CVE-2026-4046: GNU C Library iconv() DOS Vulnerability

CVE-2026-4046 Overview

The iconv() function in the GNU C Library (glibc) versions 2.43 and earlier contains a vulnerability that can cause a crash due to an assertion failure when converting inputs from the IBM1390 or IBM1399 character sets. This denial of service vulnerability can be exploited remotely to crash applications that process character set conversions using the affected function.

Critical Impact

Remote attackers can trigger an assertion failure in the iconv() function by supplying malicious input using IBM1390 or IBM1399 character sets, leading to application crashes and denial of service conditions.

Affected Products

• GNU C Library (glibc) versions 2.43 and earlier
• Applications using iconv() with IBM1390 character set support
• Applications using iconv() with IBM1399 character set support

Discovery Timeline

• 2026-03-30 - CVE-2026-4046 published to NVD
• 2026-04-01 - Last updated in NVD database

Technical Details for CVE-2026-4046

Vulnerability Analysis

This vulnerability is classified as CWE-617 (Reachable Assertion), which occurs when a program contains an assert() or similar statement that can be triggered by an attacker. In this case, the iconv() function in glibc contains an assertion that fails when processing specific character conversions from the IBM1390 or IBM1399 character sets. When triggered, the assertion causes the application to terminate abnormally, resulting in a denial of service condition.

The vulnerability is particularly concerning because iconv() is a widely-used function for character set conversion across numerous applications and services. Any application that accepts user-controlled input for character set conversion and supports IBM1390 or IBM1399 encodings is potentially vulnerable.

Root Cause

The root cause lies in improper handling of certain input sequences during character conversion operations. When the iconv() function processes specific malformed or unexpected input from the IBM1390 or IBM1399 character sets, an internal assertion is triggered rather than graceful error handling. This assertion failure causes immediate process termination, as assertions are typically compiled into production builds of glibc.

Attack Vector

The vulnerability can be exploited remotely over a network without requiring authentication or user interaction. An attacker can craft malicious input using the IBM1390 or IBM1399 character set encoding and send it to a vulnerable application that performs character conversion operations. When the application processes this input through the iconv() function, the assertion failure is triggered, causing the application to crash.

Common attack scenarios include:

• Web applications that perform character encoding conversions on user-supplied data
• Email servers processing messages with specific character encodings
• File processing services that convert document encodings
• Any network service that utilizes glibc's iconv() function with IBM1390/IBM1399 support

The attack requires minimal effort from the attacker's perspective, as the vulnerability can be triggered by simply sending specially crafted data to a vulnerable endpoint.

Detection Methods for CVE-2026-4046

Indicators of Compromise

• Unexpected application crashes or service interruptions correlated with character conversion operations
• Core dump files showing assertion failures within glibc's iconv() function
• Process termination logs indicating SIGABRT signals originating from glibc code paths
• Increased frequency of application restarts without apparent cause

Detection Strategies

• Monitor system logs for assertion failure messages related to iconv() or character set conversion functions
• Implement crash monitoring for applications that utilize glibc character conversion features
• Deploy network intrusion detection rules to identify suspicious character encoding patterns in incoming traffic
• Review application error logs for segmentation faults or abnormal terminations during encoding operations

Monitoring Recommendations

• Enable core dump collection and analysis for production systems to capture assertion failure details
• Configure application-level monitoring to detect unusual restart patterns that may indicate exploitation attempts
• Implement logging for character conversion operations, particularly those involving IBM1390 and IBM1399 encodings
• Monitor system stability metrics and correlate crashes with incoming network traffic patterns

How to Mitigate CVE-2026-4046

Immediate Actions Required

• Assess whether your systems require IBM1390 or IBM1399 character set support for legitimate operations
• Remove the IBM1390 and IBM1399 character sets from systems that do not need them as a trivial mitigation
• Update glibc to the latest patched version when available from your distribution vendor
• Implement input validation to reject or sanitize character encoding requests before processing

Patch Information

Security patches for this vulnerability are being tracked in the official glibc security advisory. For detailed patch information, refer to the Sourceware GLIBC Security Advisory. Additional technical details and bug tracking information are available at the Sourceware Bug Report #33980.

Organizations should monitor their Linux distribution's security channels for package updates containing the fix and apply them according to their patch management policies.

Workarounds

• Remove the IBM1390 and IBM1399 character set modules from systems where they are not required for business operations
• Implement application-level filtering to reject requests specifying IBM1390 or IBM1399 character encodings
• Deploy network-level controls to filter incoming requests containing suspicious character encoding specifications
• Use application sandboxing or containerization to limit the impact of potential crashes

# Configuration example - Remove IBM1390/IBM1399 character sets
# Identify and remove the character set modules if not needed
# Note: Location may vary by distribution

# Check if IBM1390/IBM1399 modules exist
ls -la /usr/lib/gconv/IBM139*

# Create a backup before making changes
cp -r /usr/lib/gconv /usr/lib/gconv.backup

# Remove the vulnerable character set modules (if not required)
rm /usr/lib/gconv/IBM1390.so
rm /usr/lib/gconv/IBM1399.so

# Update the gconv modules cache
iconvconfig