CVE-2020-10531: ICU Buffer Overflow Vulnerability

CVE-2020-10531 Overview

An integer overflow vulnerability was discovered in International Components for Unicode (ICU) for C/C++ through version 66.1. The flaw exists in the UnicodeString::doAppend() function within common/unistr.cpp, where an integer overflow can lead to a heap-based buffer overflow. This vulnerability affects a wide range of software that depends on the ICU library for Unicode text processing, including major browsers like Google Chrome, Node.js, and various Linux distributions.

Critical Impact

Successful exploitation of this vulnerability could allow an attacker to execute arbitrary code or cause denial of service through a heap-based buffer overflow triggered by specially crafted Unicode strings.

Affected Products

• ICU-Project International Components for Unicode (through version 66.1)
• Google Chrome (prior to patched versions)
• Node.js (multiple versions)
• Red Hat Enterprise Linux Desktop/Server/Workstation 6.0
• Fedora 30, 31, 33
• Debian Linux 8.0, 9.0, 10.0
• Ubuntu Linux 12.04, 14.04, 16.04, 18.04, 19.10
• openSUSE Leap 15.1
• Oracle Banking Extensibility Workbench 14.3.0, 14.4.0

Discovery Timeline

• 2020-02-24 - Chrome Stable Channel Update released addressing the vulnerability
• 2020-03-12 - CVE-2020-10531 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-10531

Vulnerability Analysis

This vulnerability resides in the core Unicode string handling functionality of ICU. The UnicodeString::doAppend() function performs string concatenation operations without proper validation of the resulting length. When the oldLength and srcLength values are added together, if both are sufficiently large, the arithmetic operation can wrap around due to integer overflow in 32-bit signed integer space. This causes newLength to become a small or negative value, leading the function to allocate an insufficient buffer size for the combined string data.

When the actual string data is subsequently written to this undersized buffer, a heap-based buffer overflow occurs. An attacker can potentially leverage this memory corruption to achieve arbitrary code execution or crash the application, resulting in denial of service.

Root Cause

The root cause is the absence of integer overflow checking when computing the new string length during append operations. The vulnerable code directly adds oldLength + srcLength without validating whether the result exceeds the maximum representable value for a 32-bit signed integer. This is a classic CWE-190 (Integer Overflow or Wraparound) vulnerability that subsequently leads to CWE-122 (Heap-based Buffer Overflow).

Attack Vector

The attack can be conducted remotely over a network. An attacker must convince a user to interact with malicious content (such as a crafted web page or document) that triggers the vulnerable code path. The attack requires:

1. Crafting input that causes the ICU library to process Unicode strings with carefully calculated lengths
2. The sum of string lengths must overflow the 32-bit integer boundary
3. The overflow results in undersized buffer allocation followed by heap corruption during the string copy operation

Applications that process untrusted Unicode text input are at risk, particularly web browsers, email clients, and document processors that utilize the ICU library.

// Security patch from ICU GitHub repository
// Source: https://github.com/unicode-org/icu/commit/b7d08bc04a4296982fcef8b6b8a354a9e4e7afca
   }
 
   int32_t oldLength = length();
-  int32_t newLength = oldLength + srcLength;
+  int32_t newLength;
+  if (uprv_add32_overflow(oldLength, srcLength, &newLength)) {
+    setToBogus();
+    return *this;
+  }
 
   // Check for append onto ourself
   const UChar* oldArray = getArrayStart();

The patch introduces safe arithmetic using uprv_add32_overflow() to detect integer overflow conditions before they can cause memory corruption. When an overflow is detected, the function safely sets the string to a bogus state and returns early.

Detection Methods for CVE-2020-10531

Indicators of Compromise

• Unexpected application crashes in processes utilizing ICU library functions, particularly string processing operations
• Memory access violations or segmentation faults originating from unistr.cpp or related ICU components
• Abnormal memory allocation patterns showing unusually small allocations followed by large write operations
• Core dumps indicating heap corruption in Unicode string handling contexts

Detection Strategies

• Deploy memory protection mechanisms such as ASLR, DEP/NX, and heap canaries to detect and mitigate exploitation attempts
• Implement application crash monitoring to identify patterns consistent with buffer overflow exploitation
• Use static code analysis tools to identify applications using vulnerable ICU library versions
• Deploy endpoint detection solutions capable of identifying memory corruption attack patterns

Monitoring Recommendations

• Monitor for unusual process behavior in applications known to use ICU libraries (browsers, Node.js applications, document processors)
• Track software inventory to identify systems running vulnerable ICU versions (66.1 and earlier)
• Implement centralized logging for application crashes and memory access violations
• Review web application firewall logs for attempts to deliver oversized or malformed Unicode content

How to Mitigate CVE-2020-10531

Immediate Actions Required

• Update ICU library to version 66.2 or later which contains the security fix
• Update Google Chrome to the latest stable release (patched in February 2020 update)
• Apply vendor-specific patches for your Linux distribution (Red Hat, Debian, Ubuntu, Fedora, openSUSE)
• Update Node.js to patched versions that include the ICU security fix
• Review and update any applications that bundle their own copy of the ICU library

Patch Information

The official fix was committed to the ICU repository and is available through multiple channels:

• ICU GitHub Commit Update - Main repository fix
• ICU GitHub Pull Request #971 - Original pull request with discussion
• Chromium ICU Dependency Commit - Chrome-specific patch

Distribution-specific advisories:

• Red Hat Security Advisory RHSA-2020:0738
• Ubuntu Security Notice USN-4305-1
• Debian Security Advisory DSA-4646
• Gentoo GLSA 202003-15

Workarounds

• If immediate patching is not possible, consider restricting access to applications that process untrusted Unicode input
• Implement input validation to reject excessively large text inputs before they reach ICU processing functions
• Deploy Web Application Firewalls (WAF) with rules to detect and block oversized or malformed Unicode payloads
• Isolate vulnerable applications using containerization or sandboxing to limit the impact of potential exploitation

# Example: Check installed ICU version on Linux systems
# Debian/Ubuntu
dpkg -l libicu*

# Red Hat/CentOS/Fedora
rpm -qa | grep icu

# Check ICU version programmatically
icuinfo 2>/dev/null || pkg-config --modversion icu-uc

# Update ICU on Debian/Ubuntu
sudo apt update && sudo apt upgrade libicu-dev libicu66

# Update ICU on Red Hat/CentOS
sudo yum update icu