Patch Apache HTTP Server to 2.4.64 to Mitigate CVE-2025-53020 DoS

The discovery of CVE-2025-53020 — a memory-management bug in Apache HTTP Server’s HTTP/2 implementation that can be turned into a denial‑of‑service by artificially inflating memory usage — is a practical wake-up call for anyone running Apache 2.4.x in production: the defect affects versions 2.4.17 through 2.4.63 and was fixed in the upstream 2.4.64 release; operators should treat exposed HTTP/2 front-ends as high-priority patch targets.

Background​

Apache published the advisory for this issue as part of the 2.4.64 release notes after a responsible disclosure process in June–July 2025. The project describes the bug as a “Late Release of Memory after Effective Lifetime” that affects the HTTP/2 path and can be exploited to increase a connection’s memory footprint until available resources are exhausted. The finder credited for the report is Gal Bar Nahum.
Independent vulnerability databases and Linux distribution trackers mirror the upstream guidance: affected versions span Apache HTTP Server 2.4.17 → 2.4.63, and the recommended remediation is upgrading to Apache HTTP Server 2.4.64 (or applying your vendor’s packaged backport). Public CVSS listings commonly place the issue in the High range (CVSS v3.1 ≈ 7.5) because the attack is network‑accessible, requires no privileges, and impacts availability.

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Why this matters right now​

• The vulnerability is remote, unauthenticated, and network-reachable, which means any attacker who can open an HTTP/2 session to an exposed Apache server can attempt to trigger the condition.
• The practical impact is availability loss: memory growth on a connection or worker can lead to process destabilization, worker exhaustion, OOM conditions or service unavailability. Multiple vendors (Ubuntu, Debian, Red Hat derivatives) issued fixes and advisories quickly after the upstream remediation, which indicates broad recognition of operational risk.
• The issue is configuration‑agnostic in the sense that any HTTP/2‑enabled instance using the affected code path could be exposed; unlike some Apache CVEs that require specific proxy or mod_ssl configurations, this vulnerability targets core HTTP/2 memory handling in the Apache stack.

Given these facts, operators should assume that publicly reachable Apache HTTP/2 endpoints running vulnerable versions are at meaningful risk until patched.

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Technical overview: what the bug is, in plain language​

At its core this is a memory-lifetime problem. Under certain HTTP/2 usage patterns an Apache process can retain more memory than intended for a connection or stream because allocated buffers are not released promptly (or the release is deferred beyond practical limits). A crafted sequence of HTTP/2 frames — notably repeated or pathological header sequences — can be used to grow the server’s memory consumption on a per‑connection basis. If an attacker opens many such connections, or keeps them active while continually enlarging buffered data, the aggregate effect is resource exhaustion and denial of service.
More concretely, mod_http2 fixes published alongside the 2.4.64 release note that the problem may be triggered by repeated request header names or by behaviors that cause the server to hold on to memory for longer than intended; the mod_http2 changelog explicitly lists the patch for CVE‑2025‑53020 as preventing a client from increasing memory consumption via repeated request header names. This points to a practical attack surface where crafted HTTP/2 header sequences drive growth in internal data structures.
Important nuance: during "normal" HTTP/2 traffic the likelihood of hitting the bug is low — it requires a pathological pattern — but the vulnerability becomes high‑impact because an attacker who can intentionally produce the pattern can do so in volume and with little effort. Several prior HTTP/2 memory and stream-handling bugs followed this same operational model: low natural likelihood, but trivial to exploit when deliberately attempted.

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Cross-checks and scoring: what the vendors and trackers say​

• The Apache Project’s official vulnerability listing documents CVE‑2025‑53020 as “HTTP/2 DoS by Memory Increase” and places it in the moderate severity category on the project page; the fix shipped in 2.4.64.
• The U.S. National Vulnerability Database (NVD) and several commercial trackers list the same affected range and note the recommended upgrade to 2.4.64; third‑party sources commonly report a CVSS v3.1 base score of 7.5 (High).
• Distribution advisories (Ubuntu, Debian, SUSE, Amazon Linux) published vendor-specific fixes or backports; package maintainers have been actively releasing updates to move users off the vulnerable upstream code. The NIST/NVD entry and vendor advisories together provide consistent, independent confirmation of the affected range and the correct remediation.

These multiple, independent confirmations are important: they allow security teams to triage and automate updates using both upstream and vendor-supplied metadata.

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Practical impact and risk analysis​

Strengths in the ecosystem response

• Upstream patch availability. Apache shipped a fix in 2.4.64 quickly after disclosure, enabling downstream vendors and distributions to follow with packaged updates.
• Broad vendor coordination. Major distributions and vulnerability trackers published indices and updates, which simplifies operational patching windows for most environments.

Operational weaknesses and risk drivers

• HTTP/2 adoption: Modern front‑end servers, cloud load balancers and container images commonly enable or pass HTTP/2 traffic. This widens the practical attack surface compared with bugs that only affect older or rare configurations.
• Container and appliance lag: Embedded appliances, marketplace images, and container base images may ship with older Apache binaries and receive updates more slowly than OS packages; this creates lingering, hard-to-patch artifacts in production fleets.
• Detection blind spots: Memory-growth issues can be mistaken for legitimate load or garbage-collection behavior; without targeted instrumentation you may miss slow resource leaks until they escalate into full outages.

Severity assessment: while the numeric CVSS and EPSS numbers vary between trackers, the correct operational posture is to treat this as a high-priority availability risk for any Apache HTTP/2 endpoint that is reachable by untrusted clients.

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Detection and hunting: how to tell if you’re exposed or under attack​

Start by confirming the obvious: which hosts run a vulnerable Apache binary or package.

• Check binary/package versions:
• apachectl -v (or httpd -v) — confirm the Apache version string.
• rpm -q httpd or dpkg -l | grep apache2 — find packaged versions in RPM/Debian systems.
• Confirm HTTP/2 support and module presence:
• apachectl -M | grep http2 (or apachectl -M | grep proxy_http2 when relevant) to list loaded modules. The presence of mod_http2 or mod_http2-related modules increases relevance.
• Look for suspicious runtime indicators:
• Repeated, unexplained memory growth in worker processes that correlates with incoming HTTP/2 connections.
• Unusual patterns of open HTTP/2 streams or long-lived connections with a small number of client IPs.
• System logs showing repeated child process crashes, OOM killer activity, or restarted services paired to slow growth patterns.
• Capture and analyze HTTP/2 frames for anomalous header patterns:
• If you can collect packet captures or proxy traces from the front-end, inspect header sequences for repeated/overly long header names or header flooding behaviour that matches the mod_http2 change descriptions. While this requires protocol-aware tooling, it is the most direct way to confirm exploit attempts.
• Hunting checklist (short):
• Inventory Apache package versions across VMs, containers, appliances and images.
• Find hosts with mod_http2 enabled.
• Correlate memory growth metrics, child restarts, and unusual HTTP/2 traffic patterns.

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Immediate mitigations (when you can’t patch immediately)​

Upgrading to the patched release (2.4.64) or applying your vendor’s security update is the only definitive fix. If you cannot patch within your next maintenance window, apply temporary mitigations to reduce exposure — but treat them as stopgaps:

• Disable HTTP/2 at the front end until patched. If HTTP/2 support is not essential, turn it off to remove the vulnerable code path. Test client compatibility before rollout.
• Use network controls to limit which clients can reach Apache’s HTTP/2 listeners (ACLs, firewall rules, cloud security groups). Restricting access from untrusted networks reduces the attack surface.
• Rate-limit or throttle new HTTP/2 connection creation and per‑connection operations at your edge or WAF layer. Modern WAFs and load balancers can implement aggressive per‑connection limits for suspicious clients.
• Enforce per-request and per-header size limits—where feasible—so attackers cannot flood header frames with pathological sequences. Recognize that some backends expect large headers, so test before broad rollouts.

Operational caveats: disabling HTTP/2 or imposing strict limits may affect legitimate traffic, so coordinate with application teams and validate in staging environments before broad changes.

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Remediation plan — step‑by‑step​

• Inventory and prioritize:
• Identify all Apache artifacts (VMs, containers, appliances, marketplace images) and mark those that run versions 2.4.17 → 2.4.63. Automated image scanning and package inventories significantly accelerate this phase.
• Confirm exposure:
• For each candidate host, confirm whether HTTP/2 is enabled and whether mod_http2 (or related modules) are loaded. If the server doesn’t accept HTTP/2 traffic, exposure risk is lower.
• Patch or rebuild:
• Apply your OS vendor’s security update that references the 2.4.64 upstream fix. For container images, rebuild base images and redeploy via CI/CD so the fix is persistent. For appliances, apply vendor firmware updates.
• Verify:
• After patching, confirm the Apache binary version (apachectl -v) and that mod_http2 behavior no longer exhibits memory-growth symptoms during controlled tests. Monitor logs for absence of assertion traces or anomalous child restarts.
• Monitor:
• Increase log verbosity for a limited window and correlate system-level process restarts against inbound HTTP/2 traffic. Apply WAF rules to detect header‑flooding patterns until you can confirm full remediation.
• Post‑remediation hardening:
• Add image-scanning gates to CI pipelines to prevent reintroduction of vulnerable Apache packages into production container images. Maintain automated vulnerability scans for packaged artifacts and images.

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Vendor and distribution responses​

The Apache Project’s 2.4.64 release included the upstream fix for CVE‑2025‑53020 and related HTTP/2 and mod_ssl issues. Downstream distributors — Ubuntu, Debian, SUSE, Amazon Linux, Red Hat derivatives and others — published vendor advisories and packaged fixes or backports following the upstream release; most vendor advisories instruct users to apply their distro updates or to upgrade to 2.4.64 where available. This broad vendor coordination makes remediation operationally practical for most environments.
A recurring operational challenge is heterogeneous artifact management: while OS package updates are straightforward, container base images, marketplace images, firmware-embedded servers and third‑party appliance images may lag. Those artifacts require rebuilds and redeploys — a process that needs CI/CD coordination and additional testing cycles.

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Attack surface hardening and longer-term lessons​

• Treat protocol parsers as stateful engines. HTTP/2’s multiplexed nature and binary framing make it easy to craft interactions that stress implementation lifecycles; defensive coding and early reclamation of temporary buffers are crucial. The CVE highlights that even long-lived, mature codebases can hide lifecycle bugs that only appear under specific protocol sequences.
• Bake artifact-level scanning into the supply chain. Don’t rely solely on vendor attestations; scan the actual images, VMs and appliances you run to find embedded vulnerable versions. Microsoft’s recent VEX/CSAF attestations are useful, but they do not replace artifact verification.
• Instrument resource metrics per connection. Visibility into per‑connection memory use, stream counts and long‑lived connection durations lets you detect anomalous behaviors earlier, enabling faster mitigation before system-wide OOMs occur.
• Make protocol hardening part of release gates. For teams shipping HTTP servers or middleboxes, add fuzzing and protocol‑stateful testing (including long-lived connection tests) into CI so lifecycle bugs are detected earlier.

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What to do right now — a short, actionable checklist​

• If you run Apache 2.4.x, immediately identify instances with versions between 2.4.17 and 2.4.63.
• For any instance that accepts HTTP/2 traffic, schedule patching to 2.4.64 or apply your vendor’s patched package in the next maintenance window. If you cannot patch quickly, disable HTTP/2 or apply strict network access controls for the affected hosts.
• Rebuild and redeploy any container images or marketplace appliance images that include the vulnerable Apache binary. Don’t assume an in-container runtime upgrade is sufficient — rebuild the image to guarantee the fix is present.
• Add detection: increase logging for a short window and watch for unusual memory growth correlated with HTTP/2 streams. Apply WAF rules to detect and throttle suspicious header sequences.

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Closing assessment​

CVE‑2025‑53020 is a clear example of the kind of reliability‑and‑availability risk that can hide inside mature server code: not an obvious remote code‑execution condition, but a practical, low‑complexity way to make services unavailable. The good news is that a fix exists and has been widely packaged; the remaining work is operational: inventory, patch, rebuild, verify.
Treat this CVE as a remediation priority for any publicly reachable Apache HTTP/2 front-end and as a timely reminder to harden protocol-facing parsers and to keep artifact inventories precise and automated. The incremental operational cost of upgrading to 2.4.64, rebuilding images, and tightening HTTP/2 exposure is far lower than the potential business cost of a sustained DoS against key web properties.

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Source: MSRC Security Update Guide - Microsoft Security Response Center