CVE-2026-55599: phpseclib is a PHP secure communications library

This week

MEDIUM 5.8REMOTEUPGRADE

CVE-2026-55599 is a medium-severity flaw (CVSS 5.8) affecting phpseclib. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

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 b

Why this priority

Public exploit reference
Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	5.8 (Medium)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	0.2% (top 89%)
CISA exploited list	No
Weakness	CWE-918
Published	2026-06-22
Last modified	2026-06-26

Publisher (CNA): GitHub, Inc. · Vendor: phpseclib

Last updated 2026-06-26 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-06-22 21:16 UTCPublished CVE record published
2026-06-26 20:10 UTCUpdated Record last modified

phpseclib phpseclib UPGRADE

Affected: >= 0.1.1 and < 1.0.30 · >= 2.0.0 and < 2.0.55 · >= 3.0.0 and < 3.0.54

Fixed in: 1.0.30, 2.0.55, 3.0.54

Safe to downgrade to: a release before 0.1.1

1. Check installed version

apt-cache policy phpseclib || rpm -q phpseclib || phpseclib --version

2. UPGRADE to 1.0.30 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade phpseclib

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade phpseclib

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

3. If no update is available yet, DOWNGRADE to a release before 0.1.1 (Debian / Ubuntu)

apt-cache madison phpseclib sudo apt install phpseclib=<VERSION_BEFORE_0.1.1> sudo apt-mark hold phpseclib

3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list phpseclib sudo dnf downgrade phpseclib-<VERSION_BEFORE_0.1.1> sudo dnf versionlock add phpseclib

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-55599?

phpseclib: upgrade to 1.0.30, 2.0.55, 3.0.54. Exact commands for each OS are on this page.

Is CVE-2026-55599 being exploited?

CVE-2026-55599 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-55599 affect?

CVE-2026-55599 affects phpseclib. Severity is medium (CVSS 5.8). 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 Auth

How urgent is CVE-2026-55599?

Priority 33/100. Public exploit reference; Remotely exploitable, no login, no user action

Software pages

PHP vulnerabilities

References

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