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CVE-2026-66046· NVD / CVE Program· CNA VulnCheck

Expat Denial of Service via storeAtts() Quadratic Complexity

Noroxi assessment

Priority: Monitor (action score 34/100)

  • The sources we track (CISA KEV, Exploit-DB, Metasploit, Nuclei, GitHub) show no public exploit code or exploitation in the wild.
  • Probability of exploitation in the next 30 days: 0.74% (FIRST EPSS); higher than 54.1% of all records.
  • Anyone who can reach it over the internet can trigger it. No account or password is required. No user action is required. No special conditions are required; it is repeatable.
  • According to CISA the attack is automatable: targets can be found by mass scanning.
  • Fixed version: azl3 expat 2.8.2-1 on Azure Linux 3.0 2.8.3-2.

Low priority for now. Upgrade to azl3 expat 2.8.2-1 on Azure Linux 3.0 2.8.3-2 or later.

Generated automatically from the record's data: CVSS (NVD and CNA), CISA KEV and SSVC, FIRST EPSS, exploit sources, fixed versions. How we calculate it

Primary sources: NVD · CVE.org · FIRST EPSS

Expat through 2.8.3 contains a denial of service vulnerability caused by quadratic algorithmic complexity in the storeAtts() function in xmlparse.c, where processing N specified attributes with non-normalized values triggers an O(N^2) linear scan of elementType->defaultAtts to determine CDATA status. A remote unauthenticated attacker can supply a single well-formed XML document of a few megabytes to an application parsing untrusted XML to cause excessive CPU consumption, resulting in denial of service without requiring authentication, external entity resolution, or non-default parser options.

HighCVSS 8.7 · v4.0—No exploit Fix available
Published
Aug 18, 2026
Updated
Oct 22, 2026
EPSS
0.7% · 54th percentile

Report tools

JSON

Action score

34

Monitor

Low priority for now.

CVSS
34 / 40 · 8.7 / 10
CISA KEV
0 / 30 · Not listed
EPSS
0 / 30 · 0.7%

CISA SSVC decision

Exploitation
none
Automatable
yes
Technical impact
partial

Vulnrichment: CISA's decision-tree inputs.

CNA vs NVD score

NVD
—
CNA · VulnCheck
8.7
NVD has not scored this yet; the score shown is the CNA’s.

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

The score given by the assigning authority versus NVD’s independent score. A gap means the severity is contested.

Get told when this record changes

  • Not in KEV: no record of exploitation in the wild
  • No public exploit code on record
  • A fix has been released

If any of these changes, we'll tell you in the daily email digest.

Follow CVE-2026-66046

When you'll hear from us

  • when it enters CISA KEV
  • when exploit code appears or is weaponized
  • when the record is updated (including a fixed version)
  • when EPSS rises 20 points in a week

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Noroxi analysis

What does it mean in your environment?

The assessment above is generated automatically from the record's data; it doesn't know your environment. Our team verifies on your systems whether this vulnerability is actually exploitable and which mitigation is enough.

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Affected systems

Is your version affected?

Pick the product and type the version you run. It's compared with this record's version ranges in your browser; the version isn't sent anywhere.

VendorProduct
libexpat projectlibexpat

Affected versions

NVD version ranges (for catalog products). Add the product to your stack with a version and matching uses these.

  • libexpat project libexpatup to 2.8.3 inclusive

Versions reported by the vendor

Affected version ranges reported by the assigning authority (VulnCheck). Independent of NVD's CPE analysis and usually ahead of it.

  • libexpat project libexpat

    • up to and including 2.8.3affected · semver

Package-level exposure

OSV and GitHub Advisory data: ecosystem, package and range. SBOM matching uses this table.

EcosystemPackageAffected rangeFix
AlmaLinux:10expatbefore 2.7.3-1.el10_2.52.7.3-1.el10_2.5
AlmaLinux:10expat-develbefore 2.7.3-1.el10_2.52.7.3-1.el10_2.5
AlmaLinux:8expatbefore 2.5.0-4.el8_102.5.0-4.el8_10
AlmaLinux:8expat-develbefore 2.5.0-4.el8_102.5.0-4.el8_10
AlmaLinux:9expatbefore 2.5.0-6.el9_8.52.5.0-6.el9_8.5
AlmaLinux:9expat-develbefore 2.5.0-6.el9_8.52.5.0-6.el9_8.5
Alpine:v3.21expatbefore 2.8.4-r02.8.4-r0
Debian:12expatbefore 2.5.0-1+deb12u42.5.0-1+deb12u4
Debian:13expatbefore 2.9.0-1~deb13u12.9.0-1~deb13u1
Debian:14expatbefore 2.8.4-12.8.4-1
openSUSE:Leap 16.0expatbefore 2.8.4-160000.1.12.8.4-160000.1.1
openSUSE:Tumbleweedexpatbefore 2.8.4-1.12.8.4-1.1
Red Hat:enterprise_linux:10.2expatbefore 0:2.7.3-1.el10_2.50:2.7.3-1.el10_2.5
Red Hat:enterprise_linux:10.2expat-debuginfobefore 0:2.7.3-1.el10_2.50:2.7.3-1.el10_2.5
Red Hat:enterprise_linux:10.2expat-debugsourcebefore 0:2.7.3-1.el10_2.50:2.7.3-1.el10_2.5
Red Hat:enterprise_linux:10.2expat-develbefore 0:2.7.3-1.el10_2.50:2.7.3-1.el10_2.5
Red Hat:enterprise_linux:8::baseosexpatbefore 0:2.5.0-4.el8_100:2.5.0-4.el8_10
Red Hat:enterprise_linux:8::baseosexpat-debuginfobefore 0:2.5.0-4.el8_100:2.5.0-4.el8_10
Red Hat:enterprise_linux:8::baseosexpat-debugsourcebefore 0:2.5.0-4.el8_100:2.5.0-4.el8_10
Red Hat:enterprise_linux:8::baseosexpat-develbefore 0:2.5.0-4.el8_100:2.5.0-4.el8_10
Red Hat:enterprise_linux:9::appstreamexpat-debuginfobefore 0:2.5.0-6.el9_8.50:2.5.0-6.el9_8.5
Red Hat:enterprise_linux:9::appstreamexpat-debugsourcebefore 0:2.5.0-6.el9_8.50:2.5.0-6.el9_8.5
Red Hat:enterprise_linux:9::appstreamexpat-develbefore 0:2.5.0-6.el9_8.50:2.5.0-6.el9_8.5
Red Hat:enterprise_linux:9::baseosexpatbefore 0:2.5.0-6.el9_8.50:2.5.0-6.el9_8.5

+7

Same product

libexpat project: all records

Other highest-scoring records for the same primary product.

  • CVE-2022-25236xmlparse.c in Expat (aka libexpat) before 2.4.5 allows attackers to insert namespace-separator characters into namespace URIs.
    50Plan
  • CVE-2022-23852Expat (aka libexpat) before 2.4.4 has a signed integer overflow in XML_GetBuffer, for configurations with a nonzero XML_CONTEXT_BYTES.
    41Plan
  • CVE-2022-22822addBinding in xmlparse.c in Expat (aka libexpat) before 2.4.3 has an integer overflow.
    41Plan

Published before and after this one in the same product

  • ← EarlierCVE-2026-56412Jun 21, 2026 · libexpat before 2.8.2 does not consider XML_TOK_DATA_CHARS in doCdataSection and thus lacks handler call depth tracking for various calls fr
    23Monitor
  • ← EarlierCVE-2026-72522Aug 10, 2026 · libexpat before 2.8.3 has an out-of-bounds read and resultant infinite loop because low surrogates are treated the same as high surrogates d
    24Monitor
  • Later →CVE-2026-76956Aug 20, 2026 · In libexpat 2.8.2 and 2.8.3 before 2.8.4, misinterpretation of getentropy's return code leads to insufficient entropy, which results in bein
    30Monitor

Remediation

Which version to upgrade to

Fix versions compiled from the vendor, package registries and Microsoft. Verify the vendor's note before upgrading.

Product / packageFixed versionSource
azl3 expat 2.8.2-1 on Azure Linux 3.02.8.3-2Microsoft (MSRC)
almalinux:expat2.5.0-4.el8_10 · AlmaLinux:8Package registry (OSV)
almalinux:expat-devel2.5.0-4.el8_10 · AlmaLinux:8Package registry (OSV)
alpine:expat2.8.4-r0 · Alpine:v3.21Package registry (OSV)
debian:expat2.5.0-1+deb12u4 · Debian:12Package registry (OSV)
debian:expat2.8.4-1 · Debian:14Package registry (OSV)
opensuse:expat2.8.4-1.1 · openSUSE:TumbleweedPackage registry (OSV)
red hat:expat0:2.8.4-0.1.hum1 · Red Hat:hummingbird:1Package registry (OSV)
red hat:expat-debuginfo0:2.5.0-4.el8_10 · Red Hat:enterprise_linux:8::baseosPackage registry (OSV)
red hat:expat-debugsource0:2.5.0-4.el8_10 · Red Hat:enterprise_linux:8::baseosPackage registry (OSV)
red hat:expat-devel0:2.8.4-0.1.hum1 · Red Hat:hummingbird:1Package registry (OSV)
red hat:expat-static0:2.8.4-0.1.hum1 · Red Hat:hummingbird:1Package registry (OSV)
rocky linux:expat0:2.5.0-4.el8_10 · Rocky Linux:8Package registry (OSV)
suse:expat2.8.4-160000.1.1 · SUSE:Linux Micro 6.2Package registry (OSV)

Exploit status

No known public exploit

No public exploit has been observed yet. That doesn't mean you're safe, only that the bar is a little higher.

Research context

For pentesters and researchers: attack profile, score disagreement, timeline, patch commits, credits, variant and chain candidates, bug bounty scope. All derived from existing data; no exploit code.

Timeline

From publication to today: proof of concept, Metasploit module, CISA KEV and fix record. Dates are as reported by the sources.

No dated events beyond publication.

EPSS, last 120 days

FIRST EPSS daily score; only changes of 0.01 or more are recorded (step chart).

Patch and commit links

Commit, PR and diff links among the references. A starting point for patch-diffing and variant hunting; fixes, not exploits.

No commit or PR link among the references.

Credits

All researchers

Finders, reporters and analysts named in the CNA record. Click a name for that researcher’s other records.

Variant candidates

Same product, same weakness class, within 18 months. If the patch missed the root cause, the sibling bug is here.

  • CVE-2026-45186100 days apartIn libexpat before 2.8.1, the computational complexity of attribute name collision checks allows a denial of service via moderately sized cr
    30Monitor
  • CVE-2025-66382263 days apartIn libexpat through 2.7.3, a crafted file with an approximate size of 2 MiB can lead to dozens of seconds of processing time.
    22Monitor

Chain candidates

An authentication bypass and a privilege-requiring bug in the same product, published close together: combined they may become an unauthenticated path.

—

Bug bounty scope

No known public program.

Source: bounty-targets-data (public HackerOne, Bugcrowd, Intigriti, YesWeHack listings).

Technical details

Attack conditions

  • Anyone who can reach it over the internet can trigger it.
  • No account or password is required.
  • No user action is required.
  • No special conditions are required; it is repeatable.

If successful

Confidentiality
Integrity
Availability
Attack vector
Network
Attack complexity
Low
Privileges required
None
User interaction
None
Scope
X

Weakness class (CWE)

CWE-407 · Inefficient Algorithmic Complexity

CVSS vector

CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

nvd-secondary

Attack context

MITRE CAPEC attack patterns and ATT&CK techniques for this weakness class (CWE). A starting point for detection rules and threat hunting.

MITRE has no CAPEC/ATT&CK mapping for this CWE.

Change log

  1. Fix✗ → ✓

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