CVE-2025-21605
Redis is an open source, in-memory database that persists on disk. In versions starting at 2.6 and prior to 7.4.3, An unauthenticated client can cause unlimited growth of output buffers, until the server runs out of memory or is killed. By default, the Redis configuration does not limit the output buffer of normal clients (see client-output-buffer-limit). Therefore, the output buffer can grow unlimitedly over time. As a result, the service is exhausted and the memory is unavailable. When password authentication is enabled on the Redis server, but no password is provided, the client can still cause the output buffer to grow from "NOAUTH" responses until the system will run out of memory. This issue has been patched in version 7.4.3. An additional workaround to mitigate this problem without patching the redis-server executable is to block access to prevent unauthenticated users from connecting to Redis. This can be done in different ways. Either using network access control tools like firewalls, iptables, security groups, etc, or enabling TLS and requiring users to authenticate using client side certificates.
| Software | From | Fixed in |
|---|---|---|
| redis / redis | 2.6.0 | 6.2.18 |
| lfprojects / valkey | 7.2.4 | 7.2.9 |
| lfprojects / valkey | 8.0.0 | 8.0.3 |
| lfprojects / valkey | 8.1.0 | 8.1.1 |
| debian / debian_linux | 11.0 | 11.0.x |
| redis / redis | 7.0 | 7.2.8 |
| redis / redis | 7.4.0 | 7.4.3 |
- https://github.com/redis/redis/releases/tag/7.4.3
- https://github.com/redis/redis/security/advisories/GHSA-r67f-p999-2gff
- https://github.com/valkey-io/valkey/releases/tag/8.1.1
- https://lists.debian.org/debian-lts-announce/2025/05/msg00014.html
- https://www.vicarius.io/vsociety/posts/cve-2025-21605-detection-script-memory-exhaustion-vulnerability-in-redis-database
- https://www.vicarius.io/vsociety/posts/cve-2025-21605-mitigation-script-memory-exhaustion-vulnerability-in-redis-database
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Frequently Asked Questions
A security vulnerability is a weakness in software, hardware, or configuration that can be exploited to compromise confidentiality, integrity, or availability. Many vulnerabilities are tracked as CVEs (Common Vulnerabilities and Exposures), which provide a standardized identifier so teams can coordinate patching, mitigation, and risk assessment across tools and vendors.
CVSS (Common Vulnerability Scoring System) estimates technical severity, but it doesn't automatically equal business risk. Prioritize using context like internet exposure, affected asset criticality, known exploitation (proof-of-concept or in-the-wild), and whether compensating controls exist. A "Medium" CVSS on an exposed, production system can be more urgent than a "Critical" on an isolated, non-production host.
A vulnerability is the underlying weakness. An exploit is the method or code used to take advantage of it. A zero-day is a vulnerability that is unknown to the vendor or has no publicly available fix when attackers begin using it. In practice, risk increases sharply when exploitation becomes reliable or widespread.
Recurring findings usually come from incomplete Asset Discovery, inconsistent patch management, inherited images, and configuration drift. In modern environments, you also need to watch the software supply chain: dependencies, containers, build pipelines, and third-party services can reintroduce the same weakness even after you patch a single host. Unknown or unmanaged assets (often called Shadow IT) are a common reason the same issues resurface.
Use a simple, repeatable triage model: focus first on externally exposed assets, high-value systems (identity, VPN, email, production), vulnerabilities with known exploits, and issues that enable remote code execution or privilege escalation. Then enforce patch SLAs and track progress using consistent metrics so remediation is steady, not reactive.
SynScan combines attack surface monitoring and continuous security auditing to keep your inventory current, flag high-impact vulnerabilities early, and help you turn raw findings into a practical remediation plan.