CVE-2026-4587: 
PHP vulnerability analysis and mitigation

CVE-2026-4587 is an improper certificate validation vulnerability in HybridAuth, an open-source PHP social sign-on library. The flaw exists in the SSL Handler component within src/HttpClient/Curl.php, where manipulation of the curlOptions argument allows improper certificate validation. All versions up to and including 3.12.2 are affected. The vulnerability was disclosed on March 23, 2026, after the reporter contacted the project maintainers on February 1, 2026, without receiving a response. It carries a CVSS v3.1 base score of 3.7 (Low) and a CVSS v4.0 base score of 6.3 (Medium) (Feedly, EUVD).

The root cause is classified under CWE-295 (Improper Certificate Validation) and CWE-287 (Improper Authentication). The vulnerability arises because the curlOptions argument in src/HttpClient/Curl.php can be manipulated in a way that disables or bypasses SSL/TLS certificate verification during outbound HTTP requests made by HybridAuth to social identity providers. This creates an opportunity for an adversary-in-the-middle (AiTM) attack, where a remote attacker with a privileged network position could intercept and potentially modify communications between the HybridAuth library and external OAuth/social API endpoints. Exploitation requires high attack complexity, as the attacker must be positioned on the network path between the affected application and the remote identity provider (Feedly, GitHub Issue).

Successful exploitation primarily affects integrity, allowing a remote attacker to intercept or tamper with OAuth tokens, authentication responses, or user profile data exchanged between HybridAuth and social identity providers such as Facebook, Twitter, or Google. Confidentiality impact is assessed as none in the CVSS v3.1 scoring, though in practice an AiTM attacker could observe sensitive authentication data in transit. Availability is not impacted. The scope is limited to the affected application's authentication flows and does not directly enable lateral movement or system-level compromise (Feedly).

No public proof-of-concept exploit code or evidence of in-the-wild exploitation has been reported for this vulnerability. The EPSS score is approximately 0.017% (0.000170), indicating a very low probability of exploitation in the near term. The attack complexity is rated High, as exploitation requires the attacker to be positioned as an adversary-in-the-middle on the network path. The vulnerability is not listed in the CISA Known Exploited Vulnerabilities (KEV) catalog, and no threat actor attribution has been identified (Feedly, EUVD).

1. Reconnaissance: Identify web applications using HybridAuth versions up to 3.12.2 for social login functionality, which can often be inferred from OAuth redirect URLs or application source code.
2. Network Positioning: Establish an adversary-in-the-middle position on the network path between the target application server and the social identity provider (e.g., via ARP spoofing, DNS poisoning, or a rogue Wi-Fi access point on the same network segment).
3. Intercept TLS Handshake: Present a fraudulent TLS certificate to the HybridAuth application during its outbound connection to the identity provider's API endpoint. Due to the improper certificate validation in Curl.php, the library may accept the invalid certificate.
4. Capture or Modify Traffic: Intercept OAuth tokens, authorization codes, or user profile data returned by the spoofed identity provider endpoint, or inject malicious responses to manipulate authentication outcomes in the target application (Feedly, GitHub Issue).

The HybridAuth project released version 3.13.0 on April 2, 2026, which is the recommended upgrade target to address this vulnerability (HybridAuth GitHub). Users should update via Composer by running composer require hybridauth/hybridauth:^3.13. As a workaround prior to upgrading, administrators can ensure that the application server enforces strict SSL/TLS certificate verification at the system or PHP cURL configuration level, and restrict outbound connections to known identity provider IP ranges. Network-level controls such as TLS inspection proxies can also help detect anomalous certificate usage.

The vulnerability was initially reported to the HybridAuth maintainers via email on February 1, 2026, but the project did not respond before the issue was publicly disclosed through a GitHub issue on February 21, 2026 (GitHub Issue). The lack of maintainer response prior to public disclosure reflects a common challenge in open-source security coordination. No significant broader media coverage or notable researcher commentary beyond the initial issue report has been identified.