs might be straightforward in some deployments, turning a UAF into reliable RCE typically requires sophisticated heap grooming. Public advisories list the worst‑case (RCE) as plausible but contingent, and most incident response playbooks treat DoS as the immediate, high‑confidence outcome.
• Availability vs. confidentiality: Even if RCE is not achieved, sustained DoS or process crashes have real business impact (outages, lost transactions, failed payments). Microsoft and other vendors classify risks that cause sustained or persistent availability loss as severe for operational systems; defenders should not conflate "no immediate RCE proof" with "no security risk." The operational effects can be critical. (User‑provided MSRC language describes this severity for availability‑class vulnerabilities.)

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Detection and hunting: telemetry to collect now​

• Inventory first (highest priority)
• Identify all hosts running PHP 8.3 or 8.4. Pay attention to packaged images (official containers, vendor appliances) and long‑lived application servers. Use package managers, container images, and runtime php -v checks to build a complete list.
• Monitor for crash signatures
• Alert on repeated PHP‑FPM or httpd/php process crashes, OOM killer events coincident with web request spikes, and core dumps. Correlate with request timestamps to find likely triggering inputs.
• Request‑level telemetry
• Capture or log request payloads (sanitized) and track unusual patterns around endpoints that use property setting / object creation. Look for long or malformed inputs sent near the time of a crash. If you have an application firewall or WAF, enable logging for relevant PHP endpoints.
• Test safely in staging
• Recreate the reported triggering patterns in a controlled staging environment using the simplified patterns described in public write‑ups (magic __set + ??= with exceptions) to confirm whether your particular application code reaches the vulnerable sequence. Do not run untrusted PoCs in production.
• EDR/Host detection
• Watch for anomalous allocator metadata writes or suspicious process behavior (unexpected writes into memory regions normally associated with freed chunks). These are advanced detections and may require heap introspection tooling or specialized host agents. Flagging frequent restarts, high error rates, and core dumps is a pragmatic first step.

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Mitigation and remediation — concrete steps for operators​

• Patch now: Upgrade to PHP 8.3.19 or PHP 8.4.5 (or later) on all systems running vulnerable series. This is the definitive fix; vendor packages for common distributions were issued shortly after the advisory. If you rely on distribution packages, install the vendor security updates — don’t rely on ad‑hoc source builds without verifying the applied patch.
• If you cannot patch immediately, implement compensating controls:
• Isolate affected instances behind strict network ACLs or reverse proxies to limit exposure to untrusted clients.
• Use a WAF with tuned rules to block suspicious payloads and to log/alert on patterns targeting endpoints that perform dynamic property assignment.
• Reduce process privileges and ensure robust crash handling (supervisor restarts, graceful failover) to limit service disruption and reduce process permanence on the host if exploited.
• Code hardening:
• Review application code for use of property overloading and null coalescing assignment in contexts where exceptions may be thrown. Where practical, prefer explicit checks and avoid relying on magic methods for critical logic paths.
• Add defensive programming: catch exceptions where appropriate and avoid state changes in destructors or in cleanup paths that leave dangling pointers. These are general best practices that reduce risk in many UAF patterns.
• Rebuild and repackage:
• If you ship custom PHP builds or container images, rebuild them using fixed upstream sources and run your full test suite. Replace any vendor images that include vulnerable PHP binaries.

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Incident response checklist (if you detect exploitation or suspicious crashes)​

• Quarantine affected hosts: Remove public network access and isolate from other services.
• Collect forensic artifacts: core dumps, php‑fpm logs, webserver logs, request payloads, memory snapshots if possible.
• Patch and replace: Apply official patched binaries or redeploy to a patched container image.
• Rotate credentials and secrets: If you suspect post‑exploitation actions, rotate service credentials and update secrets that may have been exposed.
• Rebuild from known‑good artifacts: For compromised systems, rebuild rather than attempting in‑place remediation.
• Post‑incident review: Conduct a code and dependency audit to ensure no other interpreters or embedded runtimes carry similar bugs.

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Vendor response and timeline​

• Upstream PHP released security updates that include the fix (PHP 8.3.19 and PHP 8.4.5 in the March 2025 cycle). Zend’s release notes and the PHP GitHub security advisory document the corrected code path and the patched versions.
• Linux distributions (Ubuntu, Red Hat, Rocky, and others) incorporated vendor advisories and CVE mappings into their security errata and issued fixed packages for their supported releases soon after the upstream patch. Operators should consult their distribution errata pages to identify the exact package names and rebuild instructions relevant to their environment.
• Commercial vulnerability databases and scanning vendors (OpenCVE, Wiz, Snyk, Rapid7) published alerts, risk scores, and remediation guidance; these sources are useful cross‑checks for teams building scanning rules and patch windows. Note that numeric severity (CVSS) varies by tracker depending on scoring inputs and risk models.

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Practical recommendations — prioritized checklist​

• Inventory every PHP artifact (3 hours)
• Find container images, packages, and VPS images running PHP 8.3 or 8.4. Don’t forget build pipelines and CI images.
• Patch or redeploy (24–72 hours)
• Upgrade to PHP 8.3.19 / 8.4.5 or apply vendor packages. Schedule rolling restarts to avoid simultaneous cache storms.
• Harden ingress and add monitoring (immediate)
• Limit public access, increase logging, and enable WAF rules targeted at endpoints that handle dynamic properties or accept user‑provided objects/JSON.
• Test and validate (48–96 hours)
• Recreate triggering patterns in staging, verify the patched runtime no longer crashes, and validate that the vendor patch is present in binary metadata.
• Longer term: code review and dependency hygiene (weeks)
• Audit use of magic methods and defensive exception handling patterns; incorporate specific testcases that exercise shutdown and exception cleanup paths.

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What remains uncertain (and what to watch for)​

• Public exploit availability: At the time of writing, multiple trackers report theoretical RCE possibility but do not point to a widely circulated, reliable RCE PoC in the public domain. That said, sophisticated attackers (or private proof‑of‑concepts) may exist; therefore do not delay remediation pending proof of exploit. Flag any unverified claims as conditional and act on the vendor patches as the canonical fix.
• Scoring variance: Different trackers publish a range of severity scores (from high to critical) because CVSS inputs differ — especially around attack complexity and scope of impact. Operators should prioritize their own exposure model rather than a single numeric score. If you host public PHP applications or multi‑tenant services, assume the higher severity posture until patched.

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Conclusion​

CVE‑2024‑11235 is a reminder that language runtime correctness matters. This is not a niche parser bug or a low‑impact library quirk — it touches the interpreter’s memory management and can produce fast, repeatable crashes and, under certain conditions, a pathway to memory corruption and potential RCE. The fix is available in the March 2025 PHP releases (8.3.19 and 8.4.5), and major distributors have published updates; the correct operational response is straightforward: inventory, patch, isolate, and monitor.
If you run PHP 8.3 or 8.4 in production, update immediately, verify patched binaries across your estate, and harden ingress while you validate the remediation. Treat DoS as the most likely immediate impact and RCE as a plausible worst case — both justify urgent patching and a careful post‑patch verification cycle.

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Source: MSRC Security Update Guide - Microsoft Security Response Center