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CVE-2026-63567— IesEngine block-cipher mode checks padding before MAC (CBC padding oracle)

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.

Legion of the Bouncy Castle Inc. 的 bc-csharp 库在 2.7.0 版本之前，IesEngine.DecryptBlock 方法中存在可观察到的差异。该漏洞允许远程攻击者通过捕获 IES 或 ECIES 密文，并在同一密钥对下提交修改后的密文以供解密，从而利用 CBC 模式下的填充预言机攻击（CBC padding-oracle attack）恢复明文。问题在于，在块密码模式下，引擎会在验证消息认证码（MAC）之前先解密密文并移除填充。因此，当填充失败时，系统会返回与 MAC

CVSS 8.2 · High

Possible ATT&CK Techniques 1 AI

T1530 · Data from Cloud Storage
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I. Basic Information for CVE-2026-63567

Vulnerability Information
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Vulnerability Title
IesEngine block-cipher mode checks padding before MAC (CBC padding oracle)
Source: CVE Program / CVE List V5
Vulnerability Description
Observable discrepancy in IesEngine.DecryptBlock in Legion of the Bouncy Castle Inc. bc-csharp before 2.7.0 allows a remote attacker who has captured an IES or ECIES ciphertext, and who can submit modified ciphertexts for decryption under the same key pair, to recover its plaintext via a CBC padding-oracle attack, because in block-cipher mode the engine decrypts the ciphertext and removes its padding before verifying the MAC. A padding failure is therefore reported with a different error message, and without the MAC computation, compared with a MAC failure. Only applications that construct IesEngine directly with a padded block cipher, such as AES in CBC mode with PKCS#7 padding, are affected; stream-mode IES is not.
Source: CVE Program / CVE List V5
CVSS Information
CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N
Source: CVE Program / CVE List V5
Vulnerability Type
通过差异性导致的信息暴露
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-2026-63567

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III. Intelligence Information for CVE-2026-63567

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Other References for CVE-2026-63567 (3)

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-2026-63568 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-2026-63574 8.7 HIGH Unbounded allocation from OpenPGP signature and user attribute subpacket lengths
CVE-2026-103604 8.7 HIGH Quadratic-time escaping when converting X.509 distinguished names to strings
CVE-2026-103603 8.7 HIGH Unbounded HSS public key level count allows huge array allocation during signature verific
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-2026-15999 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-2026-63578 7.1 HIGH Unbounded PBE iteration count when decrypting PKCS#8 private keys
CVE-2026-63570 7.1 HIGH Pkcs12Store.GetCertificateChain loops forever on cyclic issuer links

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

IV. Related Vulnerabilities

Same product: bc-csharp
Same vendor: Legion of the Bouncy Castle Inc.
Same weakness: CWE-203

V. Comments for CVE-2026-63567

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