Critical

PHP Session Deserializer Use-After-Free

IdentifiersCVE-2015-6835CWE-416

CVE-2015-6835 is a use-after-free vulnerability in PHP's session deserialization logic affecting PHP before 5.4.45, 5.5.x before 5.5.29, and 5.6.x before 5.6.13. The flaw is triggered when the session deserializer performs multiple php_var_unserialize calls and mishandles object lifetime, allowing freed memory to be reused while still referenced. In practice, crafted session content can cause PHP to deserialize truncated or malformed serialized data into unintended objects, creating a path to memory corruption. In exploit chains documented at the time, the bug was particularly relevant when session data was stored outside PHP's standard session storage, such as in MySQL-backed session stores, where truncation of serialized session data could transform attacker-controlled input into a dangerous deserialization state. Successful exploitation can lead to arbitrary code execution or application crashes.

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ANALYST BRIEF

Impact, mitigation & remediation

What it means. What to do now. Patch path, mitigations, and the assume-compromise checklist.

Impact

What an attacker gets, and what they’ve been doing with it.

Successful exploitation can result in arbitrary code execution in the context of the PHP process or denial of service through process crashes caused by use-after-free memory corruption. Where the vulnerable deserialization path can be influenced remotely through attacker-controlled session content, the flaw can be used as the memory-corruption component of a broader exploit chain to achieve full compromise of the web application environment.

Mitigation

If you can’t patch tonight, do this now.

Avoid storing raw serialized PHP session data in backends that can alter, truncate, or otherwise transform the serialized byte stream before it is deserialized. If a database-backed session store is required, encode session data in a form that preserves byte integrity before storage. Prefer PHP's standard session storage where feasible, as it was noted as not being affected in the same way in the documented exploit context. As a defense-in-depth measure in MySQL-backed deployments, enable strict SQL modes such as STRICT_TRANS_TABLES or STRICT_ALL_TABLES to prevent silent truncation that can help create exploitable malformed session data.

Remediation

Patch, then assume compromise.

Upgrade PHP to a fixed release: 5.4.45 or later, 5.5.29 or later, 5.6.13 or later, or a newer unaffected branch such as 7.0.0 or later. Systems using distribution-packaged PHP should apply the vendor-provided security update that includes the fix for CVE-2015-6835. Where the vulnerability is part of an application-specific exploit chain, update the affected application components as well.

PUBLIC EXPLOITS

Exploits

No valid public exploits. Mallory filtered out 1 candidate as fakes, detection scripts, or README-only repos.

VALID 0 / 1 TOTALView more in app

All candidate exploits were filtered out by Mallory's validation.

EXPOSURE SURFACE

Affected products & vendors

Products and vendors Mallory has correlated with this vulnerability. Open in Mallory to drill down to specific CPE configurations and version ranges.

VendorProductType

PHP GroupPhpapplication

Vendor-confirmed product mapping. Mallory continuously reconciles this list against your asset inventory.

ACTIVITY FEED

Recent activity

2 sources tracked across advisories, community write-ups, and news. New activity surfaces here as Mallory finds it.

2 SOURCESView more in app

calif blogNews

May 1, 2026

MAD Bugs: Finding and Exploiting a 21-Year-Old Vulnerability in PHP

A historical PHP unserialize-related use-after-free vulnerability mentioned as part of a broader lineage of similar bugs.

slideshareNews

Oct 21, 2015

脆弱性は誰のせい? PHP、MySQL、Joomla! の責任やいかに | PPTX

PHPのセッションデコード/デシリアライズ挙動に関する脆弱性で、Joomla!のCVE-2015-8562成立の直接要因として説明されている関連脆弱性。

What this page doesn’t show

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Exposure mapping

Query your assets running an affected version, and investigate the blast radius.

Threat actor evidence

Every observed campaign linking this CVE to a named adversary.

Associated malware

Malware families riding this exploit, with evidence and IOCs.

Detection signatures

YARA, Sigma, Snort, and vendor rules, auto-deployed to your SIEM.

Vendor-by-vendor mapping

Cross-references every affected SKU, including bundled OEM variants.

Social activity