CVE-2021-41991

The in-memory certificate cache in strongSwan before 5.9.4 has a remote integer overflow upon receiving many requests with different certificates to fill the cache and later trigger the replacement of cache entries. The code attempts to select a less-often-used cache entry by means of a random number generator, but this is not done correctly. Remote code execution might be a slight possibility.

Published: 2021-10-18 Last update: 2026-06-17 Assigner: [email protected] Source: [email protected]
NVD Status：Modified，CVE State: published
View at NVD, CVE.org, EUVD

CVE-2021-41991 is rated Moderate Risk (63.6/100): CVSS High severity, with high exploitation likelihood (EPSS 4.80%, 91th percentile). EPSS ranks this CVE among the most likely to be exploited in the near term. EPSS rose +1.87% over the last day, indicating growing attacker interest. High exploitation likelihood—assess exposure and prioritize remediation.

Risk is dynamic; we continuously reassess and refresh what is shown on this page as upstream context changes.

Exploit prediction scoring system (EPSS) score for CVE-2021-41991

EPSS lead: Daily EPSS estimates relative likelihood of exploitation; percentile ranks this CVE among scored vulnerabilities (higher = more severe relative rank).

# Date Old EPSS score New EPSS score Delta (New - Old)
1 2026-06-15 2.94% 4.80% +1.87%
2 2026-06-09 2.76% 2.94% +0.18%
3 2026-05-27 — 2.76% —

Full EPSS history (38 records total)

Common vulnerability scoring system (CVSS) metrics for CVE-2021-41991

CVSS metrics for this CVE.

Base score Version Severity Vector Exploitability Impact Score source
7.5 3.1 HIGH
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H Click to expand
Attack vector (AV:N)
Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:L)
Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:N)
No account or special rights needed—anonymous or random user is enough.
User interaction (UI:N)
Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:U)
Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems.
Confidentiality (C:N)
Doesn’t really leak secrets in a meaningful way.
Integrity (I:N)
Data isn’t meaningfully altered or forged.
Availability (A:H)
Could take the service down hard or make it unusable for people who depend on it.
 3.9 3.6 [email protected]
5.0 2.0 MEDIUM
AV:N/AC:L/Au:N/C:N/I:N/A:P Click to expand
Access vector (AV:N)
Can be exploited remotely over network reachability.
Access complexity (AC:L)
Exploitation conditions are straightforward and predictable.
Authentication (AU:N)
No authentication is required.
Confidentiality impact (C:N)
No confidentiality impact.
Integrity impact (I:N)
No integrity impact.
Availability impact (A:P)
Partial availability impact.
 10.0 2.9 [email protected]

Weakness enumeration for CVE-2021-41991

OS Trackers for CVE-2021-41991

vendor priority summary link
alpine high CVE-2021-41991: 1 source package rows (strongswan); 34 state rows across 9 repos (3.11-main, 3.12-main, 3.17-main, 3.18-main, 3.19-main, 3.20-main, 3.21-main, 3.22-main, edge-main); fixed 13, open 21. https://security.alpinelinux.org/vuln/CVE-2021-41991
debian not yet assigned CVE-2021-41991 not yet assigned priority: Debian including 1 source packages (strongswan), 5 status rows across 5 suites (bookworm, bullseye, forky, sid, trixie): resolved 5. https://security-tracker.debian.org/tracker/CVE-2021-41991
gentoo normal CVE-2021-41991: 1 GLSA(s) (202405-08), 1 atom(s) (net-vpn/strongswan); latest impact normal. https://bugs.gentoo.org/buglist.cgi?quicksearch=CVE-2021-41991
redhat medium — https://access.redhat.com/security/cve/CVE-2021-41991
suse high CVE-2021-41991 severity important: SUSE including 64 source package names (strongswan-4.4.0-6.36.9.1, strongswan-5.1.3-26.16.1, …), 224 product×package rows across 49 product lines (HPE Helion OpenStack 8, SUSE CaaS Platform 4.0, … (49 product lines)): Fixed 224. https://www.suse.com/security/cve/CVE-2021-41991/
ubuntu medium CVE-2021-41991 medium priority: Ubuntu including 1 source packages (strongswan), 15 status rows across 15 suites (bionic, focal, hirsute, impish, jammy, kinetic, lunar, mantic, noble, oracular, plucky, questing, trusty, upstream, xenial): released 14, needs-triage 1. https://ubuntu.com/security/CVE-2021-41991

Affected software / configurations for CVE-2021-41991

Vendor Product Version Raw CPE
strongswan strongswan >= 4.2.10, < 5.9.4 cpe:2.3:a:strongswan:strongswan:*:*:*:*:*:*:*:*
debian debian_linux 9.0 cpe:2.3:o:debian:debian_linux:9.0:*:*:*:*:*:*:*
debian debian_linux 10.0 cpe:2.3:o:debian:debian_linux:10.0:*:*:*:*:*:*:*
debian debian_linux 11.0 cpe:2.3:o:debian:debian_linux:11.0:*:*:*:*:*:*:*
fedoraproject fedora 33 cpe:2.3:o:fedoraproject:fedora:33:*:*:*:*:*:*:*
fedoraproject fedora 34 cpe:2.3:o:fedoraproject:fedora:34:*:*:*:*:*:*:*
fedoraproject fedora 35 cpe:2.3:o:fedoraproject:fedora:35:*:*:*:*:*:*:*
siemens sinema_remote_connect_server — cpe:2.3:a:siemens:sinema_remote_connect_server:-:*:*:*:*:*:*:*
siemens siplus_et_200sp_cp_1542sp-1_irc_tx_rail_firmware — cpe:2.3:o:siemens:siplus_et_200sp_cp_1542sp-1_irc_tx_rail_firmware:-:*:*:*:*:*:*:*
siemens simatic_cp_1243-1_firmware — cpe:2.3:o:siemens:simatic_cp_1243-1_firmware:-:*:*:*:*:*:*:*
siemens simatic_cp_1242-7_gprs_v2_firmware — cpe:2.3:o:siemens:simatic_cp_1242-7_gprs_v2_firmware:-:*:*:*:*:*:*:*
siemens simatic_net_cp_1243-8_irc_firmware — cpe:2.3:o:siemens:simatic_net_cp_1243-8_irc_firmware:-:*:*:*:*:*:*:*
siemens scalance_sc632-2c_firmware — cpe:2.3:o:siemens:scalance_sc632-2c_firmware:-:*:*:*:*:*:*:*
siemens siplus_et_200sp_cp_1543sp-1_isec_firmware — cpe:2.3:o:siemens:siplus_et_200sp_cp_1543sp-1_isec_firmware:-:*:*:*:*:*:*:*
siemens cp_1543-1_firmware — cpe:2.3:o:siemens:cp_1543-1_firmware:-:*:*:*:*:*:*:*
siemens simatic_net_cp_1545-1_firmware — cpe:2.3:o:siemens:simatic_net_cp_1545-1_firmware:-:*:*:*:*:*:*:*
siemens simatic_cp_1543sp-1_firmware — cpe:2.3:o:siemens:simatic_cp_1543sp-1_firmware:-:*:*:*:*:*:*:*
siemens simatic_net_cp1243-7_lte_eu_firmware — cpe:2.3:o:siemens:simatic_net_cp1243-7_lte_eu_firmware:-:*:*:*:*:*:*:*
siemens simatic_cp_1243-7_lte\/us_firmware — cpe:2.3:o:siemens:simatic_cp_1243-7_lte\/us_firmware:-:*:*:*:*:*:*:*
siemens simatic_cp_1542sp-1_firmware — cpe:2.3:o:siemens:simatic_cp_1542sp-1_firmware:-:*:*:*:*:*:*:*
siemens scalance_sc636-2c_firmware — cpe:2.3:o:siemens:scalance_sc636-2c_firmware:-:*:*:*:*:*:*:*
siemens simatic_cp_1542sp-1_irc_firmware — cpe:2.3:o:siemens:simatic_cp_1542sp-1_irc_firmware:-:*:*:*:*:*:*:*
siemens scalance_sc642-2c_firmware — cpe:2.3:o:siemens:scalance_sc642-2c_firmware:-:*:*:*:*:*:*:*
siemens scalance_sc646-2c_firmware < 2.3 cpe:2.3:o:siemens:scalance_sc646-2c_firmware:*:*:*:*:*:*:*:*
siemens scalance_sc622-2c_firmware — cpe:2.3:o:siemens:scalance_sc622-2c_firmware:-:*:*:*:*:*:*:*
siemens siplus_s7-1200_cp_1243-1_rail_firmware — cpe:2.3:o:siemens:siplus_s7-1200_cp_1243-1_rail_firmware:-:*:*:*:*:*:*:*
siemens siplus_s7-1200_cp_1243-1_firmware — cpe:2.3:o:siemens:siplus_s7-1200_cp_1243-1_firmware:-:*:*:*:*:*:*:*
siemens siplus_net_cp_1543-1_firmware — cpe:2.3:o:siemens:siplus_net_cp_1543-1_firmware:-:*:*:*:*:*:*:*
siemens siplus_et_200sp_cp_1543sp-1_isec_tx_rail_firmware — cpe:2.3:o:siemens:siplus_et_200sp_cp_1543sp-1_isec_tx_rail_firmware:-:*:*:*:*:*:*:*

References for CVE-2021-41991

URL Tags
https://cert-portal.siemens.com/productcert/pdf/ssa-539476.pdf Patch Third Party Advisory
https://github.com/strongswan/strongswan/releases/tag/5.9.4 Third Party Advisory
https://lists.debian.org/debian-lts-announce/2021/10/msg00014.html Mailing List Third Party Advisory
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/5FJSATD2R2XHTG4P63GCMQ2N7EWKMME5/
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/WQSQ3BEC22NF4NCDZVCT4P3Q2ZIAJXGJ/
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/Y3TQ32JLJOBJDB2EJKSX2PBPB5NFG2D4/
https://www.debian.org/security/2021/dsa-4989 Third Party Advisory
https://www.strongswan.org/blog/2021/10/18/strongswan-vulnerability-%28cve-2021-41991%29.html
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