Medium

CVE-2020-13943 — If an HTTP/2 client connecting to Apache Tomcat 10.0.0-M1 to 10.0.0-M7, 9.0.0.M1 to…

If an HTTP/2 client connecting to Apache Tomcat 10.0.0-M1 to 10.0.0-M7, 9.0.0.M1 to 9.0.37 or 8.5.0 to 8.5.57 exceeded the agreed maximum number of concurrent streams for a connection (in violation of the HTTP/2 protocol), it was possible that a subsequent request made on that connection could contain HTTP headers - including HTTP/2 pseudo headers - from a previous request rather than the intended headers. This could lead to users seeing responses for unexpected resources.

Published Oct 12, 2020 · Updated Oct 13, 2026 · Sources: NIST National Vulnerability Database, CVE Program (cve.org) e CISA ADP · CNA: apache

Low

Who exploits it and how urgent it is

A 0–100 score combining confirmed exploitation, ransomware and APT use, public exploits, EPSS and severity.

No exploitation signal in the sources consulted so far.

How the score was calculated

EPSS 55% (30-day exploitation probability)+15
Technical severity (medium)+3
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.

Exploitation

Not confirmed

Not in CISA KEV — an absence of record, not proof it is unexploited.

EPSS FIRST

54.97%

ahead of 99% of scored CVEs · Oct 16, 2026

Affects

Apache Software Foundation

Tomcat and 3 more

CVSS base

4.3

Medium · 1 source(s)

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.

Exploitation

None

none · proof-of-concept · active

Automatable

Yes

can an attacker script the kill-chain steps

Technical impact

Partial

partial · total control of the vulnerable component

Remediation

Fixes and mitigations documented by the sources — linked to the original advisory.

Documented patch — www.oracle.com (NIST National Vulnerability Database)

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 (Apache Software Foundation Tomcat, Debian Debian Linux, Oracle Instantis Enterprisetrack…).
Apply the fixed version or the patch documented under Remediation.
EPSS ahead of 99% of scored CVEs: prioritize it in the patch queue.
Search available logs for exploit probes, errors and authentication anomalies.

Affected scope

Vendor, product and versions as the sources word them.

Vendor	Product	Versions	Status	Source
Apache Software Foundation	Tomcat	version 9.0.34version 9.0.35version 9.0.36version 9.0.37version 10.0.0-milestone1version 10.0.0-milestone2version 10.0.0-milestone3version 10.0.0-milestone4 and 117 more version 10.0.0-milestone5version 10.0.0-milestone6version 10.0.0-milestone7version 8.5.0version 8.5.1version 8.5.2version 8.5.3version 8.5.4version 8.5.5version 8.5.6version 8.5.7version 8.5.8version 8.5.9version 8.5.10version 8.5.11version 8.5.12version 8.5.13version 8.5.14version 8.5.15version 8.5.16version 8.5.17version 8.5.18version 8.5.19version 8.5.20version 8.5.21version 8.5.22version 8.5.23version 8.5.24version 8.5.25version 8.5.26version 8.5.27version 8.5.28version 8.5.29version 8.5.30version 8.5.31version 8.5.32version 8.5.33version 8.5.34version 8.5.35version 8.5.36version 8.5.37version 8.5.38version 8.5.39version 8.5.40version 8.5.41version 8.5.42version 8.5.43version 8.5.44version 8.5.45version 8.5.46version 8.5.47version 8.5.48version 8.5.49version 8.5.50version 8.5.51version 8.5.52version 8.5.53version 8.5.54version 8.5.55version 8.5.56version 8.5.57version 9.0.0-milestone10version 9.0.0-milestone11version 9.0.0-milestone12version 9.0.0-milestone13version 9.0.0-milestone14version 9.0.0-milestone15version 9.0.0-milestone16version 9.0.0-milestone17version 9.0.0-milestone18version 9.0.0-milestone19version 9.0.0-milestone20version 9.0.0-milestone21version 9.0.0-milestone22version 9.0.0-milestone23version 9.0.0-milestone24version 9.0.0-milestone25version 9.0.0-milestone26version 9.0.0-milestone27version 9.0.0-milestone5version 9.0.0-milestone6version 9.0.0-milestone7version 9.0.0-milestone8version 9.0.0-milestone9version 9.0.1version 9.0.2version 9.0.3version 9.0.4version 9.0.5version 9.0.6version 9.0.7version 9.0.8version 9.0.9version 9.0.10version 9.0.11version 9.0.12version 9.0.13version 9.0.14version 9.0.15version 9.0.16version 9.0.17version 9.0.18version 9.0.19version 9.0.20version 9.0.21version 9.0.22version 9.0.23version 9.0.24version 9.0.25version 9.0.26version 9.0.27version 9.0.28version 9.0.29version 9.0.30version 9.0.31version 9.0.32version 9.0.33	Vulnerable	NIST National Vulnerability Database
Debian	Debian Linux	version 9.0version 10.0	Vulnerable	NIST National Vulnerability Database
Oracle	Instantis Enterprisetrack	version 17.1version 17.2version 17.3	Vulnerable	NIST National Vulnerability Database
Oracle	Sd-wan Edge	version 9.0	Vulnerable	NIST National Vulnerability Database

Attack characteristics

The CVSS vector, decoded. It describes the attack, not your exposure to it.

Attack vector

Network

Attack complexity

Low

Privileges required

Low

User interaction

None

Scope

Unchanged

Confidentiality

Low

Integrity

None

Availability

None

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
4.3	CVSS 3.1	Medium	2.8	1.4	nvd@nist.gov via NIST National Vulnerability Database
4.0	CVSS 2.0	Medium	8.0	2.9	nvd@nist.gov via NIST National Vulnerability Database

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:N (score shown at the top: nvd@nist.gov)

EPSS over time FIRST

Oct 13, 2026 → Oct 16, 2026 · 4 reading(s) · max 54.97%

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).

web/proxy/WAF logsnetwork IDS/IPS

Timeline

What happened to this CVE, newest first.

2026
SSVC assessment updated
Oct 13, 2026 · CVE Program (cve.org) e CISA ADP
Record published
Oct 13, 2026
Fix documented
Oct 13, 2026 · NIST National Vulnerability Database
CVSS changed · 4.3
Oct 13, 2026 · NIST National Vulnerability Database
Source status changed · → Modified
Oct 13, 2026 · NIST National Vulnerability Database
Record created in this database
Oct 13, 2026 · NIST National Vulnerability Database
2020
CVE published
Oct 12, 2020 · apache

Source activity

Repeated readings are counted rather than listed.

1× Official description updated · Oct 13, 2026 – Oct 13, 2026

References

7 on the record

lists.apache.org/thread.html/r4a390027eb27e4550142fac6c8317cc684b157ae314d31514747f307
Vendor Advisory, x_refsource_MISC, NIST National Vulnerability Database
www.oracle.com/security-alerts/cpuApr2021.html
Patch, Third Party Advisory, x_refsource_MISC, NIST National Vulnerability Database
lists.opensuse.org/opensuse-security-announce/2020-11/msg00021.html
Broken Link, vendor-advisory, x_refsource_SUSE, NIST National Vulnerability Database
lists.opensuse.org/opensuse-security-announce/2020-11/msg00002.html
Mailing List, Third Party Advisory, vendor-advisory, x_refsource_SUSE, NIST National Vulnerability Database
lists.debian.org/debian-lts-announce/2020/10/msg00019.html
Third Party Advisory, mailing-list, x_refsource_MLIST, NIST National Vulnerability Database
security.netapp.com/advisory/ntap-20201016-0007/
Third Party Advisory, x_refsource_CONFIRM, NIST National Vulnerability Database
www.debian.org/security/2021/dsa-4835
Third Party Advisory, vendor-advisory, x_refsource_DEBIAN, NIST National Vulnerability Database

Elsewhere on this site

Apache Software Foundation
every CVE for this vendor

Related CVEs (same products)

Critical CVE-2026-86248 Apache Tomcat: Fix for CVE-2026-34500 was incomplete. OCSP checks sometimes soft-fail with FFM even when soft-fail is disabled 23d
High CVE-2026-87022 Apache Tomcat: WebSocket message smuggling with per-message-deflate 23d
Critical CVE-2026-86350 Apache Tomcat: Regression in fix for CVE-2026-41293 can trigger request header mix-up 23d
High CVE-2026-78383 Apache Tomcat: AJP DoS via missing request body 23d
High CVE-2026-79677 Apache Tomcat: WebSocket DoS due to lost asynchronous write timeout 23d
High CVE-2026-78437 Apache Tomcat: HTTP/2 DoS via malformed request 23d

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 CWE classification.
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-2020-13943 is "If an HTTP/2 client connecting to Apache Tomcat 10.0.0-M1 to 10.0.0-M7, 9.0.0.M1 to…", rated Medium. It affects Tomcat (Apache Software Foundation); versions are in the scope table. It is not in CISA KEV, which does not rule out exploitation. CVSS 4.3 describes technical severity. EPSS 54.97% 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.

Comments

Moderated technical discussion. Read the community guidelines.

No comments yet.