SMTP request manipulation via unvalidated reverse-DNS hostname
Published Mar 24, 2026 · Updated Mar 24, 2026
CRLF injection in F5 NGINX Plus and NGINX Open Source allows remote attackers to manipulate SMTP upstream requests through DNS. ngx_mail_smtp_resolve_addr_handler accepted a reverse-DNS hostname containing CRLF and reused it in auth_http headers and SMTP XCLIENT data without RFC 1034 character validation. Exploitation requires the mail proxy, a configured resolver, and control of a client's PTR answer; the resulting impact is limited to injected headers and upstream request manipulation.
Summary
What happened
CRLF injection in F5 NGINX Plus and NGINX Open Source allows remote attackers to manipulate SMTP upstream requests through DNS. ngx_mail_smtp_resolve_addr_handler accepted a reverse-DNS hostname containing CRLF and reused it in auth_http headers and SMTP XCLIENT data without RFC 1034 character validation. Exploitation requires the mail proxy, a configured resolver, and control of a client's PTR answer; the resulting impact is limited to injected headers and upstream request manipulation.
The record
- CVE
- CVE-2026-28753
- Published
- Mar 24, 2026
- Updated
- Mar 24, 2026
- Vendor
- F5 Networks
- Product
- NGINX Open Source
- Classifications
- CWE-93, T1190
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Mar 24, 2026CVE publishedPublication date reported by the CVE source.
- Mar 24, 2026Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
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Is a vulnerable build present?
Compare these published version ranges with your installed build and any vendor patches.
- Affected versionversion=0.6.27 <1.28.3
- Affected versionversion=1.29.0 <1.29.7
What conditions does exploitation require?
What is affected?
Published CVSS scores
CVSS describes severity. EPSS estimates exploitation probability.
Attacks
What attackers are doing with it
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Weakness, pattern, technique
Public exploit references
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Technologies
Your stack
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Your stack
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