If left unpatched, CVE-2026-84308 could lead to major system disruptions or data loss.
Status: Deferred
Last updated: 🕘 09 Sep 2026, 21:09 UTC
Originally published on: 🕗 01 Sep 2026, 20:17 UTC
Time between publication and last update: 8 days
CVSS Release: version 3
security-advisories@github.com
Secondary
CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:N
CVE-2026-84308: phpseclib is a PHP secure communications library. Prior to 3.0.57 and 4.0.1, pure-PHP X25519 scalar multiplication in phpseclib/Math/PrimeField/Integer.php performs data-dependent conditional modular reductions in add() and subtract(). During the Montgomery ladder in phpseclib/Crypt/EC/BaseCurves/Montgomery.php, the reduction behavior of each step depends on the secret scalar prefix, creating per-step timing and libgmp call-count observations that can reveal a reused 251-bit clamped private scalar. The phpseclib/Crypt/EC/Formats/Keys/MontgomeryPrivate.php derivation path invokes the pure-PHP multiplication without a native-engine check, while phpseclib/Crypt/EC/Formats/Keys/PKCS8.php reaches it when ext-sodium is unavailable. Exploitation requires a reused or long-lived X25519 private key, knowledge of the corresponding public key, execution of the pure-PHP path, and a local observer capable of resolving individual ladder steps or libgmp entry-point calls. Ephemeral X25519 keys, including phpseclib's normal SSH exchange path, are not affected. Recovery of the scalar permanently compromises operations that reuse that key. This issue is fixed in versions 3.0.57 and 4.0.1.
The exploitability of CVE-2026-84308 depends on two key factors: attack complexity (the level of effort required to execute an exploit) and privileges required (the access level an attacker needs).
This vulnerability, CVE-2026-84308, requires a high level of attack complexity and low privileges, making it difficult but not impossible to exploit. Organizations should ensure robust security configurations to mitigate risks.
A lower complexity and fewer privilege requirements make exploitation easier. Security teams should evaluate these aspects to determine the urgency of mitigation strategies, such as patch management and access control policies.
Attack Complexity (AC) measures the difficulty in executing an exploit. A high AC means that specific conditions must be met, making an attack more challenging, while a low AC means the vulnerability can be exploited with minimal effort.
Privileges Required (PR) determine the level of system access necessary for an attack. Vulnerabilities requiring no privileges are more accessible to attackers, whereas high privilege requirements limit exploitation to authorized users with elevated access.
Above is the CVSS Sub-score Breakdown for CVE-2026-84308, illustrating how Base, Impact, and Exploitability factors combine to form the overall severity rating. A higher sub-score typically indicates a more severe or easier-to-exploit vulnerability.
Below is the Impact Analysis for CVE-2026-84308, showing how Confidentiality, Integrity, and Availability might be affected if the vulnerability is exploited. Higher values usually signal greater potential damage.
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