phpseclib is a PHP library for secure communications that includes AES encryption. A flaw in the AES‑CBC decryption unpadding routine allows an attacker to gain information from timing differences, enabling a padding oracle timing attack. This information‑disclosure weakness (CWE‑208) can allow full recovery of encrypted plaintexts when an adversary can manipulate or observe ciphertext processing.

The vulnerability affects the phpseclib library across versions 1.0.0 to 1.0.26, 2.0.0 to 2.0.51, and 3.0.0 to 3.0.49. Versions 1.0.27, 2.0.52, and 3.0.50 and later contain the fix.

The CVSS score of 8.2 reflects high severity. The EPSS score of less than 1 % suggests a low probability of exploitation. It is not listed in CISA’s Known Exploited Vulnerabilities catalog. Based on the description, it is inferred that an attacker would need to send crafted ciphertexts to a service performing AES‑CBC decryption and measure response times to recover plaintext. The likely attack vector is remote, through network or application interfaces that accept encrypted data.