CVE-2021-22898: Haxx Curl Information Disclosure Flaw

CVE-2021-22898 Overview

CVE-2021-22898 is an information disclosure vulnerability affecting curl versions 7.7 through 7.76.1. The vulnerability exists in the TELNET option parser when using the -t command line option (known as CURLOPT_TELNETOPTIONS in libcurl) to send variable=content pairs to TELNET servers. Due to a flaw in the option parser for sending NEW_ENV variables, libcurl could be made to pass uninitialized data from a stack-based buffer to the server, potentially revealing sensitive internal information over a clear-text network protocol.

Critical Impact

Uninitialized stack memory can be transmitted to remote TELNET servers, potentially exposing sensitive data such as memory contents, credentials, or other application data that resides in memory.

Affected Products

• Haxx curl 7.7 through 7.76.1
• Debian Linux 9.0
• Fedora 33 and 34
• Oracle Communications Cloud Native Core (Binding Support Function 1.11.0, Network Function Cloud Native Environment 1.10.0, Network Repository Function 1.15.0/1.15.1, Network Slice Selection Function 1.8.0, Service Communication Proxy 1.15.0)
• Oracle Essbase
• Oracle MySQL Server
• Siemens SINEC Infrastructure Network Services
• Splunk Universal Forwarder

Discovery Timeline

• 2021-06-11 - CVE-2021-22898 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-22898

Vulnerability Analysis

This vulnerability is classified as Uninitialized Memory Use (CWE-909) and Information Exposure (CWE-200). The flaw resides in curl's TELNET option parsing code within lib/telnet.c. When processing NEW_ENV variable assignments intended for TELNET servers, the parser uses sscanf() to extract variable name and value pairs from user-provided input.

The root issue is that the sscanf() function's return value was not properly validated. The original code only checked if sscanf() returned a truthy value (any non-zero matches), rather than verifying that exactly two fields (varname and varval) were successfully parsed. This meant that if only one field was matched, the second field (varval) would contain uninitialized stack data, which would then be transmitted to the remote TELNET server.

Root Cause

The vulnerability stems from improper return value checking of the sscanf() function in the TELNET option parser. The sscanf() function returns the number of successfully matched and assigned input items. The original code treated any successful match (return value >= 1) as valid, when it should have required exactly two matches (return value == 2) to ensure both the variable name and value were properly parsed from the input string.

Attack Vector

Exploitation requires network access and user interaction. An attacker would need to convince a user to connect to a malicious TELNET server using curl with specifically crafted -t options. The attack scenario involves:

1. User specifies a malformed variable assignment using the -t option (e.g., missing the value portion after the comma)
2. The sscanf() parser only matches the variable name, leaving the value buffer uninitialized
3. Curl transmits both the variable name and the uninitialized value buffer content to the TELNET server
4. The malicious server receives potentially sensitive stack memory contents

         size_t tmplen = (strlen(v->data) + 1);
         /* Add the variable only if it fits */
         if(len + tmplen < (int)sizeof(temp)-6) {
-          if(sscanf(v->data, "%127[^,],%127s", varname, varval)) {
+          if(sscanf(v->data, "%127[^,],%127s", varname, varval) == 2) {
             msnprintf((char *)&temp[len], sizeof(temp) - len,
                       "%c%s%c%s", CURL_NEW_ENV_VAR, varname,
                       CURL_NEW_ENV_VALUE, varval);

Source: GitHub cURL Commit

Detection Methods for CVE-2021-22898

Indicators of Compromise

• Outbound TELNET connections (port 23) from systems running vulnerable curl versions
• Curl process invocations with the -t or --telnet-option command line parameters
• Log entries showing malformed TELNET option strings with missing value components
• Network traffic containing NEW_ENV TELNET negotiations with unusual or binary data in the value fields

Detection Strategies

• Monitor for curl command execution with TELNET-related options using process auditing tools
• Implement network traffic analysis to identify TELNET protocol connections originating from unexpected sources
• Deploy file integrity monitoring on curl/libcurl binaries to ensure patched versions are in place
• Use vulnerability scanning tools to identify systems running affected curl versions (7.7 through 7.76.1)

Monitoring Recommendations

• Enable command-line auditing on endpoints to capture curl invocations with TELNET options
• Configure network security monitoring to alert on TELNET traffic to external destinations
• Establish baseline of legitimate TELNET usage and alert on deviations
• Review system logs for curl-related errors or anomalous behavior patterns

How to Mitigate CVE-2021-22898

Immediate Actions Required

• Upgrade curl to version 7.77.0 or later, which contains the security fix
• If immediate upgrade is not possible, avoid using the -t/--telnet-option or CURLOPT_TELNETOPTIONS features
• Review and restrict TELNET connectivity from systems where curl is used
• Audit systems to identify all instances of vulnerable curl/libcurl installations

Patch Information

The fix was implemented in curl commit 39ce47f219b09c380b81f89fe54ac586c8db6bde. The patch corrects the sscanf() return value check in lib/telnet.c to require exactly 2 matches before proceeding with the variable transmission. Official patches are available through:

• cURL CVE-2021-22898 Details
• GitHub cURL Commit
• Oracle CPU July 2021 Security Alert
• Siemens Product Security Advisory
• Debian Security Advisory DSA-5197

Workarounds

• Disable or block TELNET protocol usage entirely if not required for business operations
• Configure network firewalls to block outbound TELNET connections (TCP port 23)
• Use application whitelisting to prevent curl from being invoked with TELNET options
• Implement egress filtering to restrict curl network connections to approved destinations only

# Configuration example - Block outbound TELNET at firewall level
# iptables rule to block outbound TELNET connections
iptables -A OUTPUT -p tcp --dport 23 -j DROP

# Verify curl version to ensure patched release
curl --version | head -1
# Should show curl 7.77.0 or later

# Alternative: Use curl without TELNET support (compile-time option)
# ./configure --disable-telnet