CVE-2021-45105
MEDIUMCVE-2021-45105 is a medium-severity (CVSS 5.9) Improper Input Validation vulnerability in org.apache.logging.log4j:log4j-core. 4 public exploit references exist, so weaponization risk is real. O3 Security confirms whether CVE-2021-45105 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
Apache Log4j2 vulnerable to Improper Input Validation and Uncontrolled Recursion
Exploitation Status
No confirmed exploitation observed yet
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2021-45105.
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
CVE-2021-45105 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 368,770 CVEs with a current EPSS score, this one falls in the ≥ 90% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
Real-World Exposure
org.apache.logging.log4j:log4j-core☕org.apache.logging.log4j:log4j-core☕org.apache.logging.log4j:log4j-core☕org.ops4j.pax.logging:pax-logging-log4j2☕org.ops4j.pax.logging:pax-logging-log4j2☕org.ops4j.pax.logging:pax-logging-log4j2☕org.ops4j.pax.logging:pax-logging-log4j2Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Maven packages — download data is not available via public APIs for these ecosystems.
Description
Apache Log4j2 versions 2.0-alpha1 through 2.16.0 (excluding 2.12.3 and 2.3.1) did not protect from uncontrolled recursion from self-referential lookups. This allows an attacker with control over Thread Context Map data to cause a denial of service when a crafted string is interpreted. This issue was fixed in Log4j 2.17.0, 2.12.3, and 2.3.1.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| ☕Maven | org.apache.logging.log4j:log4j-core | ≥ 2.4.0&&< 2.12.3 | 2.12.3 |
| ☕Maven | org.apache.logging.log4j:log4j-core | ≥ 2.13.0&&< 2.17.0 | 2.17.0 |
| ☕Maven | org.apache.logging.log4j:log4j-core | all versions | 2.3.1 |
| ☕Maven | org.ops4j.pax.logging:pax-logging-log4j2 | ≥ 1.8.0&&< 1.9.2 | 1.9.2 |
| ☕Maven | org.ops4j.pax.logging:pax-logging-log4j2 | ≥ 1.10.0&&< 1.10.9 | 1.10.9 |
| ☕Maven | org.ops4j.pax.logging:pax-logging-log4j2 | ≥ 1.11.0&&< 1.11.12 | 1.11.12 |
Research use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for org.apache.logging.log4j:log4j-core. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update org.apache.logging.log4j:log4j-core to 2.12.3 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-45105 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether CVE-2021-45105 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to CVE-2021-45105. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Fixing This On Your OS
If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.
Red Hat Product Security has performed an analysis of this flaw and has classified the Attack Complexity(AC) as High because there are multiple factors involved which are beyond attacker's control: - The application has to use the logging configuration using a Context Map Lookup (for example, $${ctx:loginId}) which is…
Frequently Asked Questions
Is CVE-2021-45105 in your dependencies?
O3 detects CVE-2021-45105 across Maven dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.