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CVE-2026-55599

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 valid

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.

MEDIUM · CVSS 5.8
Monitor
  • No active-exploitation, high-EPSS, or public-exploit signals - routine patching cadence
No Sigma yet — build one → YARA rules0
Look this up elsewhere - one-click external pivots
NVDCVE.orgCISAExploit-DBGitHub PoCVulnCheck KEVGreyNoise
▸ How to read a CVE - triage first, then detect and patch
This page is every public fact about CVE-2026-55599, cross-linked. Its job is to answer one question fast - does this need my attention now? - and then hand you the two things you do about it. Here is how an analyst reads it.
Triage: should I act now? Four signals, and they are not interchangeable:
CVSSseverity - how bad it is IF exploited, 0-10. A high CVSS alone is not urgency; a flaw can be a perfect 10 and never actually be attacked. EPSSprobability - a model’s estimate of the chance it is exploited in the next 30 days, 0-1. This is the “will it actually happen” signal. CISA KEVconfirmed - it is being exploited in the wild right now. The strongest signal on the page; KEV beats any score. Weaponisedavailability - public exploits / PoCs, and especially Metasploit modules rated Excellent / Great. Reliable, packaged exploit code means low-skill attackers can use it today.
How they combine: KEV, or a dependable Metasploit module, means patch now regardless of CVSS. High CVSS + low EPSS + no exploit is real but not an emergency - schedule it. Low CVSS but KEV-listed still gets patched now. The verdict above already weighed these for you; this is how it got there.
Then what - two workflows:
Detectwhen you cannot patch today, follow this CVE to the ATT&CK techniques it enables, then Build a SIEM detection (the green button) - author a rule, test it in Atomic, deploy it. That buys visibility while the patch waits. PatchAffected products / packages tell you if you are exposed; Fixed versions by distribution and Vendor advisories give the exact version that closes it.
Reading order for the panels below: verdict + badges, then Public exploits / Metasploit (is it weaponised), then ATT&CK techniques + Sigma / IDS rules (can I detect it), then Affected products / packages + Fixed versions (am I exposed, what patches it), then Threat actors / IOCs (who uses it), then Scoring & timeline / references (the evidence).
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Severity & exploitation scoring

View on NVD →
CVSS base score
5.8
MEDIUMCVSS v3.1 · [email protected]
EPSS exploitation probability
n/a
No EPSS score in our data for this CVE. EPSS is published daily for scored CVEs - a very new, reserved, or rejected CVE may not have one yet.
CVSS metric silhouette
VectorComplexityPrivilegesInteractionScopeConfidentialityIntegrityAvailability
shape grows toward worst-case
SSVC triage
No SSVC vulnrichment for this CVE. CISA's Vulnrichment program scores newer CVEs (~2024 onwards) plus selected older critical ones. Use the EPSS probability + KEV status to triage instead.
CVSS vector breakdown
Exploitability - how they get in
Attack Vector
Network Adjacent Local Physical
Attack Complexity
Low High
Privileges Required
None Low High
User Interaction
None Required
Scope
Unchanged Changed
Impact - what breaks
Confidentiality
None Low High
Integrity
None Low High
Availability
None Low High
VECTORCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:N/A:N
Attack path
Full kill chain
→
◆

ATT&CK techniques

1

Techniques this CVE enables. Pills with a solid outline are high confidence - named directly in ATT&CK or Nuclei, or human-curated by CTID; the rest are inferred from the weakness type using MITRE's CVE Mapping Methodology and the CWE → CAPEC chain. Broad, generic-weakness guesses are filtered out. A small N× marks a technique that N independent sources agree on.

T1090 · Proxy
▤ Build a SIEM detection for these techniques
▤

CAPEC attack patterns

1

Attack patterns this CVE enables - the bridge from weakness to ATT&CK technique.

CAPEC-CAPEC-664 · Server Side Request Forgery
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Weakness Classification

CWE-918Server-Side Request Forgery (SSRF)
▤

Related CVEs

4
CVEs linked to this one by a shared weakness (CWE) or affected product - joins on data already in the engine, with the reason shown per row. Not a guess.
CVE-2002-1484 DB4Web server, when configured to use verbose debug messages, allows remote atta... 1 same CWE-918 CRITICAL CVE-2004-2061 RiSearch 1.0.01 and RiSearch Pro 3.2.06 allows remote attackers to use the show.... 2 same CWE-918 CRITICAL CVE-2007-6758 Server-side request forgery (SSRF) vulnerability in feed-proxy.php in extjs 5.0.... same CWE-918 HIGH CVE-2010-1637 The Mail Fetch plugin in SquirrelMail 1.4.20 and earlier allows remote authentic... same CWE-918 MEDIUM
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References & Sources

1
Source URLs (vendor pages, mailing lists, write-ups). Exploit/PoC links are in their own section above to avoid duplication.
https://github.com/phpseclib/phpseclib/security/advisories/GHSA-m557-wrgg-6rp4
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