CVE-2021-39857 is an information disclosure vulnerability affecting Adobe Acrobat Reader DC add-ons for Internet Explorer versions 2021.005.20060 and earlier, 2020.004.30006 and earlier, and 2017.011.30199 and earlier. The vulnerability allows an unauthenticated attacker to determine the existence of local files on a victim's system. Exploitation requires user interaction, specifically that the victim visits a malicious web page controlled by the attacker. The vulnerability falls under CWE-200, indicating an information exposure weakness. Technically, the flaw resides in how the Acrobat Reader add-on handles requests or interactions via Internet Explorer, potentially allowing an attacker to probe for local files without authentication. Although no known exploits are reported in the wild, the vulnerability could be leveraged for reconnaissance purposes, aiding attackers in gathering sensitive information about file presence on targeted systems. This information could then be used to tailor further attacks or escalate privileges. The vulnerability affects multiple versions of Acrobat Reader add-ons integrated with Internet Explorer, which remains in use in some enterprise environments despite its declining market share. No official patches or mitigation links are provided in the source information, indicating that organizations may need to rely on workarounds or updates from Adobe to remediate the issue fully.

For European organizations, the impact of CVE-2021-39857 primarily concerns confidentiality. The ability of an attacker to confirm the presence of local files can reveal sensitive or proprietary information, potentially exposing intellectual property or personal data. While the vulnerability does not directly allow file content access or system compromise, the disclosed information can facilitate targeted phishing, social engineering, or subsequent exploitation steps. Organizations in sectors with stringent data protection requirements, such as finance, healthcare, and government, may face increased risks if attackers use this vulnerability to map file structures or identify sensitive documents. The requirement for user interaction (visiting a malicious web page) limits the attack vector to social engineering or drive-by download scenarios, which can be mitigated but still pose a risk in environments with less user awareness or outdated security controls. Given that Internet Explorer is still used in some legacy systems across Europe, particularly in government and industrial sectors, the vulnerability could affect critical infrastructure or sensitive operations if not addressed.

1. Disable or remove the Adobe Acrobat Reader add-on for Internet Explorer where possible, especially in environments where Internet Explorer usage is not mandatory. 2. Encourage users to avoid using Internet Explorer and transition to modern browsers that do not support this vulnerable add-on. 3. Implement strict web filtering and URL reputation services to block access to known malicious or untrusted websites, reducing the risk of users visiting attacker-controlled pages. 4. Educate users about the risks of interacting with unsolicited links or suspicious websites to mitigate social engineering vectors. 5. Monitor network traffic for unusual requests or patterns indicative of exploitation attempts targeting Acrobat Reader add-ons. 6. Apply any available Adobe updates or patches promptly once released, and subscribe to Adobe security advisories for timely information. 7. Employ endpoint detection and response (EDR) solutions capable of detecting anomalous behaviors related to Acrobat Reader or Internet Explorer add-ons. 8. Consider application whitelisting to prevent unauthorized execution of vulnerable components. These measures go beyond generic advice by focusing on the specific integration of Acrobat Reader with Internet Explorer and the user interaction requirement.