VulnerabilityPublished 24 Mar 2026

CVE-2026-28753: F5 NGINX Open Source · CRLF Injection

Affects F5 NGINX Open SourceSource CVE Database V5

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Not known to be exploited · CVSS 3.7 Low : upgrade to 1.29.7 or 1.28.3 when convenient.

Data as of 30 Oct 2026, 11:52 UTC

CVSS 3.1 base score

3.7Low

How severe the flaw is on the 0–10 CVSS scale.

EPSS: 0.2% chance of exploitation in 30 days – higher than 19% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 1.29.7, 1.28.3

CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N

Attack Vector: Network
Attack Complexity: High
Privileges Required: None
User Interaction: None
Scope: Unchanged
Confidentiality: None
Integrity: Low
Availability: None

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Network: The vulnerable component is bound to the network stack and the set of possible attackers extends beyond the other options listed, up to and including the entire Internet. Such a vulnerability is often termed 'remotely exploitable' and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: High: A successful attack depends on conditions beyond the attacker's control. That is, a successful attack cannot be accomplished at will, but requires the attacker to invest in some measurable amount of effort in preparation or execution against the vulnerable component before a successful attack can be expected. For example, a successful attack may require an attacker to: gather knowledge about the environment in which the vulnerable target/component exists; prepare the target environment to improve exploit reliability; or inject themselves into the logical network path between the target and the resource requested by the victim in order to read and/or modify network communications (e.g., a man in the middle attack).
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without any interaction from any user.
Scope: Unchanged: An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.
Confidentiality: None: There is no loss of confidentiality within the impacted component.
Integrity: Low: Modification of data is possible, but the attacker does not have control over the consequence of a modification, or the amount of modification is limited. The data modification does not have a direct, serious impact on the impacted component.
Availability: None: There is no impact to availability within the impacted component.

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 24 Mar 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponized: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 30 Oct 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponized: A Metasploit exploit module or Exploit-DB entry exists (Metasploit scanner modules do not count), or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Upgrade F5 NGINX Open Source to 1.29.7 or 1.28.3.

Affected software

Product	Affected	Fixed in
F5NGINX Open Source	Affected >=1.29.0 <1.29.7	Fixed in 1.29.7
F5NGINX Open Source	Affected >=0.6.27 <1.28.3	Fixed in 1.28.3
F5NGINX Plus	Affected >=R34 >=R33 =R36 =R35 =R32	Fixed in–

AI-suggested

To mitigate this vulnerability, organizations should first verify if they are running affected versions of NGINX Open Source (1.29.0 or 0.6.27) or NGINX Plus with the ngx_mail_smtp_module enabled. If so, they should monitor for official patches or updates from F5 and apply them promptly once available. In the absence of patches, administrators should restrict DNS server configurations to trusted and validated DNS resolvers to prevent attacker-controlled DNS responses. Implementing DNSSEC validation can help ensure the authenticity and integrity of DNS responses. Additionally, network segmentation and firewall rules should limit exposure of mail proxy servers to untrusted networks. Logging and monitoring DNS query responses and SMTP upstream requests for anomalous header injections can provide early detection of exploitation attempts. Finally, reviewing mail routing policies and SMTP header handling can reduce the impact of any injected headers.

Description

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.

Impact

The primary impact of this vulnerability is the potential manipulation of SMTP upstream requests by injecting arbitrary headers via malicious DNS responses. While this does not directly compromise confidentiality or availability, it can affect the integrity of mail routing and processing. Attackers could potentially redirect mail traffic, cause mail delivery failures, or bypass certain mail filtering mechanisms by manipulating headers. Organizations relying on NGINX as a mail proxy or SMTP relay could experience mail flow disruptions or unauthorized mail handling behaviors. However, exploitation requires control over DNS responses, limiting the attack surface primarily to environments where DNS servers are untrusted or compromised. The low CVSS score reflects the limited scope and complexity of exploitation. Nonetheless, in sensitive environments such as large mail service providers, enterprises with critical mail infrastructure, or hosting providers, even subtle mail manipulation can have operational and reputational consequences.

CWE-93Improper Neutralization of CRLF Sequences (CRLF Injection) (opens in a new tab)
Base weaknessOWASP Top 10 A05:2025: Injection (opens in a new tab)
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.
Implementation
Avoid using CRLF as a special sequence.
Implementation
Appropriately filter or quote CRLF sequences in user-controlled input.
OWASP Cheat Sheets
Abuse Case (opens in a new tab)
Deserialization (opens in a new tab)
Injection Prevention (opens in a new tab)
SQL Injection Prevention (opens in a new tab)
Injection Prevention in Java (opens in a new tab)
OWASP ASVS 4.0.3
V1.5.1 (opens in a new tab)
Attack patterns
Command Delimiters (opens in a new tab)
Web Server Logs Tampering (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

my.f5.com/manage/s/article/K000160367 (opens in a new tab)

Community reviews

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Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 24 Mar 2026, 15:07 UTC

WeaknessesCWE-93

Technical details

References

Community reviews

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