PHP Standard Library (PSL) is set of APIs covering async, collections, networking, I/O, cryptography, terminal UI, etc. In versions 6.1.0, 6.1.1 and 6.2.0, the Psl\H2\ServerConnection does not validate that the total bytes received in DATA frames match the content-length header declared in the HEADERS frame, allowing request smuggling. This is in violation of RFC 9113 §8.1.1. A malicious client is able to send more DATA bytes than declared, smuggling additional content past application-level size limits and send fewer DATA bytes than declared and close the stream early, causing applications that trust the declared length to behave incorrectly. The vulnerability is only reachable for consumers using Psl\H2\ServerConnection directly to accept untrusted client traffic. Consumers of documented high-level PSL APIs are not affected. This issue has been fixed in versions 6.1.2 and 6.2.1.

Affected: Packagist: php-standard-library/h2, Packagist: php-standard-library/php-standard-library

## Impact

`Psl\H2\ServerConnection` does not validate that the total bytes received in DATA frames match the `content-length` header declared in the HEADERS frame, in violation of RFC 9113 §8.1.1.

A malicious client can: - Send more DATA bytes than declared, smuggling additional content past application-level size limits. - Send fewer DATA bytes than declared and close the stream early, causing applications that trust the declared length to behave incorrectly.

The vulnerability is only reachable for consumers using `Psl\H2\ServerConnection` directly to accept untrusted client traffic. The high-level `Psl\HTTP\Server` is in active development and was not yet released at the time of this advisory; consumers of documented high-level PSL APIs are not affected.

## Patches

Fixed in [6.1.2](https://github.com/php-standard-library/php-standard-library/releases/tag/6.1.2) and [6.2.1](https://github.com/php-standard-library/php-standard-library/releases/tag/6.2.1).

- Parses and validates the `content-length` header on incoming HEADERS (server-side only — clients do not enforce this per RFC 9110 §9.3.2). - Tracks cumulative DATA frame payload length per stream. - Throws `StreamException` on mismatch or overflow.

Regression tests landed in [#781](https://github.com/php-standard-library/php-standard-library/pull/781), 9 of the new tests fail against the pre-fix code, proving the validation boundary is enforced.

## Workarounds

None at the protocol layer. Applications using `Psl\H…

Withdrawn by the publisher.

Affected: Ubuntu:Pro:14.04:LTS: php5, Ubuntu:Pro:16.04:LTS: php7.0, Ubuntu:Pro:18.04:LTS: php7.2, Ubuntu:Pro:20.04:LTS: php7.4, Ubuntu:22.04:LTS: php8.1, Ubuntu:24.04:LTS: php8.3, Ubuntu:25.10: php8.4, Ubuntu:26.04:LTS: php8.5

PHP Standard Library (PSL) is set of APIs covering async, collections, networking, I/O, cryptography, terminal UI, etc. In versions 6.1.0, 6.1.1 and 6.2.0, the Psl\H2\ServerConnection does not validate that the total bytes received in DATA frames match the content-length header declared in the HEADERS frame, allowing request smuggling. This is in violation of RFC 9113 §8.1.1. A malicious client is able to send more DATA bytes than declared, smuggling additional content past application-level size limits and send fewer DATA bytes than declared and close the stream early, causing applications that trust the declared length to behave incorrectly. The vulnerability is only reachable for consumers using Psl\H2\ServerConnection directly to accept untrusted client traffic. Consumers of documented high-level PSL APIs are not affected. This issue has been fixed in versions 6.1.2 and 6.2.1.