Executive Summary

This vulnerability exists in the Ed25519 signature verification function of the tinyssh project, specifically in the file tinyssh/crypto_sign_ed25519_tinyssh.c. The flaw is that the verifier does not properly check that the scalar component S of a signature is less than the subgroup order L, as required by the Ed25519 specification (RFC 8032 §5.1.7). Without this check, signatures can be malleated by adding the subgroup order L to S, creating different signatures that are still accepted as valid by tinyssh.

This signature malleability breaks signature uniqueness and can cause inconsistencies across different implementations that enforce strict canonical checks. The vulnerability is exploitable only locally and is considered difficult to exploit, but an exploit has been published.

The issue was fixed by adding a validation step that rejects any signatures where S is greater than or equal to L, ensuring strict subgroup order checks and preventing acceptance of malleable signatures.

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Impact Analysis

This vulnerability can impact you by allowing an attacker with local access to create malleable Ed25519 signatures that are accepted as valid by tinyssh, even though they differ from the original signature. This breaks the uniqueness property of signatures.

Such malleability can lead to inconsistencies in signature verification across different systems or implementations, potentially undermining trust in cryptographic authentication and audit mechanisms that rely on unique, canonical signatures.

While the attack complexity is high and exploitability is difficult, the presence of a published exploit means that attackers could potentially leverage this flaw to bypass certain cryptographic checks or cause confusion in signature validation processes.

Upgrading to the fixed version 20260301 of tinyssh is recommended to mitigate this risk.

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Compliance Impact

I don't know

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Detection Guidance

This vulnerability is related to improper verification of Ed25519 cryptographic signatures within the tinyssh server, specifically allowing signature malleability due to missing checks on the scalar component S of the signature.

Detection involves verifying whether the tinyssh version in use includes the fix that rejects signatures with scalar S values greater than or equal to the Ed25519 subgroup order L.

Since the vulnerability is local and related to signature verification logic, direct network detection commands are not straightforward. However, you can check the installed tinyssh version to determine if it is vulnerable.

• Check tinyssh version installed: `tinyssh -v` or check package version via your package manager.
• Verify if the version is older than 20260301, which contains the fix.

For more technical detection, you could attempt to use a proof-of-concept malleated Ed25519 signature (S + L) against your tinyssh server to see if it accepts such signatures, indicating vulnerability. This requires custom scripts or tools that generate and test Ed25519 signatures with manipulated scalar values.

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Mitigation Strategies

The primary and recommended mitigation step is to upgrade tinyssh to version 20260301 or later, which includes the patch that properly rejects signatures with scalar S values greater than or equal to the Ed25519 subgroup order L.

This update enforces strict subgroup order checks during signature verification, preventing signature malleability and improving cryptographic robustness.

• Download and install tinyssh version 20260301 or newer.
• Apply the patch identified by commit 9c87269607e0d7d20174df742accc49c042cff17 if upgrading is not immediately possible.

Since the attack complexity is high and exploitability is difficult and local, limiting local access to the system and monitoring for suspicious local activity can also help reduce risk until the patch is applied.

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