CVE-2020-27618: GNU Glibc iconv DoS Vulnerability

CVE-2020-27618 Overview

CVE-2020-27618 is an Infinite Loop vulnerability in the iconv function of the GNU C Library (glibc) version 2.32 and earlier. When processing invalid multi-byte input sequences in specific IBM character encodings (IBM1364, IBM1371, IBM1388, IBM1390, and IBM1399), the function fails to advance the input state, causing applications to enter an infinite loop. This results in a denial of service condition that can render affected systems or applications unresponsive.

Critical Impact

Applications using glibc's iconv function with IBM character encodings are vulnerable to denial of service attacks when processing maliciously crafted input, potentially causing system resource exhaustion and service unavailability.

Affected Products

• GNU glibc (versions 2.32 and earlier)
• NetApp ONTAP Select Deploy Administration Utility
• NetApp Storage Systems (A250, 500F, H410C, H300S, H500S, H700S, H300E, H500E, H700E, H410S)
• Oracle Communications Cloud Native Core Service Communication Proxy 1.14.0
• Debian Linux 10.0

Discovery Timeline

• 2021-02-26 - CVE-2020-27618 published to NVD
• 2025-06-09 - Last updated in NVD database

Technical Details for CVE-2020-27618

Vulnerability Analysis

The vulnerability resides in glibc's iconv character set conversion function, which is a fundamental component used by countless applications for internationalization and text processing. When the function encounters invalid multi-byte input sequences while using IBM EBCDIC-based encodings (specifically IBM1364, IBM1371, IBM1388, IBM1390, and IBM1399), it fails to properly advance the input state pointer. This oversight creates a condition where the function repeatedly processes the same invalid byte sequence without making progress, resulting in an infinite loop.

This vulnerability is classified as CWE-835 (Loop with Unreachable Exit Condition), indicating a fundamental flaw in the loop control logic. The issue is distinct from the related CVE-2016-10228, which addressed a similar but different code path in the same function.

Root Cause

The root cause lies in the error handling logic within the iconv conversion routines for IBM character encodings. When the function detects an invalid multi-byte sequence, it should either report an error and advance past the problematic bytes, or skip the invalid sequence entirely. However, in the affected code path, the function signals an error but fails to update the input position pointer. As a result, subsequent iterations of the conversion loop attempt to process the exact same input position, creating an infinite loop condition that cannot be escaped without external intervention.

Attack Vector

The attack vector is local, requiring the attacker to have local access to the system or the ability to provide input to an application that uses the vulnerable iconv function with the affected IBM encodings. Attack scenarios include:

• Providing maliciously crafted text files to applications that perform character encoding conversion
• Submitting specially crafted form data to web applications running on affected systems
• Exploiting services that process user-supplied text with character set conversion
• Targeting backup or logging systems that convert character encodings during data processing

The vulnerability can be triggered by providing a carefully constructed byte sequence that appears to be a partial or malformed multi-byte character in one of the affected IBM encodings.

Detection Methods for CVE-2020-27618

Indicators of Compromise

• Processes consuming 100% CPU for extended periods during text processing operations
• Applications becoming unresponsive when handling files with IBM character encodings
• System load spikes associated with iconv or character conversion operations
• Service timeouts in applications performing internationalization tasks

Detection Strategies

• Monitor for processes with abnormally high CPU utilization that correlate with text processing activities
• Implement application-level timeouts for character encoding conversion operations
• Deploy SentinelOne Singularity platform to detect anomalous process behavior indicative of infinite loop conditions
• Use system profiling tools to identify stuck iconv calls in production environments

Monitoring Recommendations

• Enable detailed logging for applications that perform character set conversions with IBM encodings
• Configure alerting for CPU utilization thresholds on systems running text processing services
• Monitor application response times for services that handle internationalized content
• Track glibc version deployment across infrastructure to identify vulnerable systems

How to Mitigate CVE-2020-27618

Immediate Actions Required

• Upgrade glibc to a patched version that addresses the infinite loop condition
• Apply vendor-specific patches for NetApp, Oracle, and Debian systems
• Implement input validation to reject or sanitize potentially malicious multi-byte sequences
• Configure application-level timeouts to prevent indefinite blocking on conversion operations

Patch Information

GNU glibc maintainers have addressed this vulnerability in subsequent releases. System administrators should consult their distribution's package repositories for patched versions. Additional vendor-specific guidance is available from:

• Sourceware Bug Report #19519 - Original bug tracking and patch discussion
• Sourceware Bug Report #26224 - Related technical details
• NetApp Security Advisory - NetApp product-specific guidance
• Oracle Security Alert January 2022 - Oracle product patches
• Oracle Security Alert April 2022 - Additional Oracle updates
• Debian LTS Announcement - Debian package updates
• Gentoo GLSA 2021-07-07 - Gentoo security advisory

Workarounds

• Avoid using the affected IBM character encodings (IBM1364, IBM1371, IBM1388, IBM1390, IBM1399) when possible
• Implement input size limits and processing timeouts in applications using iconv
• Use alternative character conversion libraries for processing untrusted input with IBM encodings
• Deploy application sandboxing to limit the impact of resource exhaustion attacks

# Check installed glibc version
ldd --version

# On Debian/Ubuntu systems, verify glibc package version
dpkg -l libc6

# On RHEL/CentOS systems, verify glibc package version
rpm -q glibc

# Update glibc on Debian/Ubuntu
sudo apt-get update && sudo apt-get upgrade libc6

# Update glibc on RHEL/CentOS
sudo yum update glibc