CVE Intelligence
High Info disclosure

CVE-2021-25122 — Apache Tomcat h2c request mix-up

When responding to new h2c connection requests, Apache Tomcat versions 10.0.0-M1 to 10.0.0, 9.0.0.M1 to 9.0.41 and 8.5.0 to 8.5.61 could duplicate request headers and a limited amount of request body from one request to another meaning user A and user B could both see the results of user A's request.

Published Mar 1, 2021 · Updated Oct 13, 2026 · Sources: NIST National Vulnerability Database, CVE Program (cve.org) e CISA ADP · CNA: apache

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 18.1% (30-day exploitation probability)+15
  • Technical severity (high)+6
  • 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
18.11%
ahead of 97% of scored CVEs · Oct 15, 2026
Affects
Apache Software Foundation
Tomcat and 11 more
CVSS base
7.5
High · 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)
  • Documented patch — www.oracle.com (NIST National Vulnerability Database)
  • 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.

  1. Identify exposed assets running the affected vendor/product/version combinations (Apache Software Foundation Tomcat, Debian Debian Linux, Oracle Agile Product Lifecycle Management…).
  2. The attack is network-based and needs no privileges: restrict exposure (ACLs, VPN, WAF) until patched.
  3. Apply the fixed version or the patch documented under Remediation.
  4. EPSS ahead of 97% of scored CVEs: prioritize it in the patch queue.
  5. Search available logs for exploit probes matching the vulnerability class (info disclosure), errors and authentication anomalies.

Affected scope

Vendor, product and versions as the sources word them.

VendorProductVersionsStatusSource
Apache Software Foundation Tomcat version 10.0.0-milestone1version 9.0.0-milestone23version 9.0.0-milestone24version 9.0.0-milestone25version 9.0.0-milestone26version 9.0.0-milestone27version 9.0.0-milestone3version 9.0.0-milestone4
and 28 more version 9.0.0-milestone5version 10.0.0version 9.0.0-milestone22version 10.0.0-milestone10version 10.0.0-milestone2version 10.0.0-milestone3version 10.0.0-milestone4version 10.0.0-milestone5version 10.0.0-milestone6version 10.0.0-milestone7version 10.0.0-milestone8version 10.0.0-milestone9version 9.0.0-milestone13≥ 8.5.0 and ≤ 8.5.61≥ 9.0.0 and ≤ 9.0.41version 9.0.0-milestone1version 9.0.0-milestone10version 9.0.0-milestone11version 9.0.0-milestone12version 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-milestone2version 9.0.0-milestone20version 9.0.0-milestone21
Vulnerable NIST National Vulnerability Database
Apache Software Foundation Tomcat ≥ Apache Tomcat 8.5 and < 8.5.62≥ Apache Tomcat 9 and < 9.0.42≥ Apache Tomcat 10 and < 10.0.2 Vulnerable CVE Program (cve.org) e CISA ADP
Debian Debian Linux version 10.0version 9.0 Vulnerable NIST National Vulnerability Database
Oracle Agile Product Lifecycle Management version 9.3.3version 9.3.6 Vulnerable NIST National Vulnerability Database
Oracle Communications Cloud Native Core Policy version 1.14.0 Vulnerable NIST National Vulnerability Database
Oracle Communications Cloud Native Core Security Edge Protection Proxy version 1.6.0 Vulnerable NIST National Vulnerability Database
Oracle Communications Instant Messaging Server version 10.0.1.5.0 Vulnerable NIST National Vulnerability Database
Oracle Database version 21cversion 19cversion 12.2.0.1 Vulnerable NIST National Vulnerability Database
Oracle Graph Server And Client version 21.3.0< 21.3.0 Vulnerable NIST National Vulnerability Database
Oracle Instantis Enterprisetrack version 17.3version 17.2version 17.1 Vulnerable NIST National Vulnerability Database
Oracle Managed File Transfer version 12.2.1.3.0version 12.2.1.4.0 Vulnerable NIST National Vulnerability Database
Oracle Mysql Enterprise Monitor ≤ 8.0.23 Vulnerable NIST National Vulnerability Database
Oracle Siebel Ui Framework ≤ 21.9 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
None
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
None
Availability
None

Every base score collected

Sources score independently and may disagree; each row says who scored it and under which version.

ScoreVersionSeverityExpl.ImpactScored by
7.5 CVSS 3.1 High 3.9 3.6 nvd@nist.gov via NIST National Vulnerability Database
5.0 CVSS 2.0 Medium 10.0 2.9 nvd@nist.gov via NIST National Vulnerability Database
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N (score shown at the top: nvd@nist.gov)

Weakness MITRE

Weakness class according to MITRE’s CWE catalog.

CWE-200 · Exposure of Sensitive Information

Other CVEs with this weakness →

EPSS over time FIRST

Oct 13, 2026 → Oct 15, 2026 · 3 reading(s) · max 18.11%

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).
  • Look for anomalous data read/export volumes and access to sensitive records.
web/proxy/WAF logsnetwork IDS/IPSapplication logsauthentication logs

Timeline

What happened to this CVE, newest first.

  • 2026
  • Affected products added · apache software foundation/apache tomcat
    Oct 13, 2026 · CVE Program (cve.org) e CISA ADP
  • 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 · 7.5
    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
  • 2021
  • CVE published
    Mar 1, 2021 · apache

Source activity

Repeated readings are counted rather than listed.

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

References

15 on the record

Elsewhere on this site

Apache Software Foundation
every CVE for this vendor
Info disclosure
same class
CWE-200
other pages naming this weakness

Related CVEs (same products)

  • CVE-2026-87022 Apache Tomcat: WebSocket message smuggling with per-message-deflate 22d
  • 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 22d
  • CVE-2026-86350 Apache Tomcat: Regression in fix for CVE-2026-41293 can trigger request header mix-up 22d
  • CVE-2026-79677 Apache Tomcat: WebSocket DoS due to lost asynchronous write timeout 22d
  • CVE-2026-78383 Apache Tomcat: AJP DoS via missing request body 22d
  • CVE-2026-78437 Apache Tomcat: HTTP/2 DoS via malformed request 22d

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 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-2021-25122 is "Apache Tomcat h2c request mix-up", rated High. 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 7.5 describes technical severity. EPSS 18.11% 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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