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CVE-2026-103603— Unbounded HSS public key level count allows huge array allocation during signature verification

Quick assessment

Affected
Legion of the Bouncy Castle Inc. bc-csharp
Exploitation
No confirmed in-the-wild exploitation; assess based on exposure
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

在 Legion of the Bouncy Castle Inc. 提供的 bc-csharp 库（版本号低于 2.7.0）中，HSS/LMS 签名代码（具体位于 HssPublicKeyParameters 和 HssSignature 类）存在内存分配过量漏洞。远程未认证攻击者若能同时提供 HSS 公钥和签名，即可通过构造包含异常高的层数（level count）的公钥编码，导致服务因内存耗尽而拒绝服务。 原因在于：在解析 HSS 公钥时，读取到的层数 L 未与 RFC 8554 规定的最大值 8 进行校验；

CVSS 8.7 · High EPSS 0.41% · P33

Possible ATT&CK Techniques 1 AI

T1496 · Resource Hijacking

Affected Version Matrix 1

VendorProduct Version RangeStatus
Legion of the Bouncy Castle Inc. bc-csharp < 2.7.0 affected
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I. Basic Information for CVE-2026-103603

Vulnerability Information
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Vulnerability Title
Unbounded HSS public key level count allows huge array allocation during signature verification
Source: CVE Program / CVE List V5
Vulnerability Description
Memory allocation with excessive size value in the HSS/LMS signature code (HssPublicKeyParameters, HssSignature) in Legion of the Bouncy Castle Inc. bc-csharp before 2.7.0 allows a remote unauthenticated attacker who can supply both an HSS public key and a signature to cause a denial of service through memory exhaustion via a public key encoding with an excessive level count, because the level count L read when parsing an HSS public key was not checked against the RFC 8554 maximum of 8, and signature parsing then allocated an array of L - 1 entries before reading any further signature data. A single verification can commit up to about 17 GB of memory or fail with an OutOfMemoryException.
Source: CVE Program / CVE List V5
CVSS Information
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N
Source: CVE Program / CVE List V5
Vulnerability Type
未经控制的内存分配
Source: CVE Program / CVE List V5

Affected Products

Vendor Product Affected Versions CPE Subscribe
Legion of the Bouncy Castle Inc. bc-csharp 0 ~ 2.7.0 -

II. Public POCs for CVE-2026-103603

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III. Intelligence Information for CVE-2026-103603

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Patches & Fixes for CVE-2026-103603 (1)

Security Blog Posts for CVE-2026-103603 (1)

Same Patch Batch · Legion of the Bouncy Castle Inc. · 2026-10-02 · 24 CVEs total

CVE-2026-63569 9.1 CRITICAL MTI/A0 DHAgreement does not validate the peer's ephemeral value
CVE-2026-16000 8.7 HIGH KCcmBlockCipher (DSTU 7624 CCM) tag not bound to nonce when no associated data is used
CVE-2026-63571 8.7 HIGH Attribute certificate path validation does not verify the attribute certificate's signatur
CVE-2026-63566 8.7 HIGH DTLS handshake reassembler allocates buffer from unchecked 24-bit length
CVE-2026-63574 8.7 HIGH Unbounded allocation from OpenPGP signature and user attribute subpacket lengths
CVE-2026-63568 8.7 HIGH Unbounded CMP/CRMF password-based MAC iteration count allows CPU exhaustion
CVE-2026-103600 8.7 HIGH Unbounded ASN.1 nesting depth causes process-terminating stack overflow
CVE-2026-103604 8.7 HIGH Quadratic-time escaping when converting X.509 distinguished names to strings
CVE-2026-17507 8.7 HIGH MLS membership checks compare a uint32 leaf_index as signed, admitting an out-of-range sen
CVE-2026-18036 8.2 HIGH NTRU leaks private key information by reducing secret values with a non-constant-time inte
CVE-2026-63577 8.2 HIGH Name Constraints bypass: directoryName constraint matched at any position in the DN instea
CVE-2026-63576 8.2 HIGH URI name constraints checked against a mis-parsed host
CVE-2026-63573 8.2 HIGH Bleichenbacher padding oracle in CMS RSA PKCS#1 v1.5 key-transport unwrap
CVE-2026-103602 8.2 HIGH Name constraints bypass via trailing dot in rfc822Name, dNSName and URI hosts
CVE-2026-103601 8.2 HIGH CcmBlockCipher and KCcmBlockCipher leave unverified plaintext in the output buffer after a
CVE-2026-63567 8.2 HIGH IesEngine block-cipher mode checks padding before MAC (CBC padding oracle)
CVE-2026-15999 8.2 HIGH AES-CCM decryption accepts zero or out-of-range tag length, bypassing authentication
CVE-2026-16001 8.2 HIGH IesEngine stream-mode MAC forgery via length-dependent KDF split
CVE-2026-63575 7.1 HIGH PKCS#12 key derivation loops about 2^32 times on a zero or negative iteration count
CVE-2026-63578 7.1 HIGH Unbounded PBE iteration count when decrypting PKCS#8 private keys

Showing top 20 of 24 CVEs. View all on vendor page &rarr; →

IV. Related Vulnerabilities

Same product: bc-csharp
Same vendor: Legion of the Bouncy Castle Inc.
Same weakness: CWE-789

V. Comments for CVE-2026-103603

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