CVE-2026-42155: Magento LTS has Weak API Session ID — Predictable MD5 of Time-Derived Inputs

Should you act?

Critical and remotely reachable

No known exploitation yet, but it is network-reachable without credentials. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.

• Not in CISA KEV, but CISA’s SSVC assessment records a public proof-of-concept, so attack code exists.
• EPSS 0.5%, percentile 41: FIRST’s model estimates a 0.5% chance of exploitation activity in the next 30 days, higher than 41% of all scored CVEs (low). EPSS is a probability, not a verdict.
• Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
• Only applies if your dependency tree includes openmage/magento-lts (Composer (PHP)) <= 20.17.0, fixed in 20.18.0. Check your lockfile, including transitive dependencies.

What it is

Magento Long Term Support (LTS) is an unofficial, community-driven project provides an alternative to the Magento Community Edition e-commerce platform with a high level of backward compatibility. Prior to 20.18.0, the XML-RPC / SOAP API session ID is generated using an outdated, time-based construction rather than a Cryptographically Secure Pseudo-Random Number Generator (CSPRNG). All inputs to the MD5 hash are time-derived and non-secure. Because the resulting digest relies entirely on the timestamp and the PHP internal LCG state, the effective entropy is severely constrained. This violates the OWASP ASVS v4 requirement of ≥ 64 bits of entropy (V3.2.2) and NIST SP 800-63B standards. By narrowing the LCG window (via server state leaks or general predictability) and leveraging the lack of API rate-limiting, an attacker can generate a localized pool of candidate MD5 hashes and execute a high-speed online brute-force attack to hijack active API sessions. This vulnerability is fixed in 20.18.0.

From the GitHub advisory

Affected Version: OpenMage LTS ≤ 20.16.0 (confirmed on 20.16.0)

Affected File: https://github.com/OpenMage/magento-lts/blob/main/app/code/core/Mage/Api/Model/Session.php – start() method

Summary

The XML-RPC / SOAP API session ID is generated using an outdated, time-based construction rather than a Cryptographically Secure Pseudo-Random Number Generator (CSPRNG):

All inputs to the MD5 hash are time-derived and non-secure:

Weakness type: CWE-330, CWE-331, CWE-338.

Product: openmage/magento-lts (Composer (PHP)) · Assigned by: GitHub_M · Published: 2026-05-15

Affected and fixed versions

Severity scores

• Attack vector: network (remotely reachable)
• Attack complexity: low
• Attack requirements: none
• Privileges required: none (no login)
• User interaction: none

CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.

Exploitation and urgency

• CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
• CISA SSVC assessment: exploitation poc, automatable yes, technical impact total.
• FIRST EPSS (model of 2026-10-21): 0.49% probability of exploitation activity in the next 30 days; percentile 41.

Timeline

• 2026-05-05 — GitHub-reviewed advisory GHSA-2cwr-gcf9-pvxr published (critical)
• 2026-05-15 — CVE record last updated
• 2026-05-15 — CVE record published by GitHub_M

Sources and references

Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.

Vulnerability records

1. GitHub Advisory GHSA-2cwr-gcf9-pvxr github.com
2. CVE.org record CVE-2026-42155 cve.org
3. NVD entry CVE-2026-42155 nvd.nist.gov

Other references from the CVE record and advisory

1. github.com github.com

Critical Composer (PHP) advisories · Today’s Security Digest