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1Repair Windows errors before they cause bigger problems2Fix the driver behind crashes, sound loss and screen glitches3Clear out junk files and repair common Windows errorsA lenient DER parser can create a signature bypass when it accepts an encoding or structure that the signature scheme does not permit, then extracts the same digest or meaning a verifier expects. The gap is between what the signed format requires and what the implementation accepts—not a general property of ASN.1 parsers, and not a guarantee that any malformed signature will verify.
Why canonical DER matters to signature verification
ASN.1 describes data structures; BER provides encoding rules for them, and DER is a restricted BER profile that selects a single encoding for each value. That single representation matters when cryptographic verification depends on how data is encoded. Accepting alternative BER forms as though they were the required DER form can make a verifier’s acceptance rules broader than the signature format’s rules.
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RFC 7468, Appendix B, “DER Expectations,” puts the point directly: “A digital signature is (supposed to be) computed over the DER encoding of the semantic content, so providing anything other than the DER encoding is senseless.” The appendix is informative and directs readers to the relevant standards for normative requirements. It also notes that every DER encoding is a BER encoding, but only one BER encoding is the DER encoding of a given value; parsing and digesting non-DER input amounts to guesswork.
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DER’s definite-length encoding also lets a parser anticipate resource needs. But correctly decoding lengths and consuming all input is not enough to establish that a signature has the expected structure or meaning.
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How the interpretation gap can bypass a signature check
- The signature scheme defines an expected representation. In RSA PKCS#1 v1.5, for example, verification involves a padded encoded message containing a
DigestInfostructure for the expected hash. - The verifier decodes and checks that representation. It must enforce the relevant scheme’s structure and semantics, not merely find a digest value somewhere in a parseable ASN.1 object.
- A permissive implementation accepts more than the format allows. If it tolerates extra fields, non-canonical encodings, unexpected nesting, or invalid padding and still extracts the expected digest, it may accept an input a strict verifier rejects.
An attacker’s opportunity comes from that mismatch: a constructed signature block may satisfy the permissive implementation’s checks while failing the intended format’s requirements. This does not mean an attacker can alter an arbitrary byte in a valid signature and make it verify. The outcome depends on the signature scheme, parser and verifier behavior, key parameters, and deployment.
What the Forge advisory illustrates
Forge’s advisory describes a verification path in which _parseAllDigestBytes ensured that all bytes were consumed, but did not ensure that the parsed structure was the canonical, minimal DigestInfo shape expected by RFC 8017 verification semantics. The advisory also identifies missing enforcement of the specified minimum eight-byte PKCS#1 v1.5 padding string. In the tested Forge versions and defaults described by that advisory, those checks could allow a crafted signature to pass where OpenSSL rejected it.
The lesson is specific but broadly useful in code review: “no trailing bytes” is a useful check, not proof that the data has the one permitted structure. A verifier must check the expected algorithm identifiers, digest representation, nesting, and padding constraints for its scheme.
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Red Hat’s Libreswan advisory describes a flaw in validation of a DER-encoded ASN.1 digest in an IKEv2 RSASSA-PKCS1-v1_5 authentication path. It separates two consequences:
| Input or condition | Reported consequence | Qualification |
|---|---|---|
| A malformed hash shorter than expected | An assertion failure can cause the daemon to restart, resulting in denial of service. | This is the malformed-input availability impact described by Red Hat. |
A forgery attempt using a weak public RSA exponent, such as e=3 |
A Bleichenbacher-style signature forgery can enable an authentication bypass. | Red Hat says its modern enterprise policy blocks those weak exponents, affecting practical severity in that environment. |
These are not interchangeable outcomes: a crash from malformed input is not itself a signature forgery. Red Hat recommends upgrading or restricting authentication to modern algorithms. It documents ECDSA and RSASSA-PSS authentication as ways to avoid the vulnerable legacy path, with a compatibility caveat: native Windows VPN clients that do not support RSASSA-PSS may no longer work with that configuration. That guidance is specific to the documented Libreswan deployment and should not be generalized to every operating system or policy.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Parser bugs can cross trust boundaries in other ways
DER leniency is one route from parsing to failed authentication, but ASN.1 defects can also cause memory-safety or availability failures without creating a signature bypass. Other examples show why certificate and firmware validation must be reviewed beyond the low-level decoder:
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- A Microsoft Research paper explains that X.509 extensions can carry payloads inside an OCTET STRING that must then be decoded according to the extension’s object identifier. It notes that an unrecognized critical extension must be rejected. A decoder can correctly parse the outer DER while the application mishandles the extension’s semantics.
- A 2019 SSTIC paper reports that a crafted
keyUsageextension enabled a secure-boot bypass on the listed NXP processors. It also describes a Nintendo 3DS RSA PKCS#1 v1.5 issue in which unchecked bounds for the embedded signed hash changed which data the BootROM checked. These are trust-boundary failures, but they are distinct mechanisms from accepting non-canonical DER.
What to check in a verifier or parser
- Require the encoding profile the protocol specifies. Where DER is required, reject malformed and non-canonical input rather than permissively normalizing it before verification.
- Validate the exact signature structure. Check the expected
DigestInfocontents and PKCS#1 v1.5 padding constraints. Do not treat complete input consumption as a substitute for structural validation. - Check boundaries and nested meaning. Validate lengths, integer and bit-string bounds, nested payload types, resource limits, and X.509 extension semantics, including handling of unrecognized critical extensions.
- Test rejection paths. Include alternate BER encodings, trailing or embedded ASN.1 fields, malformed digest lengths, and boundary-length padding in negative tests. Confirm that invalid inputs fail closed rather than being normalized or partially interpreted.
- Compare implementations on more than parser strictness. Review canonical-encoding enforcement, scheme-level structure checks, handling of extra data, key-parameter constraints, and failure behavior separately. A memory-safe parser can still be semantically permissive, while a strict DER decoder cannot compensate for incorrect certificate-validation policy.
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