CVE-2026-115506 — libexpat through 2.9.0 Denial of Service via Billion Laughs Detection Delay
libexpat through 2.9.0 contains a denial of service vulnerability because billion laughs protection in accountingDiffTolerated() in xmlparse.c detects entity amplification too late. Attackers can supply a moderately sized XML document with a large DTD and nested entity expansions to consume tens of seconds of CPU per parse.
Who exploits it and how urgent it is
A 0–100 score combining confirmed exploitation, ransomware and APT use, public exploits, EPSS and severity.
How the score was calculated
- EPSS 0.5% (30-day exploitation probability)+6
- Technical severity (high)+6
- CISA SSVC: exploitation "poc"+5
- Automatable exploitation (SSVC)+3
The score orders the patch queue; it does not replace your asset context (internet exposure, criticality, compensating controls).
Triage
Is it exploited, how likely is exploitation, what does it touch, and how severe do the sources call it.
CISA SSVC assessment CISA
Three decision points CISA publishes for the CVEs it assesses (SSVC 2.0.3). A stakeholder decision, not a severity score.
Remediation
Fixes and mitigations documented by the sources — linked to the original advisory.
No fix documented by the sources we poll. That does not mean none exists: check the vendor advisory.
First 24 hours
Ordered from the record’s own fields — exposure first, because you cannot patch what you have not found. A rule-generated starting point.
- Identify exposed assets running the affected vendor/product/version combinations (Libexpat Libexpat).
- The attack is network-based and needs no privileges: restrict exposure (ACLs, VPN, WAF) until patched.
- No fix documented in the sources: reduce exposure and monitor the vendor advisory.
- Search available logs for exploit probes, errors and authentication anomalies.
Affected scope
Vendor, product and versions as the sources word them.
Attack characteristics
The CVSS vector, decoded. It describes the attack, not your exposure to it.
Every base score collected
Sources score independently and may disagree; each row says who scored it and under which version.
| Score | Version | Severity | Expl. | Impact | Scored by |
|---|---|---|---|---|---|
| 8.7 | CVSS 4.0 | High | — | — | disclosure@vulncheck.com via NIST National Vulnerability Database |
| 8.7 | CVSS 4.0 | High | — | — | VulnCheck via CVE Program (cve.org) e CISA ADP |
| 7.5 | CVSS 3.1 | High | 3.9 | 3.6 | disclosure@vulncheck.com via NIST National Vulnerability Database |
| 7.5 | CVSS 3.1 | High | — | — | GitHub, Inc. via GitHub Security Advisories |
| 7.5 | CVSS 3.1 | High | — | — | VulnCheck via CVE Program (cve.org) e CISA ADP |
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H (score shown at the top: disclosure@vulncheck.com) Weakness MITRE
Weakness class according to MITRE’s CWE catalog.
EPSS over time FIRST
Oct 23, 2026 → Oct 28, 2026 · 6 reading(s) · max 0.46%
Detection
Read off the CVSS vector and the weakness class. Starting points, not tested rules.
- Prioritize edge telemetry for the network-reachable component (proxy, WAF, IDS/IPS).
Timeline
What happened to this CVE, newest first.
- 2026
- SSVC assessment updated
- Source status changed · Received → Awaiting Analysis
- Affected products added · libexpat/libexpat
- Record published
- CVSS changed · 7.5
- Source status changed · → Received
- Record created in this database
- CVE published
Source activity
Repeated readings are counted rather than listed.
- 2× References added · Oct 22, 2026 – Oct 23, 2026
- 1× Official description updated · Oct 22, 2026 – Oct 22, 2026
References
7 on the record
- github.com/libexpat/libexpat/pull/1433
- www.vulncheck.com/advisories/libexpat-through-2.9.0-denial-of-service-via-billion-laugh
- nvd.nist.gov/vuln/detail/CVE-2026-115506
- github.com/libexpat/libexpat/issues/1160
- github.com/advisories/GHSA-j83x-34mv-v962
- github.com/hannob/lololol
- github.com/libexpat/libexpat
Elsewhere on this site
Related CVEs (same products)
- Medium CVE-2026-118370 libexpat through 2.9.0 Integer Overflow in storeAtts via attIndex and nPrefixes 1d
- Medium CVE-2026-117610 libexpat through 2.9.0 NULL Pointer Dereference via External Entity Subparser 1d
- Medium CVE-2026-113351 libexpat 2.7.2 through 2.9.0 Denial of Service via XML_ParserReset 9d
- High CVE-2026-77214 libexpat Heap Buffer Over-read in xmlparse.c via XML_ParseBuffer 22d
- Medium CVE-2026-102633 libexpat 2.7.2 through 2.8.5 Integer Overflow in expat_realloc Sep 29, 2026
- High CVE-2026-93990 Expat before 2.8.5 Malformed UTF-16 Acceptance via Unchecked Surrogate Sep 19, 2026
Not in any source we poll
Listed rather than left blank: an empty field and an unmeasured one look identical on screen, and only one is a reason to look elsewhere.
- No fixed version, patch or mitigation documented.
- No confirmed IOCs, IP addresses, domains, file hashes, or malware artifacts supplied.
- No organization-specific asset inventory, compensating-control status, or patch deployment evidence supplied.
- No exploit packet captures, log samples, or incident case IDs supplied.
Answered from this record
Text assembled only from the fields above — no additional claims.
What should defenders know first?
CVE-2026-115506 is "libexpat through 2.9.0 Denial of Service via Billion Laughs Detection Delay", rated High. It affects Libexpat (Libexpat); versions are in the scope table. It is not in CISA KEV, which does not rule out exploitation. CVSS 7.5 describes technical severity. EPSS 0.46% estimates the probability of exploitation in 30 days; neither replaces your asset context. First verify whether exposed or business-critical assets run the affected versions, apply vendor fixes or mitigations, and preserve logs for review.
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