Mitigating PHP2Shell: Everything You Need to Know
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Miggo AIRoot Cause AnalysisThe vulnerability, CVE-2026-85393, is an incomplete fix for a prior issue (CVE-2026-33894). The core problem lies in the RSA PKCS#1 v1.5 signature verification process within the node-forge library. Specifically, the pki.setRsaPublicKey function, which is responsible for validating the DigestInfo structure, did not perform a sufficiently strict check on the nested DigestAlgorithm sequence. The asn1.validate utility, as noted in the commit message, would ignore extra children within this nested sequence. This allowed an attacker to craft a malicious signature by inserting 'garbage bytes' into the obj.value[0].value (representing the nested DigestAlgorithm sequence) without invalidating the signature according to the existing checks. This bypass enabled signature forgery, particularly against systems using low public exponent RSA keys. The patch directly addresses this by adding a precise length check (obj.value[0].value.length !== (('parameters' in capture) ? 2 : 1)) to ensure the nested DigestAlgorithm sequence contains only the expected OID and an optional NULL parameter, thus preventing the injection of extraneous data. Therefore, the pki.setRsaPublicKey function, in its state prior to this patch, contained the exploitable flaw.
pki.setRsaPublicKeylib/rsa.js
| Package Name | Ecosystem | Vulnerable Versions | First Patched Version |
|---|---|---|---|
| node-forge | npm | <= 1.4.0 |