Vulnerability identifier: #VU53543

Vulnerability risk: High

CVSSv3.1: 7.3 [CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H/E:P/RL:O/RC:C]

CVE-ID: CVE-2021-23017

CWE-ID: CWE-193

Exploitation vector: Network

Exploit availability: Yes

Vulnerable software:
nginx
Server applications / Web servers

Vendor: NGINX

Description

The vulnerability allows a remote attacker to execute arbitrary code on the target system.

The vulnerability exists due to an off-by-one error within the ngx_resolver_copy() function when processing DNS responses. A remote attacker can trigger an off-by-one error, write a dot character (‘.’, 0x2E) out of bounds in a heap allocated buffer and execute arbitrary code on the system.

Successful exploitation of this vulnerability may result in complete compromise of vulnerable system.

The vulnerability can be triggered by a DNS response in reply to a DNS request from nginx when the resolver primitive is configured. A specially crafted packet allows overwriting the least significant byte of next heap chunk metadata with 0x2E.

Mitigation
Install updates from vendor's website.

Vulnerable software versions

nginx: 1.20.1, 1.19.0 - 1.19.10, 1.18.0, 1.17.0 - 1.17.10, 1.16.0 - 1.16.1, 1.15.0 - 1.15.12, 1.14.0 - 1.14.2, 1.12.0 - 1.12.2, 1.11.0 - 1.11.13, 1.13.0 - 1.13.12, 1.10.0 - 1.10.3, 1.0.0 - 1.0.15, 1.2.0 - 1.2.9, 1.4.0 - 1.4.7, 1.6.0 - 1.6.3, 1.8.0 - 1.8.1, 1.9.0 - 1.9.15, 1.7.0 - 1.7.12, 1.5.0 - 1.5.13, 1.3.0 - 1.3.16, 1.1.0 - 1.1.19, 0.9.0 - 0.9.7, 0.8.0 - 0.8.55, 0.7.0 - 0.7.69, 0.6.18 - 0.6.1516

---

External links
http://www.x41-dsec.de/lab/advisories/x41-2021-002-nginx-resolver-copy/
http://mailman.nginx.org/pipermail/nginx-announce/2021/000300.html

---

Q & A

Can this vulnerability be exploited remotely?

Yes. This vulnerability can be exploited by a remote non-authenticated attacker via the Internet.

Is there known malware, which exploits this vulnerability?

No. We are not aware of malware exploiting this vulnerability. However, proof of concept for this vulnerability is available.