Executive Summary

CVE-2026-48840 is an information disclosure vulnerability in Exim versions 4.88 through 4.99.3 that occurs in the handling of the PROXY protocol in certain proxy configurations.

The vulnerability arises because the proxy_protocol() function uses a stack union without initializing it and does not properly check the minimum length of incoming PROXY protocol frames for certain address families.

Specifically, malformed PROXYv2 frames with certain address families (TCPv6 and TCPv4) and insufficient length cause uninitialized stack memory to be copied and then exposed in the SMTP greeting banner.

This leads to leakage of sensitive memory content, including live userspace virtual address pointers, which can help attackers bypass security features like Address Space Layout Randomization (ASLR).

The vulnerability is exploitable only if Exim is compiled with SUPPORT_PROXY and the hosts_proxy option is set to a non-empty value allowing the attacker to send PROXY headers.

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Impact Analysis

This vulnerability can lead to the disclosure of uninitialized stack memory values to an attacker, revealing sensitive information such as live userspace virtual address pointers.

Such information disclosure can be used by attackers to defeat security mechanisms like Address Space Layout Randomization (ASLR), making it easier to craft further exploits or attacks against the system.

The impact is limited to information disclosure (confidentiality), with no direct integrity or availability impact.

Exploitation requires specific conditions: Exim must be compiled with SUPPORT_PROXY, the hosts_proxy option must be enabled with a non-empty value, and the attacker must be able to send PROXY protocol frames from an allowed source.

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Detection Guidance

This vulnerability can be detected by monitoring for unusual or malformed PROXY-protocol frames sent to the Exim server, especially those with short payloads that trigger the uninitialized stack memory disclosure.

Specifically, detection involves checking if the Exim server is receiving PROXYv2 frames with address family 0x21 (TCPv6) and length 0, or TCPv4 frames (address family 0x11) with length less than 12, which cause the leak.

You can use network packet capture tools like tcpdump or Wireshark to filter and analyze PROXY protocol traffic on the relevant ports.

• Use tcpdump to capture PROXY protocol frames on the Exim listening port (usually port 25):
• tcpdump -i <interface> port 25 -w exim_proxy_traffic.pcap
• Analyze the captured traffic with Wireshark or tshark to identify malformed PROXYv2 frames with suspicious length fields.
• Check Exim SMTP greeting banners for unexpected IPv6 or IPv4 addresses that may contain leaked memory content.

Additionally, reviewing Exim logs for proxyfail responses or unusual connection attempts from hosts allowed by hosts_proxy may help detect exploitation attempts.

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Mitigation Strategies

The primary mitigation is to upgrade Exim to version 4.99.4 or later, which includes the fix for this vulnerability.

If immediate upgrade is not possible, temporary mitigations include:

• Restrict the hosts_proxy configuration option to exact IP addresses of trusted load balancers only.
• Disable PROXY-protocol parsing entirely by unsetting the hosts_proxy option.

These steps reduce the attack surface by limiting or removing the ability of attackers to send malformed PROXY frames that trigger the memory disclosure.

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Compliance Impact

This vulnerability causes disclosure of uninitialized stack memory values to a client, potentially leaking sensitive memory content such as live userspace virtual address pointers.

Such information disclosure could impact compliance with data protection regulations like GDPR or HIPAA, which require protection of sensitive information and prevention of unauthorized data exposure.

However, the provided information does not explicitly discuss the direct impact on compliance with these standards or any regulatory consequences.

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