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Integer Overflow in OpenSSL Cipher Update Output-Length Handling
CVE-2021-23840 is an integer-overflow vulnerability in OpenSSL's EVP_CipherUpdate, EVP_EncryptUpdate, and EVP_DecryptUpdate functions. In certain cases, an input length approaching the platform's maximum signed integer value can overflow the output-length argument. The function returns 1, indicating success, while setting the output length to a negative value. Applications consuming this invalid length may behave incorrectly or crash. Affected upstream releases are OpenSSL 1.1.1 through 1.1.1i and 1.0.2 through 1.0.2x; fixes were introduced in 1.1.1j and 1.0.2y.
What it means and what to do now: patch path, interim mitigations, and the assume-compromise checklist.
What an attacker gets, and what they’ve been doing with it.
Successful exploitation can cause application malfunction or a crash, resulting in denial of service. The availability impact is rated high, with a CVSS v3.1 score of 7.5 and vector AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H. No confidentiality or integrity impact is established for this vulnerability. Actual exposure depends on whether an application allows attacker-influenced inputs to reach the affected APIs with sufficiently large input lengths.
If you can’t patch tonight, do this now.
No general OpenSSL-specific workaround is documented. Reduce exposure of affected applications by restricting access to trusted networks and hosts. For affected Mitsubishi Electric industrial deployments, isolate control networks, block access from untrusted networks, avoid direct Internet exposure, and use appropriately secured VPN access when remote connectivity is necessary. These measures reduce reachability but do not correct the integer overflow.
Patch, then assume compromise.
Install a vendor-supported OpenSSL release or distribution package containing the fix. The original upstream fixes were OpenSSL 1.1.1j and, for OpenSSL 1.0.2 premium-support customers, 1.0.2y; these are historical fixed-version thresholds, not current support recommendations. Update applications and products that bundle OpenSSL as well as system libraries. Restart linked services or reboot after updating shared libraries. Product-specific fixes include MELSOFT GT OPC UA Client 1.03D or later, GT SoftGOT2000 1.275M or later, SINEC INS V1.0.1.1 or later, and BigFix OS Deployment Bare Metal Server 310.86.
No public exploits tracked yet. Mallory keeps watching.
Products and vendors Mallory has correlated with this vulnerability. Open in Mallory to drill down to specific CPE configurations and version ranges.
22 sources tracked across advisories, community write-ups, and news. New activity surfaces here as Mallory finds it.