CVE-2026-48840: Exim Information Disclosure Vulnerability

CVE-2026-48840 Overview

CVE-2026-48840 is an information disclosure vulnerability affecting Exim mail transfer agent versions 4.88 through 4.99.3. The flaw exists in certain proxy configurations where Exim mishandles short payloads. This mishandling causes the server to leak uninitialized stack memory contents to a connecting client. The issue is classified under [CWE-839: Numeric Range Comparison Without Minimum Check]. Remote attackers can trigger the disclosure without authentication or user interaction over the network.

Critical Impact

Remote unauthenticated attackers can read uninitialized stack memory from Exim servers running in vulnerable proxy configurations, potentially exposing sensitive runtime data.

Affected Products

• Exim 4.88 through versions before 4.99.4
• Exim deployments operating in proxy protocol configurations
• Mail servers exposing Exim to network clients with proxy support enabled

Discovery Timeline

• 2026-05-30 - CVE-2026-48840 published to NVD
• 2026-06-03 - Last updated in NVD database

Technical Details for CVE-2026-48840

Vulnerability Analysis

The vulnerability resides in Exim's handling of short payloads when the server operates behind a proxy configuration. Exim fails to properly validate the length of incoming data before processing buffers allocated on the stack. When a client sends a payload below an expected size threshold, Exim reads or transmits memory regions that were never initialized. These regions may contain residual data from prior function calls or stack frames.

The disclosed memory is returned to the client during the connection handshake or proxy protocol negotiation. An attacker repeatedly connecting to the server can collect varying stack contents across sessions. The leaked bytes may include pointers, partial credentials, or fragments of previously processed mail data depending on stack reuse patterns.

Root Cause

The root cause is improper bounds checking on short-payload inputs in the proxy handling code path. Exim allocates a fixed-size stack buffer and assumes the input fully populates it. When the payload falls below the expected length, the trailing portion of the buffer retains whatever data previously occupied that stack region. The code then operates on or echoes back the entire buffer rather than only the validated bytes.

Attack Vector

Exploitation requires network access to an Exim instance configured to accept proxy protocol connections. The attacker sends a crafted short payload to the proxy listener. The server processes the truncated input and returns or acts upon a buffer containing uninitialized stack memory. The vulnerability mechanism is described in the Exim Security Advisory and discussed on the OpenWall OSS Security List.

Detection Methods for CVE-2026-48840

Indicators of Compromise

• Repeated short-payload connections to Exim proxy listeners from a single source address
• Anomalous proxy protocol handshakes that terminate before completing the expected byte sequence
• Unusual volumes of low-byte-count sessions logged in Exim main and reject logs

Detection Strategies

• Monitor Exim version banners across mail infrastructure and flag instances running 4.88 through 4.99.3
• Inspect proxy protocol traffic for malformed or truncated PROXY headers using network sensors
• Correlate short-duration TCP sessions on Exim listener ports with subsequent connection attempts from the same client

Monitoring Recommendations

• Enable verbose logging of proxy handshake events including byte counts and source addresses
• Forward Exim logs to a centralized SIEM and alert on patterns consistent with memory probing
• Track outbound bytes per session to detect responses containing larger-than-expected payloads relative to client input

How to Mitigate CVE-2026-48840

Immediate Actions Required

• Upgrade Exim to version 4.99.4 or later across all affected mail servers
• Audit Exim configurations to identify hosts with hosts_proxy or proxy protocol settings enabled
• Restrict access to Exim proxy listeners to trusted upstream load balancers and proxies only

Patch Information

The Exim development team addressed the issue in version 4.99.4. Administrators should consult the Exim Security Advisory for upgrade guidance and verify package versions through their distribution's security repository.

Workarounds

• Disable proxy protocol support in exim.conf if upstream proxy functionality is not required
• Apply firewall rules limiting connections to Exim proxy ports to known internal proxy source addresses
• Place Exim behind a network appliance that validates and normalizes proxy protocol headers before forwarding traffic

# Configuration example: disable proxy support if not required
# In /etc/exim4/exim4.conf or main configuration
hosts_proxy =

# Verify installed Exim version
exim -bV | head -n 1