phpseclib, a PHP secure communications library, did not perform padding validation in constant time when using AES in CBC mode, enabling a padding oracle timing attack (CVE-2026-32935). This vulnerability affects versions 0.1.1 through 1.0.26, 2.0.0 through 2.0.51, and 3.0.0 through 3.0.49. Remote attackers could exploit this timing discrepancy to obtain sensitive information. Ubuntu advisories USN-8769-1 and USN-8830-1 confirm the issue and provide fixed package versions for multiple Ubuntu LTS releases. The vulnerability is rated medium severity with a CVSS 3.1 score of 5.9 (AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:N).

The vulnerability allows remote attackers to conduct a padding oracle timing attack against AES in CBC mode implementations in phpseclib, potentially exposing sensitive information. There is no indication of impact on integrity or availability. No known exploits in the wild have been reported.

A fix is available. Users should update phpseclib to versions 1.0.27, 2.0.52, or 3.0.50 or later. Ubuntu users should apply the security updates provided in advisories USN-8769-1 and USN-8830-1 via standard system updates or Ubuntu Pro Extended Security Maintenance (ESM). No additional mitigation steps are required beyond applying these updates.