ESB-2025.7438

=========================================================================== AUSCERT External Security Bulletin Redistribution ESB-2025.7438.2 K000156741: BIG-IP APM vulnerability CVE-2025-53521 2 April 2026 =========================================================================== AUSCERT Security Bulletin Summary --------------------------------- Product: F5 BIG-IP APM Publisher: F5 Networks Operating System: F5 Resolution: Patch/Upgrade CVE Names: CVE-2025-53521 Original Bulletin: https://my.f5.com/manage/s/article/K000156741 Comment: CVSS (Max): 9.8 CVE-2025-53521 (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H) CVSS Source: F5 Networks Calculator: https://www.first.org/cvss/calculator/3.1#CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H The following are listed in the CISA Known Exploited Vulnerabilities (KEV) Catalog: CISA KEV CVE(s): CVE-2025-53521 CISA KEV URL: https://www.cisa.gov/known-exploited-vulnerabilities-catalog EPSS (Max): 41.4% (97th) CVE-2025-53521 2026-04-01 Note: F5 have confirmed that CVE-2025-53521 has been exploited. Previously remediated as a Denial-of-Service (DoS), this vulnerability has been re-categorized to Remote Code Execution (RCE). AUSCERT strongly advises that the recommended actions should be followed. Revision History: April 02 2026: F5 confirm that CVE-2025-53521 has been exploited - 'Comment' section updated accordingly October 16 2025: Initial Release - --------------------------BEGIN INCLUDED TEXT-------------------- K000156741: BIG-IP APM vulnerability CVE-2025-53521 Published Date: Oct 15, 2025 Updated Date: Apr 1, 2026 Security Advisory Description When a BIG-IP APM access policy is configured on a virtual server, specific malicious traffic can lead to Remote Code Execution (RCE). (CVE-2025-53521) Important: We have learned that this vulnerability has been exploited. If your BIG-IP APM system is running a vulnerable version, or was upgraded from one, please do the following actions: - Review K000160486: Indicators of Compromise for c05d5254. - If compromise is suspected, review K11438344: Considerations and guidance when you suspect a security compromise on a BIG-IP system. Impact This is an issue impacting BIG-IP APM systems. This vulnerability allows an unauthenticated attacker to perform remote code execution. The BIG-IP system in Appliance mode is also vulnerable. This is a data plane issue; there is no control plane exposure. Note: This known vulnerability was previously categorized and remediated as a Denial-of-Service (DoS) vulnerability with CVSS scores of 7.5 (CVSS v3.1) and 8.7 (CVSS v4.0). Due to new information obtained in March 2026, the original vulnerability is being re-categorized to an RCE with CVSS scores of 9.8 (CVSS v3.1) and 9.3 (CVSS v4.0). The original CVE remediation has been validated to address the RCE in the fixed versions listed below. Security Advisory Status F5 Product Development has assigned ID 1977933 (BIG-IP APM) to this vulnerability. This issue has been classified as CWE-121: Stack-based Buffer Overflow. To determine if your product and version have been evaluated for this vulnerability, refer to the evaluated products box. To determine if your release is known to be vulnerable, the components or features that are affected by the vulnerability, and for information about releases, point releases, or hotfixes that address the vulnerability, refer to the tables below. You can also use iHealth to diagnose a vulnerability for BIG-IP, BIG-IQ, and F5OS systems. For more information about using iHealth, refer to K27404821: Using F5 iHealth to diagnose vulnerabilities. For more information about security advisory versioning, refer to K51812227: Understanding security advisory versioning. In this section o BIG-IP Next o BIG-IP and BIG-IQ o F5 Distributed Cloud and NGINX Services o F5OS o NGINX o Other products BIG-IP Next +-------------+------+-----------------+----------+----------+----------------+ | | |Versions known to|Fixes |Severity/ |Vulnerable | |Product |Branch|be vulnerable^1 |introduced|CVSS score|component or | | | | |in | |feature | +-------------+------+-----------------+----------+----------+----------------+ |BIG-IP Next |All |None |Not |Not |None | |SPK | | |applicable|vulnerable| | +-------------+------+-----------------+----------+----------+----------------+ |BIG-IP Next |All |None |Not |Not |None | |CNF | | |applicable|vulnerable| | +-------------+------+-----------------+----------+----------+----------------+ |BIG-IP Next | | |Not |Not | | |for |All |None |applicable|vulnerable|None | |Kubernetes | | | | | | +-------------+------+-----------------+----------+----------+----------------+ ^1F5 evaluates only software versions that have not yet reached the End of Technical Support (EoTS) phase of their lifecycle. For more information, refer to the Security hotfixes section of K4602: Overview of the F5 security vulnerability response policy. BIG-IP and BIG-IQ Note: After F5 releases a fix for a given branch, that fix applies to all subsequent minor, maintenance, and point releases for that branch; F5 will not list additional fixes for that branch in the table. For example, when F5 releases a fix in 17.1.2.1, the fix also applies to 17.1.2.2 and all later 17.1.x releases (17.1.3.x, 17.1.4.x). For more information, refer to K51812227: Understanding security advisory versioning. +--------------+------+---------------+----------+------------+---------------+ | | |Versions known |Fixes |Severity/ |Vulnerable | |Product |Branch|to be |introduced|CVSS score^2|component or | | | |vulnerable^1 |in | |feature | +--------------+------+---------------+----------+------------+---------------+ | |17.x |17.5.0 - 17.5.1|17.5.1.3 |Critical/9.8| | | | |17.1.0 - 17.1.2|17.1.3 |^3 (CVSS | | | +------+---------------+----------+v3.1) |The apmd | |BIG-IP APM |16.x |16.1.0 - 16.1.6|16.1.6.1 |Critical/9.3|process | | +------+---------------+----------+^3 (CVSS | | | |15.x |15.1.0 - |15.1.10.8 |v4.0) | | | | |15.1.10 | | | | +--------------+------+---------------+----------+------------+---------------+ |BIG-IP (all |All |None |Not |Not |None | |other modules)| | |applicable|vulnerable | | +--------------+------+---------------+----------+------------+---------------+ |BIG-IQ | | |Not |Not | | |Centralized |All |None |applicable|vulnerable |None | |Management | | | | | | +--------------+------+---------------+----------+------------+---------------+ ^1F5 evaluates only software versions that have not yet reached the End of Technical Support (EoTS) phase of their lifecycle. For more information, refer to the Security hotfixes section of K4602: Overview of the F5 security vulnerability response policy. ^2Starting with the August 2024 Quarterly Security Notification, F5 will provide the CVSS v4.0 base score in addition to the CVSS v3.1 score, for first-party security issues only. The CVSS score link takes you to a resource outside of MyF5, and the content may be removed without our knowledge. For more information about how F5 uses CVSS v4.0, refer to K000140363: Overview of CVSS v4.0 in F5 security advisories. ^3This known vulnerability was previously categorized and remediated as a DoS vulnerability with CVSS scores of 7.5 (CVSS v3.1) and 8.7 (CVSS v4.0). Due to new information obtained in March 2026, the original vulnerability is being re-categorized to an RCE with CVSS scores of 9.8 (CVSS v3.1) and 9.3 (CVSS v4.0). The original CVE remediation has been validated to address the issue for the recategorized RCE for the fixed versions listed above. F5 Distributed Cloud and NGINX Services +-------------------------------+-----------------+---------------------------+ |Service |Severity/CVSS |Vulnerable component or | | |score |feature | +-------------------------------+-----------------+---------------------------+ |F5 Distributed Cloud (all |Not vulnerable |None | |services) | | | +-------------------------------+-----------------+---------------------------+ |F5 Silverline (all services) |Not vulnerable |None | +-------------------------------+-----------------+---------------------------+ |NGINX One Console |Not vulnerable |None | +-------------------------------+-----------------+---------------------------+ F5OS +-------+------+-------------------+-----------+-----------+------------------+ | | |Versions known to |Fixes |Severity/ |Vulnerable | |Product|Branch|be vulnerable^1 |introduced |CVSS score |component or | | | | |in | |feature | +-------+------+-------------------+-----------+-----------+------------------+ |F5OS-A |All |None |Not |Not |None | | | | |applicable |vulnerable | | +-------+------+-------------------+-----------+-----------+------------------+ |F5OS-C |All |None |Not |Not |None | | | | |applicable |vulnerable | | +-------+------+-------------------+-----------+-----------+------------------+ ^1F5 evaluates only software versions that have not yet reached the End of Technical Support (EoTS) phase of their lifecycle. For more information, refer to the Security hotfixes section of K4602: Overview of the F5 security vulnerability response policy. NGINX +-----------+------+------------------+----------+----------+-----------------+ | | |Versions known to |Fixes |Severity/ |Vulnerable | |Product |Branch|be vulnerable^1 |introduced|CVSS score|component or | | | | |in | |feature | +-----------+------+------------------+----------+----------+-----------------+ |NGINX (all |All |None |Not |Not |None | |products) | | |applicable|vulnerable| | +-----------+------+------------------+----------+----------+-----------------+ ^1F5 evaluates only software versions that have not yet reached the End of Technical Support (EoTS) phase of their lifecycle. For more information, refer to the Security hotfixes section of K4602: Overview of the F5 security vulnerability response policy. Other products +-------+------+-------------------+-----------+-----------+------------------+ | | |Versions known to |Fixes |Severity/ |Vulnerable | |Product|Branch|be vulnerable^1 |introduced |CVSS score |component or | | | | |in | |feature | +-------+------+-------------------+-----------+-----------+------------------+ |Traffix|All |None |Not |Not |None | |SDC | | |applicable |vulnerable | | +-------+------+-------------------+-----------+-----------+------------------+ |F5 AI |All |None |Not |Not |None | |Gateway| | |applicable |vulnerable | | +-------+------+-------------------+-----------+-----------+------------------+ ^1F5 evaluates only software versions that have not yet reached the End of Technical Support (EoTS) phase of their lifecycle. For more information, refer to the Security hotfixes section of K4602: Overview of the F5 security vulnerability response policy. Security Advisory Recommended Actions If you are running a version listed in the Versions known to be vulnerable column, you can eliminate this vulnerability by installing a version listed in the Fixes introduced in column. If the Fixes introduced in column does not list a version for your branch, then no update candidate currently exists for that branch and F5 recommends that you upgrade to a version with the fix (refer to the tables). If the Fixes introduced in column lists a version prior to the one you are running, in the same branch, then your version should have the fix. Important: We have learned that this vulnerability has been exploited. If your BIG-IP APM system is running a vulnerable version, or was upgraded from one, please consider the following actions: - Review K000160486: Indicators of Compromise for c05d5254. - If compromise is suspected, review K11438344: Considerations and guidance when you suspect a security compromise on a BIG-IP system. Important: F5 strongly recommends that customers consult their corporate security policy for guidelines about incident handling procedures including but not limited to forensic best practices, that are specific to their organization. More specifically, that customers review their policies to ensure that they comply with evidence collection and forensics procedures for a security incident before attempting to recover the system. Additionally, if customers do not know exactly when the system was compromised, user configuration set (UCS) backups may have been created after the compromise occurred. F5 strongly recommends that customers rebuild the configuration from a known good source because UCS files from compromised systems can contain persistent malware. Mitigation None Acknowledgments F5 would like to thank Kristian Vlaardingerbroek, Hugo Trippaers, and other people of Schuberg Philis; Bart Vrancken; Fox-IT; and the National Cyber Security Centre (NCSC) in the Netherlands for their assistance in investigating this issue and following the highest standards of coordinated disclosure. Related Content o K41942608: Overview of MyF5 security advisory articles o K12201527: Overview of Quarterly Security Notifications o K51812227: Understanding security advisory versioning o K4602: Overview of the F5 security vulnerability response policy o K4918: Overview of the F5 critical issue hotfix policy o K39757430: F5 product and services lifecycle policy index o K9502: BIG-IP hotfix and point release matrix o K13123: Managing BIG-IP product hotfixes (11.x - 17.x) o K000090258: Download F5 products from MyF5 o K9970: Subscribe to email notifications regarding F5 products and security announcements o K9957: Creating a custom RSS feed to view new and updated documents o K44525501: Overview of BIG-IP data plane and control plane o K000135931: Contact F5 Support - --------------------------END INCLUDED TEXT---------------------- You have received this e-mail bulletin as a result of your organisation's registration with AUSCERT. 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