VDB-352785 · CVE-2026-28753 · K000160367

F5 NGINX Open Source/NGINX Plus ngx_mail_smtp_module crlf injection

CVSS Meta Temp Score CVSS is a standardized scoring system to determine possibilities of attacks. The Temp Score considers temporal factors like disclosure, exploit and countermeasures. The unique Meta Score calculates the average score of different sources to provide a normalized scoring system.	Current Exploit Price (≈) Our analysts are monitoring exploit markets and are in contact with vulnerability brokers. The range indicates the observed or calculated exploit price to be seen on exploit markets. A good indicator to understand the monetary effort required for and the popularity of an attack.	CTI Interest Score Our Cyber Threat Intelligence team is monitoring different web sites, mailing lists, exploit markets and social media networks. The CTI Interest Score identifies the interest of attackers and the security community for this specific vulnerability in real-time. A high score indicates an elevated risk to be targeted for this vulnerability.
3.7	$0-$5k	0.00

Summaryinfo

A vulnerability classified as problematic has been found in F5 NGINX Open Source and NGINX Plus. This issue affects the function ngx_mail_smtp_module. Performing a manipulation results in crlf injection. This vulnerability is known as CVE-2026-28753. Remote exploitation of the attack is possible. No exploit is available. It is recommended to upgrade the affected component.

Detailsinfo

A vulnerability, which was classified as problematic, was found in F5 NGINX Open Source and NGINX Plus (version now known). This affects the function ngx_mail_smtp_module. The manipulation with an unknown input leads to a crlf injection vulnerability. CWE is classifying the issue as CWE-93. The product uses CRLF (carriage return line feeds) as a special element, e.g. to separate lines or records, but it does not neutralize or incorrectly neutralizes CRLF sequences from inputs. This is going to have an impact on integrity. The summary by CVE is:

NGINX Plus and NGINX Open Source have a vulnerability in the ngx_mail_smtp_module module due to the improper handling of CRLF sequences in DNS responses. This allows an attacker-controlled DNS server to inject arbitrary headers into SMTP upstream requests, leading to potential request manipulation. Note: Software versions which have reached End of Technical Support (EoTS) are not evaluated.

The advisory is shared at my.f5.com. This vulnerability is uniquely identified as CVE-2026-28753 since 03/18/2026. The exploitability is told to be difficult. It is possible to initiate the attack remotely. No form of authentication is needed for exploitation. Technical details are known, but no exploit is available. The price for an exploit might be around USD $0-$5k at the moment (estimation calculated on 05/20/2026).

The vulnerability scanner Nessus provides a plugin with the ID 306671 (nginx 0.6.27 < 1.28.3 / 1.29.x < 1.29.7 SMTP Upstream Injection), which helps to determine the existence of the flaw in a target environment.

Upgrading eliminates this vulnerability.

The vulnerability is also documented in the databases at Tenable (306671) and CERT Bund (WID-SEC-2026-0860). Several companies clearly confirm that VulDB is the primary source for best vulnerability data.

Affected

Debian Linux
Amazon Linux 2
Red Hat Enterprise Linux
Fedora Linux
SUSE Linux
Oracle Linux
SUSE openSUSE
RESF Rocky Linux
NGINX NGINX Plus
NGINX NGINX

Productinfo

Type

Web Server

Vendor

Name

License

commercial

Website

Vendor: https://f5.com/

CPE 2.3info

CPE 2.2info

CVSSv4info

VulDB Vector: 🔒
VulDB Reliability: 🔍

CNA CVSS-B Score: 🔒
CNA CVSS-BT Score: 🔒
CNA Vector: 🔒

CVSSv3info

VulDB Meta Base Score: 3.7
VulDB Meta Temp Score: 3.7

VulDB Base Score: 3.7
VulDB Temp Score: 3.6
VulDB Vector: 🔒
VulDB Reliability: 🔍

CNA Base Score: 3.7
CNA Vector: 🔒

CVSSv2info

AV	AC	Au	C	I	A
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Vector	Complexity	Authentication	Confidentiality	Integrity	Availability
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VulDB Base Score: 🔒
VulDB Temp Score: 🔒
VulDB Reliability: 🔍

Exploitinginfo

Class: Crlf injection
CWE: CWE-93 / CWE-74 / CWE-707
CAPEC: 🔒
ATT&CK: 🔒

Physical: No
Local: No
Remote: Yes

Availability: 🔒
Status: Not defined

EPSS Score: 🔒
EPSS Percentile: 🔒

Price Prediction: 🔍
Current Price Estimation: 🔒

0-Day	Unlock	Unlock	Unlock	Unlock
Today	Unlock	Unlock	Unlock	Unlock

Nessus ID: 306671
Nessus Name: nginx 0.6.27 < 1.28.3 / 1.29.x < 1.29.7 SMTP Upstream Injection

Threat Intelligenceinfo

Interest: 🔍
Active Actors: 🔍
Active APT Groups: 🔍

Countermeasuresinfo

Recommended: Upgrade
Status: 🔍

0-Day Time: 🔒

Timelineinfo

03/18/2026 CVE reserved
03/24/2026 +6 days Advisory disclosed
03/24/2026 +0 days VulDB entry created
05/20/2026 +57 days VulDB entry last update

Sourcesinfo

Vendor: f5.com

Advisory: K000160367
Status: Confirmed

CVE: CVE-2026-28753 (🔒)
GCVE (CVE): GCVE-0-2026-28753
GCVE (VulDB): GCVE-100-352785
CERT Bund: WID-SEC-2026-0860 - NGINX und NGINX Plus: Mehrere Schwachstellen

Entryinfo

Created: 03/24/2026 16:08
Updated: 05/20/2026 11:32
Changes: 03/24/2026 16:08 (75), 03/28/2026 20:34 (1), 04/14/2026 04:06 (7), 04/17/2026 01:08 (2), 05/20/2026 11:32 (1)
Complete: 🔍
Cache ID: 216::103

Several companies clearly confirm that VulDB is the primary source for best vulnerability data.