CVE-2026-55599 — phpseclib/phpseclib
MEDIUMCVE-2026-55599 is a medium-severity (CVSS 5.8) Server-Side Request Forgery (SSRF) vulnerability in phpseclib/phpseclib. A fix is available for phpseclib/phpseclib — see the affected versions and patch details below.
phpseclib: X.509 certificate validation sends attacker-controlled outbound requests (server-side request forgery) via Authority Information Access
Exploitation Status
Proof-of-concept exploit code exists
- CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-55599.
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
CVE-2026-55599 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 377,166 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
Real-World Exposure
phpseclib/phpseclib🐘phpseclib/phpseclib🐘phpseclib/phpseclibReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects Packagist packages — download data is not available via public APIs for these ecosystems.
Description
phpseclib is a PHP secure communications library. From 0.1.1 until 1.0.30, 2.0.55, and 3.0.54, when an application validates an untrusted X.509 certificate with phpseclib, X509::validateSignature() reads a URL out of that certificate's Authority Information Access (AIA) extension and connects to it. Attacker who supplies certificate fully controls host, port, and path of that connection. URL fetching is enabled by default, and no destination is blocked. An unauthenticated attacker can therefore make a validating server open connections to internal hosts and ports it should never reach, for example loopback 127.0.0.1, cloud metadata address 169.254.169.254, and internal-only services. This is a server-side request forgery (SSRF) caused by an insecure default. This vulnerability is fixed in 1.0.30, 2.0.55, and 3.0.54.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐘Packagist | phpseclib/phpseclib | ≥ 0.1.1&&< 1.0.30 | 1.0.30composer require phpseclib/phpseclib:^1.0.30 |
| 🐘Packagist | phpseclib/phpseclib | ≥ 2.0.0&&< 2.0.55 | 2.0.55composer require phpseclib/phpseclib:^2.0.55 |
| 🐘Packagist | phpseclib/phpseclib | ≥ 3.0.0&&< 3.0.54 | 3.0.54composer require phpseclib/phpseclib:^3.0.54 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for phpseclib/phpseclib, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update phpseclib/phpseclib to 1.0.30 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-55599 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2026-55599 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2026-55599. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2026-55599 in your dependencies?
O3 Security finds CVE-2026-55599 across Packagist dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.