Medium

Header Injection in NGINX ngx_mail_smtp_module auth_http/XCLIENT handling

IdentifiersCVE-2026-28753CWE-74

CVE-2026-28753 is an injection vulnerability in NGINX Open Source and NGINX Plus affecting the mail SMTP proxy path, specifically ngx_mail_smtp_module in configurations using auth_http together with XCLIENT in SMTP sessions. According to the provided content, the issue is caused by improper handling of CRLF sequences in DNS responses, including PTR-record-related processing used for client validation. If an attacker can control the DNS server or DNS response data consulted by the vulnerable configuration, they can inject arbitrary headers into SMTP upstream requests. This can manipulate the request data sent by nginx to the upstream SMTP service or authentication backend. Reported vulnerable versions are 0.6.27 through 1.29.6; fixed versions are 1.29.7+ and 1.28.3+.

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ANALYST BRIEF

Impact, mitigation & remediation

What it means. What to do now. Patch path, mitigations, and the assume-compromise checklist.

Impact

What an attacker gets, and what they’ve been doing with it.

Successful exploitation allows request/header injection into SMTP upstream requests in affected mail proxy deployments. This can undermine trust in client identity or metadata conveyed upstream, potentially bypassing access checks or altering authorization-relevant fields in auth_http/XCLIENT workflows. The provided content specifically notes potential request manipulation and references PTR-record-based access-check bypass scenarios. Impact is therefore primarily integrity-related, with possible secondary authentication or policy-bypass consequences depending on how the upstream SMTP or auth service consumes the injected headers.

Mitigation

If you can’t patch tonight, do this now.

If immediate upgrade is not possible, reduce exposure by disabling or avoiding the affected mail proxy configuration pattern: auth_http combined with XCLIENT in SMTP sessions, especially where client validation depends on DNS PTR lookups or other attacker-influenced DNS data. Ensure nginx does not rely on untrusted or attacker-controlled DNS infrastructure for PTR-based validation decisions. Restrict or harden upstream trust in XCLIENT-derived metadata until patched.

Remediation

Patch, then assume compromise.

Upgrade NGINX Open Source or NGINX Plus to a fixed release. The provided content identifies 1.29.7+ and 1.28.3+ as not vulnerable. Software versions that have reached End of Technical Support are not evaluated; supported fixed versions should be used.

PUBLIC EXPLOITS

Exploits

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VALID 0 / 0 TOTALView more in app

No public exploit code observed for this vulnerability.

EXPOSURE SURFACE

Affected products & vendors

Products and vendors Mallory has correlated with this vulnerability. Open in Mallory to drill down to specific CPE configurations and version ranges.

VendorProductType

F5Nginxapplication

F5Nginx Open Sourceapplication

F5Nginx Plusapplication

Vendor-confirmed product mapping. Mallory continuously reconciles this list against your asset inventory.

ACTIVITY FEED

Recent activity

6 sources tracked across advisories, community write-ups, and news. New activity surfaces here as Mallory finds it.

6 SOURCESView more in app

opennetNews

Mar 24, 2026

�� nginx 1.29.7 � 1.28.3 � �� 6 ��

A vulnerability in nginx involving DNS PTR record handling in configurations using auth_http and XCLIENT in SMTP sessions.

opennetNews

Nov 3, 2025

�� nginx 1.29.8 � �� FreeNginx 1.29.7

An access control bypass vulnerability in nginx auth_http module involving DNS PTR records when using XCLIENT in SMTP sessions with the mail proxy.

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Exposure mapping

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Threat actor evidence

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Associated malware

Malware families riding this exploit, with evidence and IOCs.

Detection signatures

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Vendor-by-vendor mapping

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Social activity3