CVE-2020-36326

Object injection in PHPMailer/PHPMailer

Workarounds

Validate paths to loaded files using the same pattern as used in isPermittedPath() before using them in any PHP file function, such as file_exists. This method can't be used directly because it is protected, but you can implement the same thing in calling code. Note that this should be applied to all user-supplied paths passed into such functions; it's not a problem specific to PHPMailer.

Credit

This issue was found by Fariskhi Vidyan, reported and managed via Tidelift.

This is a reintroduction of an earlier issue (CVE-2018-19296) by an unrelated bug fix in PHPMailer 6.1.8. An external file may be unexpectedly executable if it is used as a path to an attachment file via PHP's support for .phar files`. Exploitation requires that an attacker is able to provide an unfiltered path to a file to attach, or to trick calling code into generating one. See this article for more info.

Untrusted serialized data is processed by a deserializer that can instantiate arbitrary objects or execute code as a side effect. Typical impact: arbitrary code execution or logic abuse.

CVE-2020-36326 has a CVSS score of 9.8 (Critical). 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 (6.4.1); upgrading removes the vulnerable code path.

Kodem intelligence

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This issue was patched in the PHPMailer 6.4.1 release. This release also implements stricter filtering for attachment paths; paths that look like any kind of URL are rejected.

1. What is CVE-2020-36326? CVE-2020-36326 is a critical-severity insecure deserialization vulnerability in phpmailer/phpmailer (composer), affecting versions >= 6.1.8, < 6.4.1. It is fixed in 6.4.1. Untrusted serialized data is processed by a deserializer that can instantiate arbitrary objects or execute code as a side effect.
2. How severe is CVE-2020-36326? CVE-2020-36326 has a CVSS score of 9.8 (Critical). 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 phpmailer/phpmailer are affected by CVE-2020-36326? phpmailer/phpmailer (composer) versions >= 6.1.8, < 6.4.1 is affected.
4. Is there a fix for CVE-2020-36326? Yes. CVE-2020-36326 is fixed in 6.4.1. Upgrade to this version or later.
5. Is CVE-2020-36326 exploitable, and should I be worried? Whether CVE-2020-36326 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-2020-36326 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-2020-36326? Upgrade phpmailer/phpmailer to 6.4.1 or later.

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