CVE-2024-39420: Adobe Acrobat Race Condition Vulnerability

CVE-2024-39420 Overview

CVE-2024-39420 is a Time-of-check Time-of-use (TOCTOU) race condition vulnerability affecting Adobe Acrobat and Acrobat Reader. The flaw exists in multiple versions including 20.005.30636, 24.002.21005, 24.001.30159, 20.005.30655, 24.002.20965, 24.002.20964, 24.001.30123, and 24.003.20054. Successful exploitation leads to arbitrary code execution in the context of the current user. Exploitation requires user interaction, specifically opening a malicious file. The vulnerability is tracked under CWE-367 and was disclosed in Adobe Security Advisory APSB24-70.

Critical Impact
Attackers can achieve arbitrary code execution on affected systems by convincing a user to open a crafted PDF file, compromising confidentiality, integrity, and availability of the host.

Affected Products

Adobe Acrobat DC (Continuous track) — versions through 24.002.21005
Adobe Acrobat 2020 (Classic track) — versions through 20.005.30655
Adobe Acrobat Reader DC and Acrobat Reader 2020 on Windows and macOS

Discovery Timeline

2024-08-14 - CVE-2024-39420 published to NVD
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2024-39420

Vulnerability Analysis

CVE-2024-39420 is a Time-of-check Time-of-use (TOCTOU) race condition [CWE-367] in Adobe Acrobat and Reader. The product validates the state of a resource, then uses that resource in a subsequent operation without ensuring the state has not changed. An attacker who can influence the resource between the check and the use can substitute a malicious resource, causing Acrobat to operate on attacker-controlled content. When Acrobat processes the substituted resource, it can lead to arbitrary code execution under the identity of the user running Acrobat.

Root Cause

The root cause is a non-atomic check-then-use sequence during file or resource handling in Acrobat. The application performs a validation on a file, path, or object state and later acts on that resource without holding a lock or re-validating the resource at the point of use. This window allows a concurrent local process to swap or modify the resource, invalidating the earlier check.

Attack Vector

The attack vector is local and requires user interaction. A victim must open a malicious PDF or associated file crafted to trigger the vulnerable code path. During processing, an attacker-controlled process running on the same system races the Acrobat check-then-use sequence to substitute a malicious resource. On success, Acrobat executes attacker-supplied code in the user's context. The attack complexity is high because the race window must be reliably won on the target system.

No public proof-of-concept exploit is currently available. Technical details are documented in the Talos Intelligence Vulnerability Report TALOS-2024-2011.

Detection Methods for CVE-2024-39420

Indicators of Compromise

Unexpected child processes spawned by Acrobat.exe or AcroRd32.exe, particularly command interpreters such as cmd.exe, powershell.exe, or /bin/sh.
Suspicious file replacement activity in temporary directories used by Acrobat immediately after a PDF is opened.
PDF files delivered via email or download from untrusted sources that trigger secondary file writes at open time.

Detection Strategies

Monitor for anomalous process trees originating from Adobe Acrobat or Reader binaries, especially execution of scripting hosts or shells.
Detect rapid file create/rename/delete operations targeting paths that Acrobat has recently accessed, indicating a potential race attempt.
Alert on Acrobat processes writing to or executing binaries from user-writable directories such as %TEMP% or %LOCALAPPDATA%.

Monitoring Recommendations

Enable process creation, image load, and file operation telemetry on endpoints running Adobe Acrobat.
Correlate PDF open events with subsequent file system and process activity to identify the narrow TOCTOU exploitation window.
Track EPSS trends for CVE-2024-39420 (currently 3.492%, percentile 88.047) as an input to patch prioritization.

How to Mitigate CVE-2024-39420

Immediate Actions Required

Update Adobe Acrobat and Acrobat Reader to the fixed versions listed in Adobe Security Advisory APSB24-70.
Inventory all endpoints running affected Acrobat versions on Windows and macOS and prioritize patching for users who routinely open external PDFs.
Instruct users to avoid opening PDF files from untrusted senders until patches are deployed.

Patch Information

Adobe released fixes in advisory APSB24-70 on August 13, 2024. Administrators should deploy the updated Acrobat DC Continuous and Acrobat 2020 Classic builds published by Adobe. Enable automatic updates in Acrobat where policy allows, and validate that the installed build number exceeds the vulnerable versions enumerated in the advisory.

Workarounds

Enable Protected View and Protected Mode in Acrobat to limit the impact of malicious documents.
Restrict execution of Acrobat child processes using application control policies such as Windows Defender Application Control or AppLocker.
Block PDF attachments from untrusted external senders at the email gateway pending patch deployment.

bash

# Query installed Adobe Acrobat/Reader versions on Windows endpoints
reg query "HKLM\SOFTWARE\WOW6432Node\Adobe\Adobe ARM\1.0\ARM" /v iProductVersion