Exim PROXY Protocol Pre-Authentication Uninitialized Stack Memory Disclosure
CVE-2026-48840 is a pre-authentication information disclosure vulnerability in Exim’s PROXY protocol parser affecting Exim 4.88 through 4.99.3, fixed in 4.99.4. The flaw is in proxy_protocol(), where a stack union used to store parsed PROXYv2 address data is left uninitialized. For PROXYv2 frames, Exim validated only that the declared payload length did not exceed the size of the union, but did not enforce the minimum payload length required for the claimed address family. As a result, a malformed PROXYv2 header with an undersized payload can cause Exim to read and format uninitialized stack bytes as a source address. The documented cases include TCPv6 (family 0x21), where len=0 can lead to disclosure of 16 bytes from hdr.v2.addr.ip6.src_addr, and TCPv4 (family 0x11), where too-short lengths can disclose 4 bytes. The leaked bytes are converted with inet_ntop into an IP string and exposed to the client in the SMTP greeting/banner during pre-authentication processing.
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Impact, mitigation & remediation
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Impact
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Mitigation
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Remediation
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Exploits
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No public exploit code observed for this vulnerability.
Affected products & vendors
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Vendor-confirmed product mapping. Mallory continuously reconciles this list against your asset inventory.
Recent activity
12 sources tracked across advisories, community write-ups, and news. New activity surfaces here as Mallory finds it.
A long-standing Exim vulnerability affecting the SMTP service, reachable pre-authentication and tied to IPv6/SMTP Hello handling in builds using hosts_proxy with SUPPORT_PROXY enabled.
An information disclosure vulnerability in Exim affecting SMTP handling when PROXY protocol support is enabled, allowing leakage of up to 16 bytes of uninitialized memory and potentially weakening protections such as ASLR.
The version that knows your environment.
Query your assets running an affected version, and investigate the blast radius.
Every observed campaign linking this CVE to a named adversary.
Malware families riding this exploit, with evidence and IOCs.
YARA, Sigma, Snort, and vendor rules, auto-deployed to your SIEM.
Cross-references every affected SKU, including bundled OEM variants.
Community discussion across Reddit, Mastodon, and other social sources.