GHSA-vc8p-8pxg-rfwg

## Summary The DER parser used for application-supplied private keys did not safely validate encoded length values before converting them to `Int` values or allocating arrays. A malformed private-key file could encode a length that overflowed or wrapped around, or request an allocation much larger than the available input. This could cause parsing errors or an uncaught `OutOfMemoryError`, potentially terminating the application process. ## Details The issue was in `DerReader.readLength()` and primitive readers such as `readInteger()`. `readLength()` previously accepted up to 127 length octets and accumulated them into an `Int`: ```kotlin length = (length shl 8) or nextByte ``` This permitted integer overflow. For example: - `0x1_0000_0001` wrapped to `1`. - `0x8000_0000` wrapped to `Int.MIN_VALUE`. Primitive readers then allocated memory based on the resulting value without first checking it against the remaining input: ```kotlin val bytes = ByteArray(length) data.get(bytes) ``` A six-byte DER value declaring a 1 GiB INTEGER caused an immediate `OutOfMemoryError` when tested with a constrained JVM heap. Because `OutOfMemoryError` is not an `Exception`, it is not caught by the public-key authentication error handling and may terminate the application process. A zero-length DER INTEGER is also invalid, but it does not produce `BigInteger.ZERO`: Java throws `NumberFormatException` when constructing a `BigInteger` from an empty byte array. No weakened or usable cryptographic key has been demonstrated through this issue. ## Attack Requirements The affected DER parser processes private-key material explicitly supplied by the application through APIs such as: - `SshClient.authenticatePublicKey()` - `SshKeys.decodePemPrivateKey()` - `SshSigning.sign()` - `SshSigning.getPublicKey()` The DER input is not populated from SSH server host keys or agent-forwarding requests. Exploitation therefore requires a user or application to load an attacker-provided private-key file. The issue is not remotely exploitable by an SSH server. ## Impact Successful exploitation can cause: - Incorrect DER length interpretation due to integer wraparound - Excessive memory allocation - An uncaught `OutOfMemoryError` - Loss of availability of the affected application process There is no demonstrated confidentiality or integrity impact. ## Remediation The DER parser now: - Rejects indefinite lengths - Explicitly limits long-form lengths to `Int.SIZE_BYTES` (four octets) and rejects values above `Int.MAX_VALUE` - Accumulates long-form lengths in a `Long` before converting to `Int` - Rejects truncated and non-minimal length encodings - Checks declared lengths against the remaining input before allocation or advancing the input position - Rejects zero-length DER INTEGER, BIT STRING, and OBJECT IDENTIFIER values where an empty encoding is invalid - Rejects non-canonical DER INTEGER encodings with redundant sign octets The bounds checks are implemented in shared DER reader helpers and apply to INTEGER, OCTET STRING, BIT STRING, OBJECT IDENTIFIER, SEQUENCE, context-specific values, and skipped values. PKCS#1 RSA and SEC1 EC private keys pass application-supplied DER directly through these helpers. PKCS#8 input is parsed by the JCA provider, and OpenSSH private keys use a separate wire-format parser rather than `DerReader`.