Server-Side Request Forgery (SSRF) in phpseclib - CVE-2026-55599
Published: June 18, 2026
Vulnerability identifier: #VU134880
CSH Severity: Medium
CVSS v4.0: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:L/VI:N/VA:N/SC:L/SI:N/SA:N/E:U/U:Green
CVE-ID: CVE-2026-55599
CWE-ID: CWE-918
Exploitation vector: Remote access
Exploit availability:
No public exploit available
Vendor: phpseclib
Affected software:
phpseclib
phpseclib
Detailed vulnerability description
The vulnerability allows a remote attacker to send attacker-controlled outbound requests to internal services.
The vulnerability exists due to server-side request forgery (SSRF) in X509::validateSignature() when validating an untrusted X.509 certificate. A remote attacker can supply a crafted certificate containing an Authority Information Access URL to send attacker-controlled outbound requests to internal services.
The issue is triggered when the certificate issuer is not already trusted, and the fetched response is handled blindly rather than being exposed directly to the attacker.
How to mitigate CVE-2026-55599
Install security update from vendor's website.