CVE-2026-93990

Expat before 2.8.5 Malformed UTF-16 Acceptance via Unchecked Surrogate

Products / packages libexpat · Siemens SIMATIC S7-1500 CPU 1518-4 PN/DP MFP (6ES7518-4AX00-1AB0) · Siemens SIMATIC S7-1500 CPU 1518-4 PN/DP MFP (6ES7518-4AX00-1AC0) · 3 more

Published 19/09/2026, 22:58:11 UTC

Severity and exploitation

Not in CISA KEV

KEV coverage does not rule out exploitation.

KEV checked 14/11/2026, 21:40:35 UTC

VulnCheck · CVSS:4.0 · 8.7

Assessment vector

CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N

VulnCheck · CVSS:3.1 · 7.5

Assessment vector

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

The headline uses the highest imported catalog score. Assessments can differ by source and CVSS version.

Severity

high8.7

CVSS measures technical severity, not the likelihood of an attack.

Known exploitation

Not listed in the checked CISA KEV catalog

Absence from CISA KEV does not rule out attacks.

Public exploit

Not established by this record

Missing evidence does not mean no public exploit exists.

Remediation

Fix status not established

No structured fix or guidance is available here. Check the latest publisher advisory.

What this means

Expat before 2.8.5 fails to validate UTF-16 input, allowing malformed sequences to be accepted. Attackers can supply malformed UTF-16 encoded XML. The impact is silent replacement of input characters in some builds.

Summarized by @cf/meta/llama-3.3-70b-instruct-fp8-fast. Verify details against the original sources.

Attack conditions

No attack-condition excerpt has been identified here. Review the full source description and references for authentication, network access, and configuration requirements.

Review supporting sources →

Affected versions

Check the exact model, release, and any platform requirements against your installation.

libexpat · VulnCheck

libexpat

2 ranges

VulnCheck version evidence

Package identity

pkg:github/libexpat/libexpat

Identity supplied by VulnCheck. Version matching requires compatible range evidence.

Check a product version

1 range

siemens-SADP version evidence

Check a product version

Compare with this publisher’s ranges. This does not inspect your installation.

affected

V3.1.7 → before *

custom versions

Other versions: unknown

Automatic dependency matching is unavailable for this record. Use the product evidence and publisher’s instructions to check your installation.

What to do

Compare your installed product and version with the affected versions above. A matching product name alone does not confirm exposure.

No structured fix or remediation guidance is available here. Check the full source description and vendor’s latest advisory for a patch or workaround; do not assume another version is safe.

cve.new guidance unless attributed to a publisher. This page does not inspect your installation.

Public exploit evidence

No public exploit reference is identified in the imported evidence. This is not a claim that no exploit exists.

Original source description

Read the full publisher description

Expat before 2.8.5 fails to validate that a high surrogate in UTF-16 input is followed by a low surrogate, allowing malformed UTF-16 sequences to be accepted. Attackers can supply UTF-16 encoded XML containing lone high surrogates that consume the following code unit, causing Expat to pass unpaired surrogates to applications built with XML_UNICODE and to silently replace input characters in other builds.

References

7 sources

libexpat/libexpat · pull request #1282

PatchVulnCheck

libexpat/libexpat · commit ff6e1d7

PatchVulnCheck

blog.hartwork.org/posts/expat-2-8-5-released/

Vendor advisoryVulnCheck

libexpat/libexpat · releases/tag/R_2_8_5

Release notesVulnCheck

libexpat/libexpat · repository

Source referenceVulnCheck

www.vulncheck.com/advisories/expat-through-2.8.4-malformed-utf-16-acceptance-via-unchecked-surrogate

Security advisoryVulnCheck

View 1 more references

cert-portal.siemens.com/productcert/html/ssa-457936.html

Source referencesiemens-SADP

Inside the record

Evidence available

Package identity

Version evidence

Remediation guidance · not supplied

Weakness classification

Additional publishers

Describes the evidence in this view, not whether the product is safe.

Original records

CVE-2026-93990

View the latest unmodified CVE JSON from the official source. It may be newer than this imported page.

cve.new JSON →

Underlying weaknesses

The source’s classification of what caused the vulnerability.

CWE-176 ↗

Improper Handling of Unicode Encoding

VulnCheck

Environment requirements

These conditions apply together as specified by the publisher. A matching product name alone does not establish exposure.

VulnCheck · platform conditions

Any condition may hold

Vulnerable product

cpe:2.3:a:libexpat_project:libexpat:*:*:*:*:*:*:*:*

From (inclusive):

After:

Through (inclusive):

Before: 2.8.5

Applicability preview

[
  {
    "nodes": [
      {
        "operator": "OR",
        "negate": false,
        "cpeMatch": [
          {
            "vulnerable": true,
            "criteria": "cpe:2.3:a:libexpat_project:libexpat:*:*:*:*:*:*:*:*",
            "versionEndExcluding": "2.8.5"
          }
        ]
      }
    ]
  }
]

Contributing organizations · 3

VulnCheck

Assigning organization

2026-11-08

CISA-ADP

Additional data publisher

2026-09-21

siemens-SADP

Additional data publisher

2026-11-10

Timeline

All times UTC

19 Sept 2026, 22:58
Advisory published
Publication date supplied by the source.
10 Nov 2026, 12:49
Source updated
Latest modification date supplied by the source.
10 Nov 2026, 13:03
Last imported by cve.new
Latest catalog import, not the date we first saw this advisory.

What changed

Follow changes

CVE · CVE-2026-939902026-11-10

Observed 2026-11-10T13:03:17.894Z · upstream updated 2026-11-10T12:49:54.993Z

Product or version evidence changed

before

[
  {
    "vendor": "libexpat",
    "name": "libexpat",
    "platforms": [],
    "versions": [
      {
        "version": "0",
        "status": "affected",
        "lessThan": "2.8.5",
        "versionType": "semver"
      },
      {
        "version": "2.8.5",
        "status": "unaffected",
        "versionType": "semver"
      }
    ],
    "defaultStatus": "unaffected",
    "packageURL": "pkg:github/libexpat/libexpat",
    "cpes": [],
    "source": "VulnCheck",
    "url": "https://www.cve.org/CVERecord?id=CVE-2026-93990"
  }
]

after

[
  {
    "vendor": "libexpat",
    "name": "libexpat",
    "platforms": [],
    "versions": [
      {
        "version": "0",
        "status": "affected",
        "lessThan": "2.8.5",
        "versionType": "semver"
      },
      {
        "version": "2.8.5",
        "status": "unaffected",
        "versionType": "semver"
      }
    ],
    "defaultStatus": "unaffected",
    "packageURL": "pkg:github/libexpat/libexpat",
    "cpes": [],
    "source": "VulnCheck",
    "url": "https://www.cve.org/CVERecord?id=CVE-2026-93990"
  },
  {
    "vendor": "Siemens",
    "name": "SIMATIC S7-1500 CPU 1518-4 PN/DP MFP (6ES7518-4AX00-1AB0)",
    "platforms": [],
    "versions": [
      {
        "version": "V3.1.7",
        "status": "affected",
        "lessThan": "*",
        "versionType": "custom"
      }
    ],
    "defaultStatus": "unknown",
    "cpes": [],
    "source": "siemens-SADP",
    "url": "https://www.cve.org/CVERecord?id=CVE-2026-93990"
  },
  {
    "vendor": "Siemens",
    "name": "SIMATIC S7-1500 CPU 1518-4 PN/DP MFP (6ES7518-4AX00-1AC0)",
    "platforms": [],
    "versions": [
      {
        "version": "V3.1.7",
        "status": "affected",
        "lessThan": "*",
        "versionType": "custom"
      }
    ],
    "defaultStatus": "unknown",
    "cpes": [],
    "source": "siemens-SADP",
    "url": "https://www.cve.org/CVERecord?id=CVE-2026-93990"
  },
  {
    "vendor": "Siemens",
    "name": "SIMATIC S7-1500 CPU 1518F-4 PN/DP MFP (6ES7518-4FX00-1AB0)",
    "platforms": [],
    "versions": [
      {
        "version": "V3.1.7",
        "status": "affected",
        "lessThan": "*",
        "versionType": "custom"
      }
    ],
    "defaultStatus": "unknown",
    "cpes": [],
    "source": "siemens-SADP",
    "url": "https://www.cve.org/CVERecord?id=CVE-2026-93990"
  },
  {
    "vendor": "Siemens",
    "name": "SIMATIC S7-1500 CPU 1518F-4 PN/DP MFP (6ES7518-4FX00-1AC0)",
    "platforms": [],
    "versions": [
      {
        "version": "V3.1.7",
        "status": "affected",
        "lessThan": "*",
        "versionType": "custom"
      }
    ],
    "defaultStatus": "unknown",
    "cpes": [],
    "source": "siemens-SADP",
    "url": "https://www.cve.org/CVERecord?id=CVE-2026-93990"
  },
  {
    "vendor": "Siemens",
    "name": "SIPLUS S7-1500 CPU 1518-4 PN/DP MFP (6AG1518-4AX00-4AC0)",
    "platforms": [],
    "versions": [
      {
        "version": "V3.1.7",
        "status": "affected",
        "lessThan": "*",
        "versionType": "custom"
      }
    ],
    "defaultStatus": "unknown",
    "cpes": [],
    "source": "siemens-SADP",
    "url": "https://www.cve.org/CVERecord?id=CVE-2026-93990"
  }
]

References changed

before

[
  "https://github.com/libexpat/libexpat/pull/1282",
  "https://github.com/libexpat/libexpat/commit/ff6e1d7e750bbe245178f51a47a965dc8342861a",
  "https://github.com/libexpat/libexpat/releases/tag/R_2_8_5",
  "https://blog.hartwork.org/posts/expat-2-8-5-released/",
  "https://github.com/libexpat/libexpat",
  "https://www.vulncheck.com/advisories/expat-through-2.8.4-malformed-utf-16-acceptance-via-unchecked-surrogate"
]

after

[
  "https://github.com/libexpat/libexpat/pull/1282",
  "https://github.com/libexpat/libexpat/commit/ff6e1d7e750bbe245178f51a47a965dc8342861a",
  "https://github.com/libexpat/libexpat/releases/tag/R_2_8_5",
  "https://blog.hartwork.org/posts/expat-2-8-5-released/",
  "https://github.com/libexpat/libexpat",
  "https://www.vulncheck.com/advisories/expat-through-2.8.4-malformed-utf-16-acceptance-via-unchecked-surrogate",
  "https://cert-portal.siemens.com/productcert/html/ssa-457936.html"
]

Large field values are marked as truncated previews. Original source snapshots remain retained.

Showing up to 50 most recent observed changes.

Reading the signals

Severity · CVSS: Technical impact and attack conditions. The headline is the highest imported catalog score; sources may disagree.
Observed exploitation · KEV: An absent or unavailable KEV listing does not establish that exploitation has not occurred.
Predicted exploitation · EPSS: 0.40% estimated probability in the next 30 days. 2026-11-08 · FIRST EPSS.

CISA-ADP · SSVC

CISA Coordinator · 2026-09-21

Exploitation: none
Automatable: yes
Technical Impact: partial

SSVC describes the publisher’s decision context. Your organization’s exposure and priorities may differ.

Record identifiers

Aliases: None recorded

Source evidence

Request a correction or dispute

Start with the assigning organization for this record. Include the CVE ID, the disputed information, and supporting evidence. A dispute does not itself reject a record.

Find the responsible CNA →Official dispute and escalation process ↗

CVE CVE-2026-93990